Data Link Layer 1 Link Layer Introduction and services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet Hubs and switches
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Data Link Layer 1
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 2
Link Layer IntroductionSome terminology hosts and routers are nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
ldquo linkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
Data Link Layer 3
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
Data Link Layer 4
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
Data Link Layer 5
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
Data Link Layer 6
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 7
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
Data Link Layer 8
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 9
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)
Data Link Layer 10
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
Data Link Layer 11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 2
Link Layer IntroductionSome terminology hosts and routers are nodes communication channels
that connect adjacent nodes along communication path are links wired links wireless links LANs
layer-2 packet is a frame encapsulates datagram
ldquo linkrdquo
data-link layer has responsibility of transferring datagram from one node to adjacent node over a link
Data Link Layer 3
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
Data Link Layer 4
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
Data Link Layer 5
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
Data Link Layer 6
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 7
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
Data Link Layer 8
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 9
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)
Data Link Layer 10
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
Data Link Layer 11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 3
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
Data Link Layer 4
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
Data Link Layer 5
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
Data Link Layer 6
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 7
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
Data Link Layer 8
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 9
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)
Data Link Layer 10
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
Data Link Layer 11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 4
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
Data Link Layer 5
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
Data Link Layer 6
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 7
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
Data Link Layer 8
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 9
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)
Data Link Layer 10
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
Data Link Layer 11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 5
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
Data Link Layer 6
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 7
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
Data Link Layer 8
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 9
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)
Data Link Layer 10
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
Data Link Layer 11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 6
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 7
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
Data Link Layer 8
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 9
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)
Data Link Layer 10
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
Data Link Layer 11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 7
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
Data Link Layer 8
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 9
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)
Data Link Layer 10
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
Data Link Layer 11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 8
Parity Checking
Single Bit ParityDetect single bit errors
Two Dimensional Bit ParityDetect and correct single bit errors
0 0
Data Link Layer 9
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)
Data Link Layer 10
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
Data Link Layer 11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 9
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)
Data Link Layer 10
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
Data Link Layer 11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 10
Checksumming Cyclic Redundancy Check view data bits D as a binary number choose r+1 bit pattern (generator) G goal choose r CRC bits R such that
ltDRgt exactly divisible by G (modulo 2) receiver knows G divides ltDRgt by G If non-zero
remainder error detected can detect all burst errors less than r+1 bits
widely used in practice (ATM HDCL)
Data Link Layer 11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 11
CRC ExampleWant
D2r XOR R = nGequivalently
D2r = nG XOR R equivalently if we divide D2r by
G want remainder R
R = remainder[ ]D2r
G
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 12
CRC ExampleLet the message M= 1010001110 and the generator polynomial is x5+x4+x2+1 Find the FCS and the transmitted frame
1
32
1
116
112
sPolynomialGenerator Popular
245
78101112
1622232632
51216
21516
231112
xxxx
xxxxx
xxxxxCRC
xxxCCITTCRC
xxxCRC
xxxxxCRC-
1010001110000001101011101 0 11 1 1 01 11 10101
1 1
10111
10
11 0101
1 1 0111 01 01
1101010100
1
00
0FCS=00100
T=101000111000100
011 01 01
100
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 13
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 14
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
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 15
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
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 16
MAC Protocols a taxonomy
Three broad classes Channel Partitioning (Static Bandwidth Allocation)
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Random Access (Dynamic Bandwidth Allocation) channel not divided allow collisions ldquorecoverrdquo from collisions
ldquoTaking turnsrdquo (Dynamic Bandwidth Allocation) Nodes take turns but nodes with more to send can take
longer turns
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 17
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
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 18
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
frequ
ency
bands time
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 19
Channel Partitioning MAC protocols WDM
WDM wavelength division multiplexing multiple beams are combined onto a single shared
fiber for transmission each beam is with its energy at a different
wavelength at the far end the beam is split up over as many
fibers as there were on the input side 8 channels of 25Gbps per channel by 1990 40
channels of 25Gbps per channel by 1998 96 channels of 10Gbps per channel by 2001 DWDM (dense WDM)combine
r spliter
long-haul fiber
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 20
CDMA code division multiple access used in several wireless broadcast channels
(cellular satellite etc) standards unique ldquocoderdquo assigned to each user ie code
set partitioning all users share same frequency but each user
has own ldquochippingrdquo sequence (ie code) to encode data
encoded signal = (original data) X (chipping sequence)
