Link Layer and LANs

Chapter 5
Link Layer and LANs

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Computer Networking:
A Top Down Approach
Featuring the Internet,
3rd edition.
Jim Kurose, Keith Ross
Addison-Wesley, July
2004.

All material copyright 1996-2006
J.F Kurose and K.W. Ross, All Rights Reserved
5: DataLink Layer

5-1


Chapter 5: The Data Link Layer

Our goals:
Ì understand principles behind data link layer

services:
r
r
r
r

error detection, correction
sharing a broadcast channel: multiple access
link layer addressing
reliable data transfer, flow control: done!

Ì instantiation and implementation of various link

layer technologies

5: DataLink Layer

5-2


Link Layer
Ì 5.1 Introduction and
Ì
Ì
Ì
Ì

services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet

Ì 5.6 Hubs and switches
Ì 5.7 PPP
Ì 5.8 Link Virtualization:

ATM and MPLS

5: DataLink Layer

5-3


Link Layer: Introduction
Some terminology:

“link”

Ì hosts and routers are nodes
Ì communication channels that

connect adjacent nodes along
communication path are links

r
r
r

wired links
wireless links
LANs

Ì layer-2 packet is a frame,

encapsulates datagram

data-link layer has responsibility of
transferring datagram from one node
to adjacent node over a link
5: DataLink Layer

5-4


Link layer: context
Ì Datagram transferred by

different link protocols
over different links:
r

e.g., Ethernet on first link,
frame relay on
intermediate links, 802.11

on last link

Ì Each link protocol

provides different
services
r

e.g., may or may not
provide rdt over link

transportation analogy
Ì trip from Princeton to

Lausanne
r limo: Princeton to JFK
r plane: JFK to Geneva
r train: Geneva to Lausanne

Ì tourist = datagram
Ì transport segment =

communication link
Ì transportation mode =
link layer protocol
Ì travel agent = routing
algorithm
5: DataLink Layer

5-5

Link Layer Services
Ì Framing, link access:
r
r
r

encapsulate datagram into frame, adding header, trailer
channel access if shared medium
“MAC” addresses used in frame headers to identify
source, dest
• different from IP address!

Ì Reliable delivery between adjacent nodes
r we learned how to do this already (chapter 3)!
r seldom used on low bit error link (fiber, some twisted
pair)
r wireless links: high error rates
• Q: why both link-level and end-end reliability?
5: DataLink Layer

5-6


Link Layer Services (more)
Ì

Flow Control:
r

Ì

pacing between adjacent sending and receiving nodes

Error Detection:
r
r

errors caused by signal attenuation, noise.
receiver detects presence of errors:
• signals sender for retransmission or drops frame

Ì Error Correction:
r receiver identifies and corrects bit error(s) without
resorting to retransmission
Ì

Half-duplex and full-duplex
r

with half duplex, nodes at both ends of link can transmit,
but not at same time
5: DataLink Layer

5-7


Adaptors Communicating
datagram

sending
node

rcving
node

link layer protocol

frame

frame

adapter

adapter

Ì link layer implemented in Ì receiving side
r looks for errors, rdt, flow
“adaptor” (aka NIC)
control, etc
r Ethernet card, PCMCI
r extracts datagram, passes
card, 802.11 card
to rcving node
Ì

sending side:
r

r

encapsulates datagram in a Ì
frame
adds error checking bits,
Ì
rdt, flow control, etc.

adapter is semiautonomous
link & physical layers

5: DataLink Layer

5-8


Link Layer
Ì 5.1 Introduction and
Ì
Ì
Ì
Ì

services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet

Ì 5.6 Hubs and switches
Ì 5.7 PPP
Ì 5.8 Link Virtualization:

ATM

5: DataLink Layer

5-9


Error Detection
EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields
• Error detection not 100% reliable!
• protocol may miss some errors, but rarely
• larger EDC field yields better detection and correction

5: DataLink Layer

5-10


Parity Checking
Single Bit Parity:
Detect single bit errors

Two Dimensional Bit Parity:
Detect and correct single bit errors

0

0

5: DataLink Layer

5-11


Internet checksum
Goal: detect “errors” (e.g., flipped bits) in transmitted
segment (note: used at transport layer only)
Sender:
Ì treat segment contents as

sequence of 16-bit
integers
Ì checksum: addition (1’s
complement sum) of
segment contents
Ì sender puts checksum
value into UDP checksum
field

Receiver:
compute checksum of received
segment
Ì check if computed checksum
equals checksum field value:

r NO - error detected
r YES - no error detected. But
maybe errors nonetheless?
More later ….
Ì

5: DataLink Layer

5-12


Checksumming: Cyclic Redundancy Check
Ì view data bits, D, as a binary number
Ì choose r+1 bit pattern (generator), G
Ì goal: choose r CRC bits, R, such that
r
r

r

<D,R> exactly divisible by G (modulo 2)
receiver knows G, divides <D,R> by G. If non-zero remainder:
error detected!
can detect all burst errors less than r+1 bits

