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A Novel Topology-blind Fair Medium Access Control for Wireless LAN and Ad Hoc

Networks

Z. Y. Fang and B. BensaouComputer Science Department

Hong Kong University of Science and Technology

ICC 2003

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OutlineOutline

IntroductionIntroduction IEEE 802.11 DCFIEEE 802.11 DCF Improving the Fairness of the Improving the Fairness of the

Backoff AlgorithmBackoff Algorithm SimulationsSimulations ConclusionConclusion

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Introduction 802.11Introduction 802.11 DCFDCF

Carrier sensing / Random backoff / Virtual Carrier sensing / Random backoff / Virtual backoff / Collision avoidancebackoff / Collision avoidance

GuaranteesGuarantees that ultimately a node will have that ultimately a node will have a chance of attempting to access the channela chance of attempting to access the channel Since the backoff timers are frozen when a node Since the backoff timers are frozen when a node

defers defers No guaranteeNo guarantee the success of such attempts the success of such attempts

In presence of hidden terminalsIn presence of hidden terminals When the traffic When the traffic loadload is is highhigh and the node and the node

densitydensity is is non uniformnon uniform Station succeeding a large number of transmission Station succeeding a large number of transmission

consecutively in an asymmetric networkconsecutively in an asymmetric network

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IntroductionIntroduction

Problem of fairnessProblem of fairness There are There are NN network nodes, optimal network nodes, optimal

success probability ofsuccess probability of 1/N 1/N This This unfairnessunfairness can be very marked can be very marked

when the traffic load is highwhen the traffic load is high

A B C

1/2 1/3 1/2

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IntroductionIntroduction

Scheduled schemesScheduled schemes NeedNeed coordinatorscoordinators to dictate the behavior of to dictate the behavior of

nodesnodes Minimum bandwidth guaranteeMinimum bandwidth guarantee

NeedNeed to incorporate a to incorporate a weightweight allocationallocation method as well as the means to distribute such method as well as the means to distribute such information among nodesinformation among nodes

Measurement-base fair backoff algorithmMeasurement-base fair backoff algorithm Can alleviate the fairness problemCan alleviate the fairness problem Need to Need to exchangeexchange

WeightsWeights allocation information allocation information Maintain Maintain topologytopology knowledge knowledge

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IntroductionIntroduction

Blind backoff schemeBlind backoff scheme WithoutWithout need for any need for any exchangeexchange of of

informationinformation WithoutWithout any any assumptionassumption Self adaptingSelf adapting to changes in the traffic to changes in the traffic

load as well as topological changesload as well as topological changes Novel backoff algorithmNovel backoff algorithm

To propose a new backoff mechanism for IEEE 802.11 for a fair channel access

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Improving the Fairness of Improving the Fairness of the Backoff Algorithmthe Backoff Algorithm

Deals with how to estimate tp: Counts the number of successful

transmissions in a period Increases contention window size in

case of more successful transmission Decreases contention window size to be

more aggressive in case of successive failures

Adjusts tp when contention window hits the maximum or the minimum bound since it is either too large or too small

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Improving the Fairness of Improving the Fairness of the Backoff Algorithmthe Backoff Algorithm

tpacket

RT

S CT

S AC

K

DATA

tunit

tp= k * tunit

DIFS SIFSIf k=2

RT

S CT

S AC

K

DATAOther STA packet

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if ( >1 )

if ( == CWmax )

= - +1

else = 2 *

else

if ( == 0)

if( == CWmin )

= + 1

else = / 2

endif

endif

= 0

Timer Event: ( TP )

wini

t iunitt i

p = k i *

wini wini

wini

winiwini

n i

n i

n i

n iikik

ikikUse current contention window to access channelif ( successfully received ACK)

= + 1 Update Update

Init : t ipacket= 0

k i= 1

t iunitt i

p = k i *

Channel Access :

t iunit

t ipacket

wini

n in i

ni= 0 // the number of successful transmission

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A A B A A B

STA A : CW=32 slot, Tp=1STA B : CW=32 slot, Tp=1

STAA

STAB

CW=32 Tp = 1

CW=32 Tp = 1

CW=32 Tp = 2

CW=32 Tp = 1

CW=32 Tp = 1

CW=32 Tp = 2

CW=64 Tp = 2

CW=32 Tp = 2

CW=32 Tp = 2

CW=64 Tp = 2

CW=32 Tp = 3

AB

A B C

1/2 1/3 1/2Monitor

1

Monitor 1

CW=32 Tp = 2

CW=32 Tp = 2

TP

TUINT

AA

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SimulationsSimulations

Network topology

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FMAC

802.11

FMACFMAC

FMAC

802.11

802.11802.11

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FMAC

802.11

FMAC

802.11

FMAC

802.11

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ConclusionConclusion

Improve fairnessImprove fairness of IEEE 802.11 of IEEE 802.11 would have to pay a price for this is would have to pay a price for this is terms of total network terms of total network throughput throughput degradationdegradation

Most of the time less than Most of the time less than 50%50% throughputthroughput is is lostlost due to due to introduction of fairnessintroduction of fairness

SatisfactionSatisfaction is the driving force is the driving force behind any behind any fairnessfairness oriented work oriented work

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