Call Admission Control for IEEE 802.11 Contention Access Mec

Abstract — This paper proposes an admission control algorithm that enables the upcoming IEEE 802.11e contention based Enhanced Distributed Channel Access (EDCA) to provide quantitative bandwidth guarantees for Wireless Local Area Networks (WLANs), rather

CallAdmissionControlforIEEE802.11

ContentionAccessMechanism

DennisPongandTimMoors

SchoolofElectricalEngineeringandTelecommunications,TheUniversityofNewSouthWales,Australia

Email:dennisp@student.unsw.edu.auandt.moors@unsw.edu.au

Abstract—ThispaperproposesanadmissioncontrolalgorithmthatenablestheupcomingIEEE802.11econtentionbasedEn-hancedDistributedChannelAccess(EDCA)toprovidequanti-tativebandwidthguaranteesforWirelessLocalAreaNetworks(WLANs),ratherthanarelativeprioritizedservice.Thealgo-rithmestimatesthethroughputthat owswouldachieveifanew owwithcertainparameterswasadmitted,andsoindicateswhethersuchanew owcanbeadmittedwhilepreservingtheQualityofService(QoS)ofexisting ows.ThealgorithmdealswiththeEDCAparametersofminimumcontentionwindowsizeandtransmissionopportunityduration,andindicateswhatvaluesshouldbeusedfordifferent ows.Simulationresultscon rmtheaccuracyofthethroughputestimatesandtheeffectivenessoftheadmissioncontrolalgorithm.

I.INTRODUCTION

TheIEEE802.11MACusesthecontentionbasedDis-tributedCoordinationFunction(DCF)asthebasicaccessmechanismandapollingbasedPointCoordinationFunction(PCF)toprovidecontentionfreechannelaccess.IEEE802.11taskgroupEisde ningmechanismstoenhancetheQoSoftheoriginalMACstandard,includinganewcontentionaccessschemecalledEnhancedDistributedChannelAccess(EDCA)[1]andanimprovedpollingschemebasedonPCF.

Contentionbasedaccessmechanismsaresimpleandro-bust,buttheycannotguaranteedelayandthroughputbe-causeoftheirprobabilisticnature.Withsmallmodi cationtotheDCFaccessmechanism,whichvirtuallyallwirelessLANcardsimplement,relativepriorityserviceisprovided.ThispaperfocusesonthecontentionbasedEDCAaccessscheme,whichprovidesaprioritizedQoSserviceusinganindependenttransmitqueueandchannelaccessfunctionforeachtraf cclass.Traf cbelongingtoahigherpriorityclasshasahigherprobabilityoftransmission,thusachievingahigherthroughputwhencompetingwithlowerprioritytraf c.However,noassurancecanbegiventohigherprioritytraf cintermsofthroughputanddelayperformance.Theproblemisespeciallyapparentwhenthewirelesschannelisoverloadedcausingthebandwidthshareofeach owtodiminish.ThislimitstheuseofEDCAformanymultimediaapplicationsthatarenotcapableofdynamicallyadaptingtotheavailablebandwidthatanyinstant.Sincetheyrequireabsoluteband-widthprovisionfromthenetworks,resourcereservationandmanagementbecomesasigni cantissueinWLANsespeciallywhenbandwidthisrelativelyscarceandhastobesharedbymanyusers.AdmissioncontrolisanimportanttooltomaintainQoSexperiencedbyusers.Inthispaper,wepropose

To be published in Globecom, San Francisco, 1-5 Dec 2003

anadmissioncontrolalgorithmforthe802.11eEDCAthatwilltakeaccountofdynamicnetworkconditionssuchasthenumberofactive owsandtheparametersadoptedforthesedata ows.Bypredictingtheachievablethroughputofthedata owsandavoidingchanneloverloading,theQoSofexisting owscanbemaintained.WeincorporatetheconceptsofEDCAintotheadmissioncontrolalgorithmandextenditsabilitytoprovidebandwidthguarantees,insteadofprovidingarelativeprioritizedservice.Simulationresultsdemonstratethatbyimplementingtheproposedadmissionalgorithm,thethroughputofexisting owscanbeprotectedandresourcescanbemanagedeffectivelyaccordingtonetworkconditions.Thealgorithmisabletoselectsuitableparametersforgoodbandwidthanddelayperformance.

