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Physical Media

physical link: transmitted data bit propagates across link

guided media: signals propagate in

solid media: copper, fiber

unguided media: signals propagate

freely, e.g., radio

Twisted Pair (TP) two insulated copper

wires Category 3: traditional

phone wires, 10 Mbps ethernet

Category 5 TP: 100Mbps ethernet

2

Physical Media: coax, fiber

Coaxial cable: wire (signal carrier)

within a wire (shield) baseband: single

channel on cable broadband: multiple

channel on cable

bidirectional common use in

10Mbs Ethernet

Fiber optic cable: glass fiber carrying

light pulses high-speed operation:

100Mbps Ethernet high-speed point-to-

point transmission (e.g., 5 Gps)

very low error rate

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Physical media: radio

signal carried in electromagnetic spectrum

no physical “wire” bidirectional propagation

environment effects: reflection obstruction by objects interference

Radio link types: microwave

e.g. up to 45 Mbps channels

LAN (e.g., waveLAN) 2Mbps, 11Mbps

wide-area (e.g., cellular) e.g. CDPD, 10’s Kbps

satellite up to 50Mbps channel (or multiple

smaller channels) 270 Msec end-end delay geosynchronous versus LEOS (low

earth orbit)

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The Data Link LayerOur goals: understand principles

behind data link layer services: error detection,

correction sharing a broadcast

channel: multiple access

link layer addressing instantiation and

implementation of various link layer technologies

Overview: link layer services error detection, correction multiple access protocols

and LANs link layer addressing specific link layer

technologies: Ethernet

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Link Layer: setting the context

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Link Layer: setting the context two physically connected devices:

host-router, router-router, host-host

unit of data: frame

applicationtransportnetwork

linkphysical

networklink

physical

M

M

M

M

Ht

HtHn

HtHnHl MHtHnHl

framephys. link

data linkprotocol

adapter card

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Link Layer Services Framing, link access:

encapsulate datagram into frame, adding header, trailer

implement channel access if shared medium, ‘physical addresses’ used in frame headers to

identify source, destination • different from IP address!

Reliable delivery between two physically connected devices: seldom used on low bit error link (fiber, some twisted

pair) wireless links: high error rates

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Link Layer Services (more)

Flow Control: pacing between sender and receivers

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

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Link Layer: Implementation implemented in “adapter”

e.g., PCMCIA card, Ethernet card typically includes: RAM, DSP chips, host bus

interface, and link interface

applicationtransportnetwork

linkphysical

networklink

physical

M

M

M

M

Ht

HtHn

HtHnHl MHtHnHl

framephys. link

data linkprotocol

adapter card

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Error DetectionEDC= Error Detection and Correction bits (redundancy)D = Data protected by error checking, may include header fields

• Error detection not 100% reliable! Q: why?• protocol may miss some errors, but rarely• larger EDC field yields better detection and correction

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Parity Checking

Single Bit Parity:Detect single bit errors

Two Dimensional Bit Parity:Detect and correct single bit errors

0 0

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Internet checksum

Sender: treat segment contents

as sequence of 16-bit integers

checksum: addition (1’s 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?

Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)

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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

<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

widely used in practice (ATM, HDCL)

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CRC ExampleWant:

D.2r XOR R = nGequivalently:

D.2r = nG XOR R equivalently: if we divide D.2r by

G, want reminder R

R = remainder[ ]D.2r

G

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Multiple Access Links and Protocols

Three types of “links”: point-to-point (single wire, e.g. PPP, SLIP) broadcast (shared wire or medium; e.g,

Ethernet, Wavelan, etc.)

switched (e.g., switched Ethernet, ATM etc)

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Contexts

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Contexts

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

interference only one node can send successfully at a time

multiple access protocol: distributed algorithm that determines how stations share

channel, i.e., determine when station can transmit communication about channel sharing must use channel itself! what to look for in multiple access protocols:

• synchronous or asynchronous • information needed about other stations • robustness (e.g., to channel errors) • performance

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MAC Protocols: a taxonomy

Three broad classes: Channel Partitioning

divide channel into smaller “pieces” (time slots, frequency)

allocate piece to node for exclusive use

Random Access allow collisions “recover” from collisions

“Taking turns” tightly coordinate shared access to avoid collisions

Goal: efficient, fair, simple, decentralized

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MAC Protocols: Goal

Channel Rate = R bps Efficient:

Single user: Throughput R Fairness

N usersMin. user throughput R/N

Decentralized Fault tolerance

Simple

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The parameter ‘a’

The number of packets sent by a source before the farthest station receives the first bit

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Base technologies

Isolates data from different sources Three basic choices

Frequency division multiple access (FDMA) Time division multiple access (TDMA) Code division multiple access (CDMA)

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FDMA

Simplest Best suited for analog links Each station has its own frequency

band, separated by guard bands Receivers tune to the right frequency Number of frequencies is limited

reduce transmitter power; reuse frequencies in non-adjacent cells

example: voice channel = 30 KHz 833 channels in 25 MHz band

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TDMA

All stations transmit data on same frequency, but at different times

Needs time synchronization Pros

users can be given different amounts of bandwidth

mobiles can use idle times to determine best base station

can switch off power when not transmitting Cons

synchronization overhead

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CDMA

Users separated both by time and frequency Send at a different frequency at each time slot

(frequency hopping) Or, convert a single bit to a code (direct

sequence) receiver can decipher bit by inverse process

Pros hard to spy immune from narrowband noise no need for all stations to synchronize no hard limit on capacity of a cell all cells can use all frequencies

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CDMA

Cons implementation complexity need for a large contiguous frequency band

(for direct sequence)

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FDD and TDD

Two ways of converting a wireless medium to a duplex channel

In Frequency Division Duplex, uplink and downlink use different frequencies

In Time Division Duplex, uplink and downlink use different time slots

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Outline

Contexts for the problem Choices and constraints Performance metrics Base technologies Centralized schemes Distributed schemes

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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

TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

FDM (Frequency Division Multiplexing): frequency subdivided.

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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, 1,3,4 have pkt, frequency bands 2,5,6 idle

TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load.

FDM (Frequency Division Multiplexing): frequency subdivided.

frequ

ency

bands time

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TDMA & FDMA: Performance

Channel Rate = R bps Single user

Throughput R/N Fairness

Each user gets the same allocationDepends on maximum number of users

Decentralized Requires division

Simple

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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 and CSMA/CD

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Slotted Aloha

time is divided into equal size slots (= pkt trans. time)

node with new arriving pkt: transmit at beginning of next slot

if collision: retransmit pkt in future slots with probability p, until successful.

Success (S), Collision (C), Empty (E) slots

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Slotted Aloha efficiencyQ: what is max fraction slots successful?A: Suppose N stations have packets to send

each transmits in slot with probability p prob. successful transmission S is:

by single node: S= p (1-p)(N-1)

by any of N nodes

S = Prob (only one transmits) = N p (1-p)(N-1)

… choosing optimum p as N -> infty ...

= 1/e = .37 as N -> infty

At best: channeluse for useful transmissions 37%of time!

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Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization pkt needs transmission:

send without awaiting for beginning of slot

collision probability increases: pkt sent at t0 collide with other pkts sent in [t0-1,

t0+1]

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Pure Aloha (cont.)P(success by given node) = P(node transmits) .

P(no other node transmits in [t0-1,t0] .

P(no other node transmits in [t0,t0+1]

= p . (1-p)N-1 . (1-p)N-1

P(success by any of N nodes) = N p . (1-p)N-1 . (1-p)N-1

… choosing optimum p as N -> infty ...

= 1/(2e) = .18

S =

thro

ughput

=

“goodput”

(

succ

ess

rate

)

G = offered load = Np0.5 1.0 1.5 2.0

0.1

0.2

0.3

0.4

Pure Aloha

Slotted Alohaprotocol constrainseffective channelthroughput!

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Aloha: Performance

Channel Rate = R bps Single user

Throughput R ! Fairness

Multiple usersCombined throughput only 0.37*R

Decentralized Slotted needs slot synchronization

Simple

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CSMA: Carrier Sense Multiple Access)

CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission

Persistent CSMA: retry immediately with probability p when channel becomes idle (may cause instability)

Non-persistent CSMA: retry after random interval human analogy: don’t interrupt others!

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CSMA collisions

collisions can occur:propagation delay means two nodes may not yethear each other’s transmissioncollision:entire packet transmission time wasted

spatial layout of nodes along ethernet

note:role of distance and propagation delay in determining collision prob.

