5: DataLink Layer 5-1 Data Communication and Networks Lecture 12 Data Link Layer December 2, 2004
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5 DataLink Layer 5-1
Data Communication and NetworksLecture 12
Data Link Layer
December 2 2004
5 DataLink Layer 5-2
Our goals
understand principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing reliable data transfer flow control done
instantiation and implementation of various link layer technologies
5 DataLink Layer 5-3
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-4
Link Layer Services Framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
Reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-5
Link Layer Services (more)
Flow Control pacing between adjacent sending and receiving nodes
Error Detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correction receiver identifies and corrects bit error(s) without
resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
5 DataLink Layer 5-6
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in
a frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-8
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-9
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-10
Internet checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-11
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence (FCS)
Transmit k+n bits which is exactly divisible by some number
Receiver divides frame by that number If no remainder assume no error may be possible to correct error
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-2
Our goals
understand principles behind data link layer services error detection correction sharing a broadcast channel multiple access link layer addressing reliable data transfer flow control done
instantiation and implementation of various link layer technologies
5 DataLink Layer 5-3
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-4
Link Layer Services Framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
Reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-5
Link Layer Services (more)
Flow Control pacing between adjacent sending and receiving nodes
Error Detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correction receiver identifies and corrects bit error(s) without
resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
5 DataLink Layer 5-6
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in
a frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-8
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-9
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-10
Internet checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-11
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence (FCS)
Transmit k+n bits which is exactly divisible by some number
Receiver divides frame by that number If no remainder assume no error may be possible to correct error
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-3
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-4
Link Layer Services Framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
Reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-5
Link Layer Services (more)
Flow Control pacing between adjacent sending and receiving nodes
Error Detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correction receiver identifies and corrects bit error(s) without
resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
5 DataLink Layer 5-6
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in
a frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-8
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-9
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-10
Internet checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-11
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence (FCS)
Transmit k+n bits which is exactly divisible by some number
Receiver divides frame by that number If no remainder assume no error may be possible to correct error
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-4
Link Layer Services Framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to identify
source dest bull different from IP address
Reliable delivery between adjacent nodes we learned how to do this already (chapter 3) seldom used on low bit error link (fiber some twisted
pair) wireless links high error rates
bull Q why both link-level and end-end reliability
5 DataLink Layer 5-5
Link Layer Services (more)
Flow Control pacing between adjacent sending and receiving nodes
Error Detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correction receiver identifies and corrects bit error(s) without
resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
5 DataLink Layer 5-6
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in
a frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-8
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-9
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-10
Internet checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-11
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence (FCS)
Transmit k+n bits which is exactly divisible by some number
Receiver divides frame by that number If no remainder assume no error may be possible to correct error
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-5
Link Layer Services (more)
Flow Control pacing between adjacent sending and receiving nodes
Error Detection errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame
Error Correction receiver identifies and corrects bit error(s) without
resorting to retransmission
Half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
5 DataLink Layer 5-6
