Link Layer
All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved
George Parisis
School of Engineering and Informatics
University of Sussex
Link Layer 5-2
Link layer
Objectives:
v understand principles behind link layer
services:
§ error detection, correction
§ sharing a broadcast channel: multiple access
§ link layer addressing
§ Ethernet
v instantiation, implementation of link layer
technologies
Link Layer 5-3
Outline
v introduction, services
v error detection, correction
v multiple access protocols
v LANs
§ addressing, ARP
§ Ethernet
§ switches
Link Layer 5-4
Link layer: introduction
terminology:
v hosts and routers: nodes
v communication channels
that connect adjacent
nodes along
communication path: links
§ wired links
§ wireless links
§ LANs
v layer-2 packet: frame,
encapsulates datagram
data-link layer has responsibility of
transferring datagram from one
node
to physically adjacent node over a
link
global ISP
Link Layer 5-5
Link layer: context
v datagram transferred by different link protocols over
different links:
§ e.g., Ethernet on first link, frame relay on intermediate
links, 802.11 on last link
v each link protocol provides different services
§ e.g., may or may not provide rdt over link
Link Layer 5-6
Link layer services
v framing, link access:
§ 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!
v reliable delivery between adjacent nodes
§ we learned how to do this already!
§ seldom used on low bit-error link (fiber, some
twisted pair)
§ wireless links: high error rates
• Q: why both link-level and end-end reliability?
Link Layer 5-7
v flow control:
§ pacing between adjacent sending and receiving nodes
v error detection:
§ errors caused by signal attenuation, noise.
§ receiver detects presence of errors:
• signals sender for retransmission or drops frame
v error correction:
§ receiver identifies and corrects bit error(s) without resorting
to retransmission
Link layer services
(more)
Link Layer 5-8
Where is the link layer
implemented?
v in each and every host
v link layer implemented in
“adaptor” (aka network
interface card NIC) or on
a chip
§ Ethernet card, 802.11
card; Ethernet chipset
§ implements link,
physical layer
v attaches into host’s
system buses
v combination of hardware,
software, firmware
controller
physical
transmission
cpu memory
host
bus
(e.g., PCI)
network adapter
card
application
transport
network
link
link
physical
Link Layer 5-9
Adaptors communicating
v sending side:
§ encapsulates
datagram in frame
§ adds error checking
bits, rdt, flow control,
etc.
v receiving side
§ looks for errors, rdt,
flow control, etc
§ extracts datagram,
passes to upper layer
at receiving side
controller controller
sending host receiving host
datagram datagram
datagram
frame
Link Layer 5-10
Outline
v introduction, services
v error detection, correction
v multiple access protocols
v LANs
§ addressing, ARP
§ Ethernet
§ switches
Link Layer 5-11
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
otherwise
Link Layer 5-12
Parity checking
single bit parity
(even/odd):
v detect single bit
errors
two-dimensional bit parity:
v detect and correct single bit errors
0 0
If the probability of bit
errors is small and errors can
be assumed to occur
independently from one bit to
the next, the probability of
multiple bit errors in a packet
would be extremely small.
Link Layer 5-13
Internet checksum (review)
sender:
v treat segment contents
as sequence of 16-bit
integers
v checksum: addition
(1’s complement sum)
of segment contents
v sender puts checksum
value into checksum
field
receiver:
v compute checksum of
received segment
v check if computed
checksum equals
checksum field value:
§ NO – error detected
§ YES – no error
detected. But maybe
errors nonetheless?
goal: detect “errors” (e.g., flipped bits) in transmitted
packet (note: used at transport layer only)
Link Layer 5-14
Cyclic redundancy check
v more powerful error-detection coding
v view data bits, D, as a binary number
v choose r+1 bit pattern (generator), G
v goal: choose r CRC bits, R, such that
§
§ receiver knows G, divides
remainder: error detected!