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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 persistent or non-persistent retransmission

collision detection: easy in wired LANs: measure signal strengths,

compare transmitted, received signals difficult in wireless LANs: receiver shut off while

transmitting

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CDMA/CD

Channel Rate = R bps Single user

Throughput R Fairness

Multiple usersDepends on Detection Time

Decentralized Completely

Simple Needs collision detection hardware

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“Taking Turns” MAC protocolschannel partitioning MAC protocols:

share channel efficiently 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” protocolslook for best of both worlds!

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“Taking Turns” MAC protocolsPolling: master node

“invites” slave nodes to transmit in turn

Request to Send, Clear to Send msgs

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)

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Summary of MAC protocols

What do you do with a shared media? Channel Partitioning, by time, frequency or

code• Time Division,Code Division, Frequency Division

Random partitioning (dynamic), • CSMA, CSMA/CD• carrier sensing: easy in some technologies (wire),

hard in others (wireless)• CSMA/CD used in Ethernet

Taking Turns• polling from a central cite, token passing

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Performance metrics

Normalized throughput fraction of link capacity used to carry non-

retransmitted packets example

• with no collisions, 1000 packets/sec• with a particular scheme and workload, 250

packets/sec• => goodput = 0.25

Mean delay amount of time a station has to wait before it

successfully transmits a packet• depends on the load and the characteristics of the

medium

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Performance metrics

Stability with heavy load, is all the time spent on resolving

contentions? => unstable with a stable algorithm, throughput does not

decrease with offered load if infinite number of uncontrolled stations share a

link, then instability is guaranteed but if sources reduce load when overload is

detected, can achieve stability Fairness

no single definition ‘no-starvation’: source eventually gets a chance to

send

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Centralized access schemes

One station is master, and the other are slaves slave can transmit only when master allows

Natural fit in some situations wireless LAN, where base station is the only

station that can see everyone cellular telephony, where base station is the

only one capable of high transmit power

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Centralized access schemes Circuit mode When station wants to transmit, it sends

a message to master using packet mode

Master allocates transmission resources to slave

Slave uses the resources until it is done No contention during data transfer Used primarily in cellular phone systems

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Centralized access schemes Polling Centralized packet-mode multiple

access schemes Polling

master asks each station in turn if it wants to send (roll-call polling)

inefficient if only a few stations are active, overhead for polling messages is high, or system has many terminals

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Centralized access schemes Reservation-based schemes When ‘a’ is large(mainly for satellite links),

can’t use a distributed scheme for packet mode (too many collisions)

Instead master coordinates access to link using reservations

Some time slots devoted to reservation messages can be smaller than data slots => minislots

Stations contend for a minislot (or own one) Master decides winners and grants them

access to link Packet collisions are only for minislots, so

overhead on contention is reduced

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Centralized access schemes

Pros simple master provides single point of coordination

Cons master is a single point of failure

• master is involved in every single transfer => added delay

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Outline

Contexts for the problem Choices and constraints Performance metrics Base technologies Centralized schemes Distributed schemes

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LAN technologies

Data link layer so far: services, error detection/correction, multiple

access

Next: LAN technologies addressing Ethernet hubs, bridges, switches 802.11 PPP ATM

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LAN Addresses

32-bit IP address: network-layer address used to get datagram to destination network

LAN (or MAC or physical) 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

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LAN AddressesEach adapter on LAN has unique LAN address

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LAN Address (more)

MAC address allocation administered by IEEE

manufacturer buys portion of MAC address space (to assure uniqueness)

MAC flat address => portability can move LAN card from one LAN to another

IP hierarchical address NOT portable depends on network to which one attaches

ARP protocol translates IP address to MAC address

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Ethernet“dominant” LAN technology: cheap $20 for 100Mbs! first widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 1, 10, 100, 1000 Mbps

Metcalfe’s Etheretsketch

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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