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in
a frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-8
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-9
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-10
Internet checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-11
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence (FCS)
Transmit k+n bits which is exactly divisible by some number
Receiver divides frame by that number If no remainder assume no error may be possible to correct error
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-6
Adaptors Communicating
link layer implemented in ldquoadaptorrdquo (aka NIC) Ethernet card PCMCI card
80211 card
sending side encapsulates datagram in
a frame adds error checking bits
rdt flow control etc
receiving side looks for errors rdt flow
control etc extracts datagram
passes to rcving node
adapter is semi-autonomous
link amp physical layers
sendingnode
frame
rcvingnode
datagram
frame
adapter adapter
link layer protocol
5 DataLink Layer 5-7
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-8
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-9
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-10
Internet checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-11
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence (FCS)
Transmit k+n bits which is exactly divisible by some number
Receiver divides frame by that number If no remainder assume no error may be possible to correct error
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-7
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-8
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-9
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-10
Internet checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-11
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence (FCS)
Transmit k+n bits which is exactly divisible by some number
Receiver divides frame by that number If no remainder assume no error may be possible to correct error
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-8
Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking may include header fields
bull Error detection not 100 reliablebull protocol may miss some errors but rarelybull larger EDC field yields better detection and correction
5 DataLink Layer 5-9
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-10
Internet checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-11
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence (FCS)
Transmit k+n bits which is exactly divisible by some number
Receiver divides frame by that number If no remainder assume no error may be possible to correct error
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-9
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
5 DataLink Layer 5-10
Internet checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-11
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence (FCS)
Transmit k+n bits which is exactly divisible by some number
Receiver divides frame by that number If no remainder assume no error may be possible to correct error
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-10
Internet checksum
Sender treat segment contents
as sequence of 16-bit integers
checksum addition (1rsquos complement sum) of segment contents
sender puts checksum value into UDP checksum field
Receiver compute checksum of
received segment check if computed checksum
equals checksum field value NO - error detected YES - no error detected
But maybe errors nonetheless More later hellip
Goal detect ldquoerrorsrdquo (eg flipped bits) in transmitted segment (note used at transport layer only)
5 DataLink Layer 5-11
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence (FCS)
Transmit k+n bits which is exactly divisible by some number
Receiver divides frame by that number If no remainder assume no error may be possible to correct error
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-11
Cyclic Redundancy Check
For a block of k bits transmitter generates n bit sequence (FCS)
Transmit k+n bits which is exactly divisible by some number
Receiver divides frame by that number If no remainder assume no error may be possible to correct error
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-12
Cyclic Redundancy Check
Example 16-Bit CRC for HDLC Performed on Address Control Data Fields Detects a Variety of Error Conditions Data Message is Represented as a Polynomial
Mi(x) = polynomial of order i =
a0 + a1 x + a2 x2 + a3 x3 + + ai xi where ai = 0 1
Data Message = ai ai-1 hellip a3 a2 a1 a0
StartFrame
Delimiter
SourceAddress
DestinationAddress
FrameControl
Data CRC
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-13
CRC Generation
Given a Specific Polynomial G(x) called the generator and a Data Message M(x) Calculate a Frame Check Sequence (FCS) or Checksum
Append the FCS (Checksum) to the Data Message for Transmission
At the Receiver Recalculate the Checksum and Check with the Transmitted Value
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-14
CRC Algorithm
Multiply xr M(x) where r = degree of G(x) Add r Zeros to M(x) or Shift M(x) Left
Divide xr M(x)G(x) = R(x) R(x) = Reminder of Divide Operation
Transmit xr M(x) + R(x) = T(x) XOR the Shifted Message and R(x)
At the Receiver Recalculate the R(x) and Check Equal to the Transmitted FCS
Errors Occurred in Transmission if Not equal
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-15
CRC Error Detection
All Single Bit Errors All Double Bit Errors Any Odd Number of Errors Any Burst Error for Which the Length of
the Burst Error is Less Than the Length of the Divisor Polynomial G(x)
