PowerPoint Presentation
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
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Link layer
Objectives:
understand principles behind link layer services:
error detection, correction
sharing a broadcast channel: multiple access
link layer addressing
Ethernet
instantiation, implementation of link layer technologies
Link Layer
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Link Layer
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Outline
introduction, services
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
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Link layer: introduction
terminology:
hosts and routers: nodes
communication channels that connect adjacent nodes along communication path: links
wired links
wireless links
LANs
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
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Link layer: context
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
each link protocol provides different services
e.g., may or may not provide rdt over link
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Link layer services
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!
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?
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flow control:
pacing between adjacent sending and receiving nodes
error detection:
errors caused by signal attenuation, noise.
receiver detects presence of errors:
signals sender for retransmission or drops frame
error correction:
receiver identifies and corrects bit error(s) without resorting to retransmission
Link layer services (more)
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Where is the link layer implemented?
in each and every host
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
attaches into host’s system buses
combination of hardware, software, firmware
controller
physical
transmission
cpu
memory
host
bus
(e.g., PCI)
network adapter
card
application
transport
network
link
link
physical
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Adaptors communicating
sending side:
encapsulates datagram in frame
adds error checking bits, rdt, flow control, etc.
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
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Outline
introduction, services
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
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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
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Parity checking
single bit parity (even/odd):
detect single bit errors
two-dimensional bit parity:
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.
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Internet checksum (review)
sender:
treat segment contents as sequence of 16-bit integers
checksum: addition (1’s complement sum) of segment contents
sender puts checksum value into 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?
goal: detect “errors” (e.g., flipped bits) in transmitted packet (note: used at transport layer only)
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Cyclic redundancy check
more powerful error-detection coding
choose r+1 bit pattern (generator), G
can detect all burst errors less than r+1 bits
view data bits, D, as a binary number
goal: choose r CRC bits, R, such that
receiver knows G, divides
widely used in practice (Ethernet, 802.11 WiFi)
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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 to satisfy:
R = remainder[ ]
D.2r
G
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Outline
introduction, services
error detection, correction
multiple access protocols
LANs
addressing, ARP
Ethernet
switches
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Multiple access links, protocols
two types of “links”:
point-to-point
PPP for dial-up access
point-to-point link between Ethernet switch, host
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)
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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, i.e., determine when node can transmit
communication about channel sharing must use channel itself!
no out-of-band channel for coordination
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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
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MAC protocols: taxonomy
channel partitioning
divide channel into smaller “pieces” (time slots, frequency)
allocate piece to node for exclusive use
random access
channel not divided, collisions allowed
“recover” from collisions
taking turns
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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, 1,3,4 have pkt, slots 2,5,6 idle
1
3
4
1
3
4
6-slot
frame
6-slot
frame
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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, 1,3,4 have pkt, frequency bands 2,5,6 idle
frequency bands
time
FDM cable
Channel partitioning MAC protocols: FDMA
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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 ➜ “collision”,
random access MAC protocol specifies:
how to detect collisions
how to recover from collisions (e.g., via delayed retransmissions)
examples of random access MAC protocols:
slotted ALOHA
ALOHA
CSMA, CSMA/CD, CSMA/CA
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Slotted ALOHA
assumptions:
all frames same size
time divided into equal size slots (time to transmit 1 frame)
nodes start to transmit only at slot beginning
nodes are synchronized
if 2 or more nodes transmit in the slot, all nodes detect collision
operation:
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
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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
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
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suppose: N nodes with many frames to send, each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency: find p* that maximizes
Np(1-p)N-1
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
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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 [t0-1,t0+1]
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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!
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CSMA (carrier sense multiple access)
CSMA: listen before transmit:
if channel sensed idle: transmit entire frame
if channel sensed busy, defer transmission
human analogy: don’t interrupt others!
Got rid of collisions?
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CSMA collisions
collisions can still occur: propagation delay means two nodes may not hear each other’s transmission
collision: entire packet transmission time wasted
distance & propagation delay play role in determining collision probability
spatial layout of nodes
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CSMA/CD (collision detection)
CSMA/CD: 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: received signal strength overwhelmed by local transmission strength
human analogy: the polite conversationalist
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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
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CSMA/CD efficiency
tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame
efficiency goes to 1
as tprop goes to 0
as ttrans goes to infinity
better performance than ALOHA – and simple, cheap, decentralized!
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“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!
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polling:
master node “invites” slave nodes to transmit in turn
typically used with “dumb” slave devices
concerns:
polling overhead
latency
single point of failure (master)
master
slaves
“Taking turns” MAC protocols
poll
data
data
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token passing:
control token passed from one node to next sequentially.
token message
concerns:
token overhead
latency
single point of failure (token)
T
data
(nothing
to send)
T
“Taking turns” MAC protocols
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Summary of MAC protocols
Link layer services
Error detection and correction
channel partitioning, by time, frequency or code
Time Division, Frequency Division
random access protocols
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trans
prop
/t
t
efficiency
5
1
1
+
=