II.NEWFEATURESIN802.11EEDCA

TheEDCAaccessmechanismsupportsrelativepriorityservicethroughtheintroductionofAccessCategories(ACs).InsteadofusingasinglequeueandonechannelaccessfunctionasinDCF,eachstationimplementsmultipleACs.EachACconsistsofanindependenttransmitqueueandachannelaccessfunctionwithitsownparameters,thatincludeminimumandmaximumContentionWindows(CWmin,CWmax),ArbitrationInterframeSpace(AIFS)andTransmissionOpportunity(TXOP)duration.OperationofeachchannelaccessfunctionissimilartoDCF.DatatransmissionbeginswhenthemediumisidleformorethantheAIFStime,withAIFS≥DIFS(DCFInterframeSpace,seeFig.1).Ifthemediumisdeterminedtobebusy,theaccessfunctionsetsitsbackofftimertoaninitialvalueofrandom(0,CWmin[AC])×slottime.ThebackofftimerisdecrementedbyaslottimeforeachtimeslotafterthemediumissensedidleforanAIFStimeandstopsdecrementingwhenatransmissionisdetected.ItresumeswhenthemediumissensedidleagainforanAIFStime.Achannelaccessfunctioncanbegintransmissiononthechannelassoonasthebackofftimerreacheszero.Anacknowledgementframeissenttothesendertoindicateasuccessfultransmissionofadataframe.IfnoacknowledgementisreceivedafteraShortIFS(SIFS),acollisionispresumedtohaveoccurred.Thebackofftimerafteracollisionischosentoberandom(0,(CWmin[AC]+1)×2i-1)×slottimeforeachretransmissionattempti.Inotherwords,thecontentionwindowsizeisdoubledforeachretransmissiontoreducetheprobabilityofcollision.HigherpriorityACsadoptlowervaluesforCWminandAIFStoyieldahigher

Abstract — This paper proposes an admission control algorithm that enables the upcoming IEEE 802.11e contention based Enhanced Distributed Channel Access (EDCA) to provide quantitative bandwidth guarantees for Wireless Local Area Networks (WLANs), rather

probabilityofsuccessfullycontendingforchannel

access.

Fig.1:Backofftimer,IFSandCWinEDCAaccess

Anothernewfeaturein802.11eistheTXOPwhichistheperiodachannelaccessfunctionhastherighttoaccessthemediumafterasuccessfulcontention,withthemaximumdurationde nedinTXOPLimit[AC].AnEDCA-TXOPisobtainedthroughcontentionchannelaccess.AchannelaccessfunctionisallowedtocontinuetransmissionafterwaitingaSIFSfollowingthesuccessfulcompletionofaframeexchangesequence,aslongasthetotaltransmissiontimedoesnotexceedTXOPLimit[AC].

III.MAINTAININGQOSINEDCA

Realtimemultimediatraf cgeneratedbyapplicationslikevideoconferencing,videoondemandandvoiceoverIPrequirescertainlevelofQoS.InaWLAN,itiscrucialtorestrictthevolumeoftraf cinordertomaintainservicequalityofcurrentservingtraf c.Iftherearenorestrictionstolimitthevolumeoftraf cbeingintroducedtotheserviceset,throughputdegradationandhighmediumaccessdelaywillresultduetoincreasingbackofftime.Thisisundesirableforinelastictraf csuchasvideoandvoiceapplicationsthatrequiresacertainlevelofbandwidthanddelayguarantees.InEDCAthereistheproblemofthespillovereffectwhentraf cisoverloadedinanAC,performanceinotherACswillalsobeaffected.Unlikewirednetworkswherebandwidthprovisioncanbemanagedatacentralizedpoint,usuallylocatedattheboundaryofasubnetwork,transmissionqueuesinaWLANaredistributedandsharethesamemedium.TheextentoftheeffectwilldependontheparametersadoptedfortheACsandthecurrentnetworkcondition.ThisproblemishighlightedinFig.2whichshowsasimulationof8ConstantBitRate(CBR)streamsat1.5Mbps-4streamseachinbothAC[1](lowestpriority)andAC[3](highestpriority).TwostreamsinAC[2]starttransmittingat1.5Mbpsandincreasetheirbitrateby400kbpsevery5seconds.Withoutadmissioncontrol,thechannelbecomesoverloadedafter13seconds.TheeffectonthroughputperformanceofAC[1]andAC[3]isdifferentduetodifferentparametersbeingselected(seesectionV).Bylimitingthetotaladmittedtraf cintheWLANbelowthetotalachievablethroughput,currentadmittedtraf ccanbeprotectedandchannelutilizationwillnotdegradesigni cantly.Thedif cultyofimplementingthisapproachin802.11liesinestimatingthevalueoftheachievablethroughputintheWLAN.Thisvaluedependsonseveraltimevaryingfactorsincludingthenumberofactivestations,theofferedtraf cvolumeandeachAC’sparameters(i.e.CWmin,CWmax,