decoding inner-product of encoded signal and chipping sequence allows multiple users to ldquocoexistrdquo and transmit simultaneously with minimal interference (if codes are ldquoorthogonalrdquo)
Channel Partitioning MAC protocols CDMA
X
PN 1
X
PN 2
X
PN 3
X
PN 4
+
頻道1
頻道2
頻道3
頻道4
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 21
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
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 22
Slotted ALOHAAssumptions 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
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 23
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
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 24
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]
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 25
CSMA (Carrier Sense Multiple Access)
CSMA listen before transmit
If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy donrsquot interrupt others
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 26
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
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 27
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
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 28
CSMACD collision detection
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 29
ldquo Taking 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
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 30
ldquo Taking 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)
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 31
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
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 32
LAN technologies
Data link layer so far services error detectioncorrection multiple
access
Next LAN technologies addressing Ethernet hubs switches PPP
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 33
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 34
MAC Addresses and ARP32-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
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 35
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)
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 36
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
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 37
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)
A
RB
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 38
A sends a packet to 10023 can it be reachable If yes what are the source IP and MAC addresses of the receiving packet at the host 10023
B sends a frame to 8012AE30132D can it be reachable
Routing to another LANExample
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 39
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 40
Ethernet
ldquo dominantrdquo 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
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 41
Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices hub or switch (more later)
hub orswitch
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 42
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
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 43
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
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 44
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
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 45
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
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 46
Ethernet CSMACD algorithm1 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
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 47
Ethernetrsquos CSMACD (more)
Jam Signal make sure all other transmitters are aware of collision 48 bits
Bit time 01 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
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 48
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
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 49
HubsHubs are essentially physical-layer repeaters
bits coming from one link go out all other links at the same rate no frame buffering no CSMACD at hub adapters detect collisions provides net management functionality
twisted pair
hub
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 50
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
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 51
Link Layer
Introduction and servicesError detection and correction Multiple access protocolsLink-Layer AddressingEthernetHubs and switches
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 52
Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large
collision domain Canrsquot interconnect 10BaseT amp 100BaseT
hub
hubhub
hub
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 53
Switch Link layer device
stores and forwards Ethernet frames examines frame header and selectively forwards
frame based on MAC dest address when frame is to be forwarded on segment uses
CSMACD to access segment transparent
hosts are unaware of presence of switches plug-and-play self-learning
switches do not need to be configured
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 54
Forwarding
bull How to determine onto which LAN segment to forward framebull Looks like a routing problem
hub
hubhub
switch1
2 3
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 55
Self learning
A switch has a switch table entry in switch table
(MAC Address Interface Time Stamp) stale entries in table dropped (TTL can be 60
min) switch learns which hosts can be reached through
which interfaces when frame received switch ldquolearnsrdquo location
of sender incoming LAN segment records senderlocation pair in switch table
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 56
Switch exampleSuppose C sends frame to D
Switch receives frame from from C notes in bridge table that C is on interface 1 because D is not in table switch forwards frame into
interfaces 2 and 3
frame received by D
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEG
1123
12 3
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 57
Switch exampleSuppose D replies back with frame to C
Switch receives frame from from D notes in bridge table that D is on interface 2 because C is in table switch forwards frame only to
interface 1
frame received by C
hub
hub hub
switch
A
B CD
EF
G H
I
address interface
ABEGC
11231
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 58
Switch traffic isolation switch installation breaks subnet into LAN
segments switch filters packets
same-LAN-segment frames not usually forwarded onto other LAN segments
segments become separate collision domains
hub hub hub
switch
collision domaincollision domain
collision domain
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 59
Switches dedicated access Switch with many
interfaces Hosts have direct
connection to switch No collisions full duplex
Switching A-to-Arsquo and B-to-Brsquo simultaneously no collisions
switch
A
Arsquo
B
Brsquo
C
Crsquo
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 60
Institutional network
hub
hubhub
switch
to externalnetwork
router
IP subnet
mail server
web server
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 61
Switches vs Routers both store-and-forward devices
routers network layer devices (examine network layer headers) switches are link layer devices
routers maintain routing tables implement routing algorithms
switches maintain switch tables implement filtering learning algorithms
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
Data Link Layer 62
Summary comparison hubs routers switches
traffi c isolation
no yes yes
plug amp play yes no yes
optimal routing
no yes no
cut through
yes no yes
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