Most Larger Burst Errors Several G(x) Polynomials Have Been
Used
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-16
Example of G(x) Polynomials
CRC-12 X12 + X11 + X3 + X2 + X + 1
CRC-16 X16 + X15 + X2 + 1
CRC-CCITT X16 + X15 + X5 + 1
CRC-32 X32 + X26 + X23 + X22 + X16 + X12 + X11 + X10
+ X8 + X7 + X5 + X4 + X2 + X + 1 CRCrsquos Are Implemented in Shift
registers
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-17
Example
The generator polynomial P = X2 + 1 Receiver receives a frame F = 111001 1101 -------101 111001 101 --- 100 101 --- 101 101 --- 0 remainder is zero F is without error
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-18
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-19
Multiple Access Links and Protocols
Two types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch and host
broadcast (shared wire or medium) traditional Ethernet upstream HFC 80211 wireless LAN
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-20
Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
interference collision if node receives two or more signals at the same
time
multiple access protocol distributed algorithm that determines how nodes
share channel ie determine when node can transmit
communication about channel sharing must use channel itself no out-of-band channel for coordination
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-21
Ideal Mulitple Access Protocol
Broadcast channel of rate R bps1 When one node wants to transmit it can send
at rate R2 When M nodes want to transmit each can
send at average rate RM3 Fully decentralized
no special node to coordinate transmissions no synchronization of clocks slots
4 Simple
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-22
MAC Protocols a taxonomy
Three broad classes Channel Partitioning
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo Nodes take turns but nodes with more to send can
take longer turns
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-23
Channel Partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt slots 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-24
Channel Partitioning MAC protocols FDMA
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example 6-station LAN 134 have pkt frequency bands 256 idle
TDM (Time Division Multiplexing) channel divided into N time slots one per user inefficient with low duty cycle users and at light load
FDM (Frequency Division Multiplexing) frequency subdivided
frequ
ency
bands time
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-25
Random Access Protocols
When node has packet to send transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo random access MAC protocol specifies
how to detect collisions how to recover from collisions (eg via delayed
retransmissions)
Examples of random access MAC protocols slotted ALOHA ALOHA CSMA CSMACD CSMACA
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-26
Slotted ALOHA
Assumptions all frames same size time is divided into
equal size slots time to transmit 1 frame
nodes start to transmit frames only at beginning of slots
nodes are synchronized if 2 or more nodes
transmit in slot all nodes detect collision
Operation when node obtains fresh
frame it transmits in next slot
no collision node can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-27
Slotted ALOHA
Pros single active node can
continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-28
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately
collision probability increases frame sent at t0 collides with other frames sent in [t0-
1t0+1]
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-29
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmitIf channel sensed idle transmit entire frame If channel sensed busy defer transmission
Human analogy donrsquot interrupt others
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-30
CSMA collisions
collisions can still occurpropagation delay means two nodes may not heareach otherrsquos transmissioncollisionentire packet transmission time wasted
spatial layout of nodes
noterole of distance amp propagation delay in determining collision probability
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-31
CSMACD (Collision Detection)CSMACD carrier sensing deferral as in CSMA
collisions detected within short time colliding transmissions aborted reducing channel
wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs receiver shut off while transmitting
human analogy the polite conversationalist
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-32
CSMACD collision detection
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-33
ldquoTaking Turnsrdquo MAC protocolschannel partitioning MAC protocols
share channel efficiently and fairly at high load
inefficient at low load delay in channel access 1N bandwidth allocated even if only 1 active node
Random access MAC protocols efficient at low load single node can fully
utilize channel high load collision overhead
ldquotaking turnsrdquo protocolslook for best of both worlds
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-34
ldquoTaking Turnsrdquo MAC protocolsPolling master node
ldquoinvitesrdquo slave nodes to transmit in turn
concerns polling overhead latency single point of
failure (master)
Token passing control token passed
from one node to next sequentially
token message concerns
token overhead latency single point of failure
(token)
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-35
Summary of MAC protocols
What do you do with a shared media Channel Partitioning by time frequency or
codebull Time Division Frequency Division