TXOP).Otherfactorssuchaschannelinterference,signalstrengthandmultipathfadingwillnotbeconsideredheresincetheycannotbeknownapriori.

400000AC[3]AC[3]AC[3]AC[2] medium priority,350000

AC[3]increasing bit rate

AC[1]AC[1]AC[1]300000AC[1]AC[2]AC[2]

)

ces250000

/setyb( tu200000AC[3] highest priority

phguorh150000

T100000

AC[1] lowest priority

50000

05101520

253035404550

Time (sec)

Fig.2:Effectofchanneloverloading

Duetothenon-deterministicnatureofpacketarrivalsandtherandomnatureofbackofftimervalues,itisdif culttoprovideabsoluteguaranteesondelayandthroughputonasmalltimescale.However,itispossibletoderiveboundsonachievablethroughputoverthelongertermaschannelstatisticsarecollectedandaveraged.Theachievablethroughputcanbeestimatedwithminimalsignalingcostandcomplexity.Wearguethatbykeepingthetraf c owbelowthisthroughputlimit,QoScanbemaintained.Videoandvoicetraf c,thatgenerallyhavelong(>60seconds)connectiontimeswithsimilarpacketstatisticsforeachconnection,willbene tfromthisapproach.Admissioncontrolwillhelptomaintaingooduserperceivedqualitybyavoidingchanneloverloading.

IV.ADMISSIONCONTROLFOREDCA

A.Estimationofachievablethroughput

Thissectionintroducesa owbasedthroughputestimationalgorithmforEDCA.A ow,inthecontextofthisdiscussion,isde nedasasetofpacketsbelongingtothesameACofastationandusesthesamesetofparametersandbackofftimer.Thealgorithmutilizescollisionstatisticsofeach owtopredicttheachievablethroughput.Theestimationofachiev-ablethroughputforeachdata owisbasedonthe802.11MACanalyticalmodelproposedin[2].Weextendthisworktoestimatethethroughputof ows,eachwithdifferentchannelaccessparametersandincorporatetheconceptofTXOPintothealgorithm.Inthismodel,thetransmissionprobabilityofa owcanbederivedwiththefollowingformula:

P(txinaslot| ow=i)=

2(1 2pi)

(1 2pi)(W+1)+piW(1 (2pi))

(1)

pi=longtermcollisionprobabilityof owiW=CWminsizeusedfor owib=

maximumbackoffstage

withCWmax=(CWmin+1)×2b 1

Themodelassumesthatthepacketcollisionprobabilityisconstantandindependentofthetransmissionstate.Traf csourcesareassumedtohaveunlimiteddatatosend.Sinceweareprimarilyinterestedinpredictingachievablethroughput

Abstract — This paper proposes an admission control algorithm that enables the upcoming IEEE 802.11e contention based Enhanced Distributed Channel Access (EDCA) to provide quantitative bandwidth guarantees for Wireless Local Area Networks (WLANs), rather

Achievablethroughput[i]=

P(successfultransmission| ow=i)×Datapayloadsize

P(collision)×Durationcollision+P(slotisidle)×aSlotTime+P(successfultransmission)×Durationsuccess

(2)

