Chapter 5 Link Layer Computer Networking: A Top Down Approach 4 th edition. Jim Kurose, Keith Ross Addison-Wesley, July 2007.
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Chapter 5 Link Layer
Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July
2007.
Link Layer: Introduction Layer-2 packet is a frame,
encapsulates datagram Link layer has responsibility
of transferring datagram from one node to adjacent node over a link
Datagram may be carried by different link-layer protocols on different links in the path. Services provided by link layer
protocols may be different May or may not provide
reliable data transfer over link
Link layer and Network Layer
Transportation Analogy
Travel Agent planning a trip for a tourist Trip from Princeton to Lausanne
Car: Princeton to JFK Plane: JFK to Geneva Train: Geneva to Lausanne
Tourist = Datagram Transportation Mode = Link layer protocol Travel Agent = Routing Algorithm
Link Layer Services Framing:
Encapsulate datagram into frame, adding header, trailer
The structure of the frame is specified by the Link Layer Protocol
Link Access Channel access if shared medium Medium Access Control (MAC) protocol specifies the
rules by which a frame is transmitted onto the link Reliable Delivery between Adjacent Nodes
Link Layer like Transport Layer provides reliability with ACKs and retransmissions.
Wireless links: high error rates Seldom used on low bit-error link (fiber, some twisted
pair)
Link Layer Services
Flow Control: Pacing between adjacent sending and receiving
nodes
Error Detection: Errors caused by signal attenuation, noise. Receiver detects presence of errors
Error correction: Receiver identifies and corrects bit error(s) without
resorting to retransmissions Transport layer provides reliable delivery between
processes on an end-to-end basis Link layer provides reliability between two nodes
connected by a single link.
Where is the link layer implemented? In each and every host Link layer implemented in
“adaptor” ( Network Interface Card NIC)
Attaches into host’s system buses
Network Adaptor Link Layer controller
Single special purpose chip Implements many of the
link layer services Mostly in hardware (Framing,
flow control, error detection etc)
Software Component Receiving the datagram,
addressing, activating the controller hardware
controller
physicaltransmission
cpu memory
host bus (e.g., PCI)
network adapter card
host
applicationtransportnetwork
link
linkphysical
Adaptors Communicating
Sending Side: Encapsulates datagram
in frame Adds error checking
bits, flow control, etc.
Receiving Side looks for errors, flow
control, etc extracts datagram,
passes to upper layer at receiving side
frame
controller controller
sending host receiving host
datagram datagram
datagram
Error Detection
EDC= Error Detection and Correction bits D = Data protected by error checking, including header fields
otherwise
Parity Checking Simplest Technique (Even or Odd) Even Parity
Sender simply includes one bit such that Total number of ones d+1 is even
Receiver counts the number of 1s in d+1 bits Detect single bit errors
Two Dimensional Parity and Cyclic Redundancy Check (CRC) Assignment 3
Multiple Access Links and Protocols
Two types of “links”: Point-to-Point
Single sender at one end and single receiver at the other end of the link
Many link layer protocols are designed for point-to-point links e.g. PPP, HDLC etc.
Broadcast link: All nodes connected to same single, shared, broadcast
channel When one node transmits a frame, the channel
broadcasts it and every node receives a copy. Ethernet and wireless LANs
How to coordinate the access of multiple sending and receiving nodes to a shared broadcast medium?
Human analogy for a broadcast channel is classroom or a party room Who gets to talk and when
Multiple Access Links and Protocols
Collision
Multiple Access Links and Protocols
Computer Networks similarly have protocols called Multiple Access Control (MAC) protocols.oWired /wireless local area networks
All nodes are capable of transmitting frameso More than two nodes can transmit
frames at the same timeo Collision: None of the receiving nodes can
make any sense of frames that were transmittedoFrames involved in the collision are lostoBroadcast channel is wasted during the
collision interval Coordinate the transmission of active nodes
oResponsibility of multiple access protocol
MAC Protocols:
Three broad classes: Channel Partitioning
Divide channel into smaller “pieces” (time slots, frequency)
Allocate piece to node for exclusive use Random Access
Channel not divided, allow collisions “Recover” from collisions
Taking Turns Nodes take turns, but nodes with more to send can
take longer turns
Ideal Multiple Access Protocol
Broadcast channel of rate R bps
1. When one node wants to transmit, it can send at rate R bps.
2. When M nodes want to transmit, each can send at average rate of R/M bps
3. Fully decentralized: There is no master node that can fail and bring down
the entire system
4. Simple and inexpensive to implement
Channel Partitioning MAC protocols: TDMA
TDMA: Time Division Multiple Access TDMA divides time into slots and each slot is
assigned to one of the N nodes. Slot size are chosen so that a single packet
can be transmitted during a slot time. TDMA eliminates collision and is perfectly fair Drawbacks
Wasteful during silent periods A node must always wait for its turn in the
transmission sequence.
