Performance Anomaly of Performance Anomaly of 802.11b802.11b

Martin Heusse, Franck Rousseau, Gilles BeMartin Heusse, Franck Rousseau, Gilles Berger-Sabbatel, Andrzej Dudarger-Sabbatel, Andrzej Duda

IEEE INFOCOM 2003

OutlineOutline

► Introduction Introduction ►Performance of IEEE 802.11b DCF Performance of IEEE 802.11b DCF

access methodaccess method►Performance anomaly of IEEE 802.11b Performance anomaly of IEEE 802.11b ►Simulation Simulation ►Performance measurementsPerformance measurements►ConclusionsConclusions

IntroductionIntroduction

►802.11 uses the 802.11 uses the CSMA/CACSMA/CA protocol to protocol to shareshare the radio channel in a the radio channel in a fair wayfair way

►Anomaly is the basic CSMA/CA methodAnomaly is the basic CSMA/CA method When a host detects repeated When a host detects repeated

unsuccessful frameunsuccessful frame transmissions, it transmissions, it decreases its bit ratedecreases its bit rate

Least oneLeast one host with a lower rate host with a lower rate►Throughput : Throughput : higher rate host = lower rate higher rate host = lower rate

hosthost

Performance of IEEE 802.11b DCF access Performance of IEEE 802.11b DCF access methodmethod

No collisions proportion p

t t rX

1500

1534=0.7

t t r + t OVPHY p MAC p

= 11 M/s * 0.7 = 7.74 Mb/s

Collisions proportion p(N)

=

No collisions Throughput

t t rt t r + t OV + tcont(N)

= Fast host : TfSlow host : Ts

Sd

R

Performance anomaly of IEEE Performance anomaly of IEEE 802.11b802.11b

U f = Tf

( N -1) Tf + Ts + Pc(N) * tjam * Ncollisions

( N -1) Tf + Ts + Pc(N) * tjam * N

Tf Ts

N-1

Performance anomaly of IEEE Performance anomaly of IEEE 802.11b802.11b

Throughput X f = U f * p f(N) * R

= U f *SdRTf

* R

= ( N -1) Tf + Ts + Pc(N) * tjam * N

Sd

Xf = Xs = X

Performance anomaly of IEEE Performance anomaly of IEEE 802.11b802.11b

►When fast and slow hosts When fast and slow hosts share the radio share the radio channelchannel of a 802.11b cell, throughput of a 802.11b cell, throughput

► The The fair accessfair access to the channel provided by to the channel provided by CSMA/CACSMA/CA

► Transmitting at 1 Mb/s to Transmitting at 1 Mb/s to capturecapture the the channel eleven times longer than hosts channel eleven times longer than hosts emitting at 11 Mb/semitting at 11 Mb/s

N * X R * pf(N) Large number of fast hosts

SimulationSimulation

Performance measurementsPerformance measurementsdifferent rates, no mobilitydifferent rates, no mobility

11Mb/s 11Mb/s

5.5 Mb/s 5.5 Mb/s

2 Mb/s 2 Mb/s1 Mb/s 1 Mb/s

Performance measurementsPerformance measurementsdifferent rates, real mobilitydifferent rates, real mobility

Keeps bit rate 11Mb/s

ConclusionsConclusions

►802.11b throughput is much 802.11b throughput is much smaller thansmaller than the nominal bit ratethe nominal bit rate

►Slow hostSlow host may considerably may considerably limit the throlimit the throughputughput of other hosts roughly to the level of other hosts roughly to the level of the lower rateof the lower rate

►DeploymentDeployment of 802.11 access points of 802.11 access points