§ can detect all burst errors less than r+1 bits
v widely used in practice (Ethernet, 802.11 WiFi)
Link Layer 5-15
Outline
v introduction, services
v error detection, correction
v multiple access protocols
v LANs
§ addressing, ARP
§ Ethernet
§ switches
Link Layer 5-16
Multiple access links, protocols
two types of “links”:
v point-to-point
§ PPP for dial-up access
§ point-to-point link between Ethernet switch, host
v broadcast (shared wire or medium)
§ old-fashioned Ethernet
§ 802.11 wireless LAN
shared wire (e.g.,
cabled Ethernet)
shared RF
(e.g., 802.11 WiFi)
shared RF
(satellite)
humans at a
cocktail party
(shared air, acoustical)
Link Layer 5-17
Multiple access protocols
v single shared broadcast channel
v two or more simultaneous transmissions by nodes:
interference
§ collision if node receives two or more signals at the
same time
multiple access protocol
v distributed algorithm that determines how nodes share
channel, i.e., determine when node can transmit
v communication about channel sharing must use channel
itself!
§ no out-of-band channel for coordination
Link Layer 5-18
An ideal multiple access protocol
given: 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:
• no special node to coordinate transmissions
• no synchronization of clocks, slots
4. simple
Link Layer 5-19
MAC protocols: taxonomy
v channel partitioning
§ divide channel into smaller “pieces” (time slots,
frequency)
§ allocate piece to node for exclusive use
v random access
§ channel not divided, allow collisions
§ “recover” from collisions
v taking turns
Link Layer 5-20
Channel partitioning MAC protocols:
TDMA
TDMA: time division multiple access
v access to channel in “rounds”
v each station gets fixed length slot (length =
pkt trans time) in each round
v unused slots go idle
v example: 6-station LAN, 1,3,4 have pkt, slots
2,5,6 idle
1 3 4 1 3 4
6-slot
frame
6-slot
frame
Link Layer 5-21
FDMA: frequency division multiple access
v channel spectrum divided into frequency bands
v each station assigned fixed frequency band
v unused transmission time in frequency bands go idle
v example: 6-station LAN, 1,3,4 have pkt, frequency bands
2,5,6 idle
fr
eq
ue
nc
y
ba
nd
s
time
FDM cable
Channel partitioning MAC protocols:
FDMA
Link Layer 5-22
Outline
v introduction, services
v error detection, correction
v multiple access protocols
v LANs
§ addressing, ARP
§ Ethernet
§ switches
Link Layer 5-23
Random access protocols
v when node has packet to send
§ transmit at full channel data rate R.
§ no a priori coordination among nodes
v two or more transmitting nodes ➜ “collision”,
v random access MAC protocol specifies:
§ how to detect collisions
§ how to recover from collisions (e.g., via delayed
retransmissions)
v examples of random access MAC protocols:
§ slotted ALOHA
§ ALOHA
§ CSMA, CSMA/CD, CSMA/CA
Link Layer 5-24
Slotted ALOHA
assumptions:
v all frames same size
v time divided into equal
size slots (time to
transmit 1 frame)
v nodes start to transmit
only slot beginning
v nodes are synchronized
v if 2 or more nodes
transmit in slot, all nodes
detect collision
operation:
v when node obtains fresh
frame, transmits in next slot
§ if no collision: node can
send new frame in next
slot
§ if collision: node
retransmits frame in
each subsequent slot
with prob. p until success
Link Layer 5-25
Pros:
v single active node can
continuously transmit at
full rate of channel
v highly decentralized:
only slots in nodes need
to be in sync
v simple
Cons:
v collisions, wasting slots
v idle slots
v nodes may be able to
detect collision in less
than time to transmit
packet
v clock synchronization
Slotted ALOHA
1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C C S S S E E E
Link Layer 5-26
v suppose: N nodes with
many frames to send, each
transmits in slot with
probability p
v prob that given node has
success in a slot = p(1-
p)N-1
v prob that any node has a
success = Np(1-p)N-1
v max efficiency: find p*
that maximizes
Np(1-p)N-1
v for many nodes, take
limit of Np*(1-p*)N-1 as N
goes to infinity, gives:
max efficiency = 1/e = .