Table1:Cycledurationofdifferentaccessschemes

foreachtraf c owundersaturationconditions,thestatedassumptionswillnothaveagreatimpactontheaccuracyoftheresults.Thealgorithmusesparametersdecidedatrun-timeincludingCWmin,CWmax,physicallayertransmissionrateandTXOPduration.Giventheseparametersandthemonitoredcollisionrateofeach ow,thetransmissionprobabilityfor owiatsaturationconditioniscalculatedusing(1).Inordertoestimatethesaturationthroughput,threepiecesofinformationarederived-theprobabilitiesoftransmission,collisionandidleinaslot.Theprobabilityofasuccessfultransmissionwith owiisgivenbytheproductoftheprobabilityof owitransmittingandallother owsnottransmitting.Acollisionoccurswhenmultiple owstransmitinthesameslotwiththeprobabilitygivenbythedifferencebetweenprobabilityofoneormoretransmissionsinaslotandonlyonetransmissioninaslot.Theslotisidlewhenallstationsarenottransmitting.Assumingthattherearemactive ows,theprobabilitiesarecalculatedasfollows:

P(successfultx| ow=i)

=P(txinaslot| ow=i)×(1-P(txinaslot| ow=i))P(successfultxinaslot)=

m

i=1

P(successfultx| ow=i)

P(1ormoretxinaslot)=1-

m

i=1

(1-P(txinaslot| ow=i))

P(collision m

inaslot)=P(1ormoretxinaslot)-i=1P(successfultxinaslot| ow=i)P(slotisidle)=1-P(1ormoretxinaslot)

Withtheseprobabilities,theachievablethroughputof ow

iistheproportionoftimefortransmittingdatapayloadinrespecttoidle,collisionandheadertransmissiontime,duringacycleofframeexchange.Acycleofframeexchangeconsistsofseveralcollisioncyclesandonesuccessfuldataframetransmissionplusheadertransmissionandidletimes.Theachievablethroughputfor owiiscalculatedasinequation(2).Thecycledurationtimeisthetimerequiredtotransmittheframesequenceplustheassociatedpreamblesandphysicallayerheadersinthecycle(pleaserefertotable1).B.ImplementationofAdmissionControl

Theproposedadmissioncontrolusestheaboveachievablebandwidthestimationalgorithm.Themainobjectiveistopreventchanneloverloadandprotectadmitted ows.Thisisparticularlyimportanttoinelasticmultimediatraf cthataresensitivetobandwidth uctuations.Thedatastreamwillnotbeadmittedifthecontrolalgorithmdeterminesthatthere

isinsuf cientbandwidthtoservicethenewstream.Thealgorithmalsoaidsintheselectionofparameterssuchascontentionwindowsizeandtransmissionopportunityduration,as ndingtherightparameterscanbedif cult[3].

TheadmissioncontrolisimplementedintheAccessPoint(AP)operatingininfrastructuremodewiththeresponsibilityofcollisionmonitoring,throughputestimationandmakingadmissiondecisions.Eachactive owneedsacountertokeeptrackofthecollisionrate.Thecollisionrateiscalculatedeveryupdateperiodusingexponentialweightedaverageaccordingto(3)toremoveshortterm uctuationsduetointerference.Weadoptedavalueof0.8forαand1secondforupdateperiodinoursimulation.Thesetwoparameterscontrolthevariabilityoftheestimatedthroughput.Webelievetheval-ueschosenprovideagoodbalancebetweenremovingshortterm uctuationsandre ectingthelongtermtrend.Furtheroptimizationmaybeneededtodeterminethebestparametersaccordingtotraf cbehaviour.

pi,newaverage=(1 α)pi,current+αpi,previousaverage

(3)

Fromthisinformation,theadmissioncontrollerisabletopredicttheachievablethroughputofthe ows.Stationsneedtomakearequesttotheadmissioncontrollertoobtainthedesiredbandwidthbeforetransmission.Ifadmittingthenew owcausestotaladmittedtraf ctoexceedthelimit,therequestisrejected.TheadmissionalgorithmsearchesforthebestparametersforCWminandTXOPduration(ifused),giventherequiredbandwidthofthenewstream.Whenanew owrequestismade,thecollisionrateisinitializedtotheaveragecollisionrateoftheexisting owhavingasimilarachievablethroughputasthedesiredbandwidthofthenew ow.Thebasicoperationinvolvesiterativelyreducingthecontentionwindowsizeandestimatingtheachievablethroughputuntilitmatchesthebandwidthdescriptionofthenew ow.Alternatively,ifTXOPisused,theTXOPdurationcanbeincreasedtoyieldahigherachievablethroughputforthe ow.