Random partitioning (dynamic) bull ALOHA S-ALOHA CSMA CSMACDbull carrier sensing easy in some technologies (wire)
hard in others (wireless)bull CSMACD used in Ethernetbull CSMACA used in 80211
Taking Turnsbull polling from a central site token passing
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-36
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-37
MAC Addresses and ARP
32-bit IP address network-layer address used to get datagram to destination IP subnet
MAC (or LAN or physical or Ethernet) address used to get datagram from one interface to
another physically-connected interface (same network)
48 bit MAC address (for most LANs) burned in the adapter ROM
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-38
LAN Addresses and ARPEach adapter on LAN has unique LAN address
Broadcast address =FF-FF-FF-FF-FF-FF
= adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-39
LAN Address (more)
MAC address allocation administered by IEEE manufacturer buys portion of MAC address
space (to assure uniqueness) Analogy (a) MAC address like Social Security
Number (b) IP address like postal address MAC flat address portability
can move LAN card from one LAN to another
IP hierarchical address NOT portable depends on IP subnet to which node is attached
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-40
ARP Address Resolution Protocol
Each IP node (Host Router) on LAN has ARP table
ARP Table IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineMAC address of Bknowing Brsquos IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
237196723
237196778
237196714
237196788
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-41
ARP protocol Same LAN (network) A wants to send datagram
to B and Brsquos MAC address not in Arsquos ARP table
A broadcasts ARP query packet containing Bs IP address Dest MAC address = FF-
FF-FF-FF-FF-FF all machines on LAN
receive ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their ARP
tables without intervention from net administrator
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-42
Routing to another LANwalkthrough send datagram from A to B via R assume A knowrsquos B IP address
Two ARP tables in router R one for each IP network (LAN)
In routing table at source Host find router 111111111110 In ARP table at source find MAC address E6-E9-00-17-BB-4B etc
A
RB
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-43
A creates datagram with source A destination B A uses ARP to get Rrsquos MAC address for 111111111110 A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram Arsquos adapter sends frame Rrsquos adapter receives frame R removes IP datagram from Ethernet frame sees its
destined to B R uses ARP to get Brsquos MAC address R creates frame containing A-to-B IP datagram sends to B
A
RB
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-44
Link Layer
Services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-45
Ethernet
ldquodominantrdquo wired LAN technology cheap $20 for 100Mbs first widely used LAN technology Simpler cheaper than token LANs and ATM Kept up with speed race 10 Mbps ndash 10 Gbps
Metcalfersquos Ethernetsketch
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-46
Star topology
Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-47
Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
Preamble 7 bytes with pattern 10101010 followed by one
byte with pattern 10101011 used to synchronize receiver sender clock
rates
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-48
Ethernet Frame Structure (more) Addresses 6 bytes
if adapter receives frame with matching destination address or with broadcast address (eg ARP packet) it passes data in frame to net-layer protocol
otherwise adapter discards frame
Type indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk)
CRC checked at receiver if error is detected the frame is simply dropped
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-49
Unreliable connectionless service Connectionless No handshaking between
sending and receiving adapter Unreliable receiving adapter doesnrsquot send
acks or nacks to sending adapter stream of datagrams passed to network layer can
have gaps gaps will be filled if app is using TCP otherwise app will see the gaps
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-50
Ethernet uses CSMACD
No slots adapter doesnrsquot
transmit if it senses that some other adapter is transmitting that is carrier sense
transmitting adapter aborts when it senses that another adapter is transmitting that is collision detection
Before attempting a retransmission adapter waits a random time that is random access
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-51
Ethernet CSMACD algorithm
1 Adaptor receives datagram from net layer amp creates frame
2 If adapter senses channel idle it starts to transmit frame If it senses channel busy waits until channel idle and then transmits
3 If adapter transmits entire frame without detecting another transmission the adapter is done with frame
4 If adapter detects another transmission while transmitting aborts and sends jam signal
5 After aborting adapter enters exponential backoff after the mth collision adapter chooses a K at random from 012hellip2m-1 Adapter waits K512 bit times and returns to Step 2
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-52
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 1 microsec for 10 Mbps Ethernet for K=1023 wait time is about 50 msec
Exponential Backoff Goal adapt retransmission
attempts to estimated current load heavy load random wait
will be longer first collision choose K
from 01 delay is K 512 bit transmission times