Ì widely used in practice (ATM, HDLC)

5: DataLink Layer

5-13

CRC Example
Want:

D.2r XOR R = nG
equivalently:

D.2r = nG XOR R
equivalently:
if we divide D.2r by
G, want remainder R

R = remainder[

D.2r
G

]

5: DataLink Layer

5-14


Link Layer
Ì 5.1 Introduction and
Ì
Ì
Ì

Ì

services
5.2 Error detection
and correction
5.3Multiple access
protocols
5.4 Link-Layer
Addressing
5.5 Ethernet

Ì 5.6 Hubs and switches
Ì 5.7 PPP
Ì 5.8 Link Virtualization:

ATM

5: DataLink Layer

5-15


Multiple Access Links and Protocols
Two types of “links”:
Ì point-to-point
r PPP for dial-up access
r point-to-point link between Ethernet switch and host
Ì broadcast (shared wire or medium)
r Old-fashioned Ethernet
r upstream HFC

r 802.11 wireless LAN

5: DataLink Layer

5-16


Multiple Access protocols
Ì single shared broadcast channel
Ì two or more simultaneous transmissions by nodes:

interference
r

collision if node receives two or more signals at the same time

multiple access protocol
Ì distributed algorithm that determines how nodes
share channel, i.e., determine when node can transmit
Ì communication about channel sharing must use channel
itself!
r

no out-of-band channel for coordination

5: DataLink Layer

5-17

Ideal Multiple Access Protocol
Broadcast channel of rate R bps
1. When one node wants to transmit, it can send at
rate R.
2. When M nodes want to transmit, each can send at
average rate R/M
3. Fully decentralized:
r
r

no special node to coordinate transmissions
no synchronization of clocks, slots

4. Simple

5: DataLink Layer

5-18


MAC Protocols: a taxonomy
Three broad classes:
Ì Channel Partitioning
r

r

divide channel into smaller “pieces” (time slots, frequency,
code)
allocate piece to node for exclusive use

Ì Random Access
r channel not divided, allow collisions
r “recover” from collisions
Ì “Taking turns”
r Nodes take turns, but nodes with more to send can take
longer turns

5: DataLink Layer

5-19


Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access
Ì access to channel in "rounds"
Ì each station gets fixed length slot (length = pkt

trans time) in each round
Ì unused slots go idle
Ì example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6
idle

5: DataLink Layer

5-20


Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access

Ì channel spectrum divided into frequency bands
Ì each station assigned fixed frequency band
Ì unused transmission time in frequency bands go idle
Ì example: 6-station LAN, 1,3,4 have pkt, frequency

frequency bands

bands 2,5,6 idle

time

5: DataLink Layer

5-21


Random Access Protocols
Ì When node has packet to send
r transmit at full channel data rate R.
r no a priori coordination among nodes
Ì two or more transmitting nodes ➜ “collision”,
Ì random access MAC protocol specifies:
r how to detect collisions
r how to recover from collisions (e.g., via delayed
retransmissions)
Ì Examples of random access MAC protocols:
r slotted ALOHA
r ALOHA
r CSMA, CSMA/CD, CSMA/CA
5: DataLink Layer

5-22


Slotted ALOHA
Assumptions
Ì all frames same size
Ì time is divided into
equal size slots, time to
transmit 1 frame
Ì nodes start to transmit
frames only at
beginning of slots
Ì nodes are synchronized
Ì if 2 or more nodes
transmit in slot, all
nodes detect collision

Operation
Ì when node obtains fresh
frame, it transmits in next
slot
Ì no collision, node can send
new frame in next slot
Ì if collision, node
retransmits frame in each
subsequent slot with prob.
p until success

5: DataLink Layer

5-23


Slotted ALOHA

Pros
Ì single active node can
continuously transmit
at full rate of channel
Ì highly decentralized:
only slots in nodes
need to be in sync
Ì simple

Cons
Ì collisions, wasting slots
Ì idle slots
Ì nodes may be able to
detect collision in less
than time to transmit
packet
Ì clock synchronization
5: DataLink Layer

5-24


Slotted Aloha efficiency
Efficiency is the long-run

fraction of successful slots
when there are many nodes,
each with many frames to send
Ì Suppose N nodes with

many frames to send,
each transmits in slot
with probability p
Ì prob that node 1 has
success in a slot
= p(1-p)N-1

Ì prob that any node has
a success = Np(1-p)N-1

Ì For max efficiency

with N nodes, find p*
that maximizes
Np(1-p)N-1
Ì For many nodes, take
limit of Np*(1-p*)N-1 as
N goes to infinity,
gives 1/e = .37

At best: channel
used for useful
transmissions 37%
of time!
5: DataLink Layer

5-25