37
efficiency: long-run
fraction of successful
slots
(many nodes, all with
many frames to send)
at best: channel
used for useful
transmissions
37%
of time!
!
Slotted ALOHA: efficiency
Link Layer 5-27
Pure (unslotted) ALOHA
v unslotted Aloha: simpler, no synchronization
v when frame first arrives
§ transmit immediately
v collision probability increases:
§ frame sent at t0 collides with other frames sent in
[t0-1,t0+1]
Link Layer 5-28
Pure ALOHA efficiency
P(success by given node) = P(node transmits) .
P(no other node transmits in [t0-1,t0] .
P(no other node transmits in [t0-1,t0]
= p . (1-p)N-1 . (1-p)N-1
= p . (1-p)2(N-1)
… choosing optimum p and then letting n
= 1/(2e) = .18
even worse than slotted Aloha!
Link Layer 5-29
CSMA (carrier sense multiple
access)
CSMA: listen before transmit:
if channel sensed idle: transmit entire frame
v if channel sensed busy, defer transmission
v human analogy: don’t interrupt others!
Got rid of collisions?
Link Layer 5-30
CSMA collisions
v collisions can still
occur: propagation
delay means two
nodes may not hear
each other’s
transmission
v collision: entire packet
transmission time
wasted
§ distance & propagation
delay play role in in
determining collision
probability
spatial layout of nodes
Link Layer 5-31
CSMA/CD (collision detection)
CSMA/CD: carrier sensing, deferral as in CSMA
§ collisions detected within short time
§ colliding transmissions aborted, reducing channel
wastage
v collision detection:
§ easy in wired LANs: measure signal strengths,
compare transmitted, received signals
§ difficult in wireless LANs: received signal strength
overwhelmed by local transmission strength
v human analogy: the polite conversationalist
Link Layer 5-32
Ethernet CSMA/CD algorithm
1. NIC receives datagram
from network layer,
creates frame
2. If NIC senses channel
idle, starts frame
transmission. If NIC
senses channel busy,
waits until channel idle,
then transmits.
3. If NIC transmits entire
frame without detecting
another transmission,
NIC is done with
frame !
4. If NIC detects another
transmission while
transmitting, aborts
5. After aborting, NIC enters
binary (exponential)
backoff:
§ after mth collision, NIC
chooses K at random
from {0,1,2, …, 2m-1}.
NIC waits K·512 bit
times, returns to Step 2
§ longer backoff interval
with more collisions
Link Layer 5-33
CSMA/CD efficiency
v Tprop = max prop delay between 2 nodes in LAN
v ttrans = time to transmit max-size frame
v efficiency goes to 1
§ as tprop goes to 0
§ as ttrans goes to infinity
v better performance than ALOHA – and simple, cheap,
decentralized!
transprop /tt
efficiency
51
1
+
=
Link Layer 5-34
“Taking turns” MAC protocols
channel partitioning MAC protocols:
§ share channel efficiently and fairly at high load
§ inefficient at low load: delay in channel access, 1/
N 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
“taking turns” protocols
look for best of both worlds!
Link Layer 5-35
polling:
v master node
“invites” slave nodes
to transmit in turn
v typically used with
“dumb” slave
devices
v concerns:
§ polling overhead
§ latency
§ single point of
failure (master)
master
slaves
poll
data
data
“Taking turns” MAC protocols
Link Layer 5-36
token passing:
v control token passed
from one node to next
sequentially.
v token message
v concerns:
§ token overhead
§ latency
§ single point of failure
(token)
T
data
(nothing
to send)
T
“Taking turns” MAC protocols
Link Layer 5-37
Summary of MAC protocols
v Link layer services
v Error detection and correction
v channel partitioning, by time, frequency or
code
§ Time Division, Frequency Division
§ random access protocols