after second collision choose K from 0123hellip
after ten collisions choose K from 01234hellip1023
Seeinteract with Javaapplet on AWL Web sitehighly recommended
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-53
10BaseT and 100BaseT 10100 Mbps rate latter called ldquofast ethernetrdquo T stands for Twisted Pair Nodes connect to a hub ldquostar topologyrdquo 100
m max distance between nodes and hub
twisted pair
hub
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-54
Manchester encoding
Used in 10BaseT Each bit has a transition Allows clocks in sending and receiving nodes to
synchronize to each other no need for a centralized global clock among nodes
Hey this is physical-layer stuff
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-55
Gbit Ethernet
uses standard Ethernet frame format allows for point-to-point links and shared
broadcast channels in shared mode CSMACD is used short
distances between nodes required for efficiency
uses hubs called here ldquoBuffered Distributorsrdquo Full-Duplex at 1 Gbps for point-to-point links 10 Gbps now
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-56
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Interconnections Hubs and switches
57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-57
Link Layer
51 Introduction and services
52 Error detection and correction
53Multiple access protocols
54 Link-Layer Addressing
55 Ethernet
56 Hubs and switches 57 PPP 58 Link Virtualization
ATM
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-58
Point to Point Data Link Control one sender one receiver one link easier than
broadcast link no Media Access Control no need for explicit MAC addressing eg dialup link ISDN line
popular point-to-point DLC protocols PPP (point-to-point protocol) HDLC High level data link control (Data link
used to be considered ldquohigh layerrdquo in protocol stack
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-59
PPP Design Requirements [RFC 1557]
packet framing encapsulation of network-layer datagram in data link frame carry network layer data of any network layer
protocol (not just IP) at same time ability to demultiplex upwards
bit transparency must carry any bit pattern in the data field
error detection (no correction) connection liveness detect signal link failure to
network layer network layer address negotiation endpoint can
learnconfigure each otherrsquos network address
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-60
PPP non-requirements
no error correctionrecovery no flow control out of order delivery OK no need to support multipoint links (eg
polling)
Error recovery flow control data re-ordering all relegated to higher layers
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-61
PPP Data Frame
Flag delimiter (framing) Address does nothing (only one option) Control does nothing in the future possible
multiple control fields Protocol upper layer protocol to which frame
delivered (eg PPP-LCP IP IPCP etc)
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-62
PPP Data Frame
info upper layer data being carried check cyclic redundancy check for error
detection
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-63
Byte Stuffing ldquodata transparencyrdquo requirement data field
must be allowed to include flag pattern lt01111110gt Q is received lt01111110gt data or flag
Sender adds (ldquostuffsrdquo) extra lt 01111110gt byte after each lt 01111110gt data byte
Receiver two 01111110 bytes in a row discard first
byte continue data reception single 01111110 flag byte
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-64
Byte Stuffing
flag bytepatternin datato send
flag byte pattern plusstuffed byte in transmitted data
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
5 DataLink Layer 5-65
PPP Data Control ProtocolBefore exchanging network-
layer data data link peers must
configure PPP link (max frame length authentication)
learnconfigure network layer information
for IP carry IP Control Protocol (IPCP) msgs (protocol field 8021) to configurelearn IP address
- Data Communication and Networks
- Our goals
- Link Layer
- Link Layer Services
- Link Layer Services (more)
- Adaptors Communicating
- Slide 7
- Error Detection
- Parity Checking
- Internet checksum
- Cyclic Redundancy Check
- Slide 12
- CRC Generation
- CRC Algorithm
- CRC Error Detection
- Example of G(x) Polynomials
- Example
- Slide 18
- Multiple Access Links and Protocols
- Multiple Access protocols
- Ideal Mulitple Access Protocol
- MAC Protocols a taxonomy
- Channel Partitioning MAC protocols TDMA
- Channel Partitioning MAC protocols FDMA
- Random Access Protocols
- Slotted ALOHA
- Slide 27
- Pure (unslotted) ALOHA
- CSMA (Carrier Sense Multiple Access)
- CSMA collisions
- CSMACD (Collision Detection)
- CSMACD collision detection
- ldquoTaking Turnsrdquo MAC protocols
- Slide 34
- Summary of MAC protocols
- Slide 36
- MAC Addresses and ARP
- LAN Addresses and ARP
- LAN Address (more)
- ARP Address Resolution Protocol
- ARP protocol Same LAN (network)
- Routing to another LAN
- PowerPoint Presentation
- Slide 44
- Ethernet
- Star topology
- Ethernet Frame Structure
- Ethernet Frame Structure (more)
- Unreliable connectionless service
- Ethernet uses CSMACD
- Ethernet CSMACD algorithm
- Ethernetrsquos CSMACD (more)
- 10BaseT and 100BaseT
- Manchester encoding
- Gbit Ethernet
- Slide 56
- Slide 57
- Point to Point Data Link Control
- PPP Design Requirements [RFC 1557]
- PPP non-requirements
- PPP Data Frame
- Slide 62
- Byte Stuffing
- Slide 64
- PPP Data Control Protocol
-
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