Channel Partitioning MAC protocols: FDMA
FDMA: Frequency Division Multiple Access
Channel spectrum divided into frequency bands
Each station assigned fixed frequency band
FDMA eliminates collision and is fair Drawback
Wasteful during silent periods
Random Access Protocols When node has packet to send
Transmit at full channel data rate R. Two or more transmitting nodes ➜ “collision”, When collision each node
Repeatedly retransmit until it gets through Doesn’t retransmit the frame right away Waits for a random delay before retransmitting
Examples of random access MAC protocols: Slotted ALOHA ALOHA CSMA, CSMA/CD
Slotted ALOHA
All frames of exactly L bits
Time divided into equal size slots (time to transmit one frame)
Nodes start to transmit only at the beginning of the slot
Nodes are synchronized so that each node knows when the slot begins
Operation: When node obtains
fresh frame, waits until the beginning of the next slot If there is no collision:
Node has successfully transmittedCan send new frame in
next slot If collision: Node
retransmits frame in each subsequent slot until success
Slotted Aloha
Slotted ALOHA
Pros Single active node
can continuously transmit at full rate of channel
Decentralized: Each node detects collision and independently decides when to retransmit
Cons Collisions, wasting slots Idle slots Slotted Aloha is inefficient
(and rude!): doesn't listen before talking!
Slotted ALOHA
1 2&3 2Time
Collision
Retransmission Retransmission
3
Slot
Node 1 Packet
Nodes 2 & 3 Packets
Collision mechanism in Slotted ALOHA
Pure (unslotted) ALOHA Unslotted Aloha: Simpler, no synchronization
Unslotted and fully decentralized
When frame first arrives Transmit immediately
Collision probability increases
Inefficient than Slotted Aloha Slotted Aloha does not allow the nodes to transmit any
time The nodes have to wait for the beginning of the next slot The wasted time is confined to one time slot
Pure Aloha
Pure ALOHA
1 2 3 3 2Time
Collision
Retransmission Retransmission
Node 1 Packet
Collision mechanism in ALOHA
Waiting a random time
Node 2 Packet
Node 3 Packet
CSMA Collisions
If all nodes performs carrier sensing, do collision occurs in the first place? oCollisions can still occur:
o Propagation delay means two nodes may not hear each other’s transmission
oAdding collision detection oHelps improve performance by not
transmitting a useless frame in its entirety.
oCSMA/CD used in EthernetoCover in later slides
“Taking Turns” MAC protocols
Polling Protocol: Master node
“invites” slave nodes to transmit in turn
Concerns: Polling overhead latency single point of
failure (master)
master
slaves
poll
data
data
“Taking Turns” MAC protocolsToken Passing Protocol: No master node Special frame called
token is exchanged among the nodes in a fixed order.
Node receives a token Holds only if has
something to transmit
Otherwise forwards token to the next node
Concerns: Token overhead Accidently neglects to
release the token
T
data
(nothingto send)
T
Switch Link-layer Device
Store, forward Ethernet frames Examine incoming frame’s MAC address Selectively forward frame to the outgoing
link Switch itself is transparent to the nodes
Nodes are unaware of presence of switches Plug-and-Play
Switches do not need to be configured Two important functions:
Filtering• Determines whether a frame should be forwarded
to some interface or should be dropped. Forwarding
• Determine the interface to which a frame should be directed
Switch: Allows Multiple Simultaneous Transmissions
Hosts have dedicated, direct connection to switch
Switches buffer packets Ethernet protocol used on
each incoming link, but no collisions; full duplex
Switching: A-to-A’ and B-to-B’ simultaneously, without collisions
A
A’
B
B’
C
C’
switch with six interfaces(1,2,3,4,5,6)
1 23
45
6
Switch Table
Q: How does switch know that A’ reachable via interface 4, B’ reachable via interface 5?
A: Each switch has a switch table, each entry: MAC address of a host Interface that leads
towards the host Time at which the entry
for the node as placed in the table
How is the table created?
A
A’
B
B’
C
C’
switch with six interfaces(1,2,3,4,5,6)
1 23
45
6
Switch: self-learning
switch learns which hosts can be reached through which interfaces when frame received,
switch “learns” location of sender: incoming LAN segment
records sender/location pair in switch table
A
A’
B
B’
C
C’
1 23
45
6
A A’
Source: ADest: A’
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Self-Learning,Forwarding:Example
A
A’
B
B’
C
C’
1 23
45
6
A A’
Source: ADest: A’
MAC addr interface TTL
Switch table (initially empty)
A 1 60
A A’A A’A A’A A’A A’
frame destination unknown:flood
A’ A
destination A location known:
A’ 4 60
selective send
Switch: Frame Filtering/Forwarding
When frame received:
1. Record link associated with sending host2. Index switch table using MAC destination address3. if entry found for destination
then {forward the frame on interface indicated
} else flood
forward on all but the interface on which the frame arrived
Interconnecting Switches
switches can be connected together
A
B
Q: sending from A to G - how does S1 know to forward frame destined to G via S4 and S3?
A: self learning! (works exactly the same as in single-switch case!)
S1
C D
E
FS2
S4
S3
H
I
G
Institutional Network
to externalinternet
router
mail server
web server
Switch: Advantages Elimination of Collisions:
Switches buffer frames and never transmit more than one frame on any segment at one time
Modern switches are Full Duplex Heterogeneous Links
Switch isolates one link from anotherDifferent links can operate at different
speeds and over different media
Difference between Hubs and Switches?
Difference between Switches and Routers? (Home Assignment)
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