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Ethernet Frame Structure (more) Addresses: 6 bytes, frame is received by all

adapters on a LAN and dropped if address does not match

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

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Ethernet: uses CSMA/CD

A: sense channel, if idle then {

transmit and monitor the channel; If detect another transmission then { abort and send jam signal;

update # collisions; delay as required by exponential backoff algorithm; goto A}

else {done with the frame; set collisions to zero}}

else {wait until ongoing transmission is over and goto A}

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Ethernet Technologies: 10Base2 10: 10Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology

repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

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10BaseT and 100BaseT

10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Hub to which nodes are connected by twisted

pair, thus “star topology” CSMA/CD implemented at hub

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10BaseT and 100BaseT (more) Max distance from node to Hub is 100 meters Hub can disconnect “jabbering adapter Hub can gather monitoring information,

statistics for display to LAN administrators

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Ethernet

The most widely used LAN Standard is called IEEE 802.3 Uses CSMA/CD with exponential backoff Also, on collision, place a jam signal on wire, so

that all stations are aware of collision and can increment timeout range

‘a’ small =>time wasted in collision is around 50 microseconds

Ethernet requires packet to be long enough that a collision is detected before packet transmission completes (a <= 1) packet should be at least 64 bytes long for longest

allowed segment Max packet size is 1500 bytes

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Evaluating Ethernet

Pros easy to setup requires no configuration

Problems at heavy loads, users see large delays

because of backoff nondeterministic service doesn’t support priorities

But, very successful because problems only at high load can segment LANs to reduce load

66

CSMA/CA

Used in wireless LANs So, need explicit acks But this makes collisions more

expensive Can’t detect collision because

transmitter overwhelms colocated receiver => try to reduce number of collisions

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CSMA/CA algorithm

First check if medium is busy If so, wait for medium to become idle Wait for interframe spacing Set a contention timer to an interval randomly

chosen in the range [1, CW] On timeout, send packet and wait for ack If no ack, assume packet is lost

try again, after doubling CW

If another station transmits while counting down, freeze CW and unfreeze when packet completes transmission

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Dealing with hidden terminals

CSMA/CA works when every station can receive transmissions from every other station

Not always true Hidden terminal

some stations in an area cannot hear transmissions from others, though base can hear both

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Hidden terminals A and C cannot hear each other. A sends to B, C cannot receive A. C wants to send to B, C senses a “free”

medium (CS fails) Collision occurs at B. A cannot receive the collision (CD fails). A is “hidden” for C.

Solution? Hidden terminal is peculiar to wireless (not found in wired) Need to sense carrier at receiver, not sender! “virtual carrier sensing”: Sender “asks” receiver whether

it can hear something. If so, behave as if channel busy.

Hidden Terminal Problem

BA C

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Dealing with hidden terminals CSMA/CA doesn’t work

with hidden terminal, collision because carrier not detected

Two solutions Busy Tone Multiple Access (BTMA)

uses a separate “busy-tone” channel when station is receiving a message, it places a tone

on this channel everyone who might want to talk to a station knows

that it is busy• even if they cannot hear transmission that station

hears

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802.11 avoids the problem of hidden terminals A and C want to send to B A sends RTS to B B sends CTS to A C “overhears” CTS from B C waits for duration of A’s transmission

RTS/CTS

A B C

RTS

CTSCTS

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Multiple Access Collision Avoidance solution2

Instead, use a single frequency band, but use explicit messages to tell others that receiver is busy

In MACA, before sending data, send a Request to Sent (RTS) to intended receiver

Station, if idle, sends Clear to Send (CTS)

Sender then sends data If station overhears RTS, it waits for

other transmission to end (why does this work?)

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Token passing

In distributed polling, every station has to wait for its turn

Time wasted because idle stations are still given a slot

What if we can quickly skip past idle stations? This is the key idea of token ring Special packet called ‘token’ gives station the

right to transmit data When done, it passes token to ‘next’ station

=> stations form a logical ring

No station will starve

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Logical rings

Can be on a non-ring physical topology

76

Single and double rings

With a single ring, a single failure of a link or station breaks the network => fragile

With a double ring, on a failure, go into wrap mode

Used in FDDI

77

Evaluating token ring

Pros medium access protocol is simple and explicit no need for carrier sensing, time synchronization or

complex protocols to resolve contention guarantees zero collisions can give some stations priority over others

Cons token is a single point of failure

• lost or corrupted token trashes network• need to carefully protect and, if necessary, regenerate

token all stations must cooperate

• network must detect and cut off unresponsive stations stations must actively monitor network

• usually elect one station as monitor

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Fiber Distributed Data Interface FDDI is the most popular token-ring

base LAN Dual counterrotating rings, each at 100

Mbps Uses fiber links Supports both non-realtime and

realtime traffic Supports both single attached and dual

attached stations single attached (cheaper) stations are

connected to only one of the rings