foreachupdate{if(nonewflowrequests){

updateparametersandcollisionprobinrecord;runachievablebandwidthestimationalgorithm;}

elseif(newflowrequestexists){

collision=collisionofflowwithsimilarthroughput;fori=1:numactive+1

Abstract — This paper proposes an admission control algorithm that enables the upcoming IEEE 802.11e contention based Enhanced Distributed Channel Access (EDCA) to provide quantitative bandwidth guarantees for Wireless Local Area Networks (WLANs), rather

calculateachievablethroughput[i];

fori=1:numflows+1{

if(requestbw[i]>achievable{reduceCWorincreaseTXOP;

re-estimateachievablethroughput;}

if(CWorTXOPlimitisreached){rejectnewflow;

restoretooriginalparameters;exitprogram;}}

admitnewflow;

distributenewparameterstostations;}}

Fig.3:Pseudocodeofadmissioncontrol

V.SIMULATIONRESULTS

Simulationswereconductedusingthenetworksimulator(ns-2)[4].Inthesesimulations,nohiddenstationswerepresentandthechannelwasassumedtobeerrorfree.Allstationsoperatedat36Mbpscomplyingwiththe802.11aphysicallayerstandardandhadoneactive owatatime.A.AchievableThroughputofaFlow

We rstverifytheaccuracyofthroughputestimationatsaturationlevel.ThreeACsareusedwithCWminsizesof15,31and63.EachACconsistsof4CBR owsfrom4stationswithunlimiteddatasourceandthereareatotalof12active ows.CWmaxsizesforallthreeACsaresetto1023.AIFSissetequaltoDIFS.FivesimulationrunsareconductedwithvariedMACservicedataunit(MSDU)sizesfrom256to2048B.TheachievablethroughputforeachACisestimatedandcomparedwithsimulationresults.ThroughputofonlyonestreamforeachACisshowninFig.4toeasecomparison.

450000Simulation AC[2]400000Simulation AC[1]Estimate AC[3]Estimate AC[2]Estimate AC[1]

350000)

ces/s300000etyb( tup250000hguorh200000t noitaru150000tas10000050000

0200

400600800

100012001400160018002000

MSDU size (bytes)

Fig.4:Estimatedachievablethroughputcloselymatchessimulation

resultsfordifferentACs.

Asseenfromtheseresults,thealgorithmprovidesagoodestimationoftheachievablethroughputofanAC.ItisclearthathighpriorityACs,withsmallercontentionwindowsandlargerMSDUsizes(orTXOPdurations),willhaveahigherthroughputwhenthechannelisfullyutilized.

B.EffectivenessofAdmissionControlAlgorithm

Thesimulationstartswith15stationsand15CBR ows,5at1.8Mbpswith1000BpacketsandCWmin=13,5at500kbpswith500BpacketsandCWmin=31,and5at80kbpswith100BpacketsandCWmin=40.TheCWminvaluesaredeterminedbytheadmissioncontrolalgorithm.RTS/CTSmechanismisused.These15 owsareadmittedtotheservicesetandtheyreachtheachievablethroughputlimitindicatedbytheadmissioncontrolalgorithm.ThereasonCBRtraf cischoseninthesesimulationsisbecausewecanaccuratelyevaluatetheachievablethroughputlimitsestimatedbythealgorithm,bysettingeach owtransmittingclosetothepredictedthroughputlimit.Incaseofvariablebitratetraf c,similarperformancecanbeachievedbylimitingtheofferedtraf cofeach owbelowitsestimatedlimit.Dependingonthenetworksituation,furthertraf cengineeringtoolscanbeapplied(e.g.leakybucket)toreducetheburstinessandallowtraf ctotransmitabovethelimitsmomentarily,therebyincreasingef ciency.Wenowfocusontheregionwhenthechannelisfullyutilized.Wewouldliketostudytheeffectofadmittinganew owwhentheadmissioncontrolindicatesweshouldnotdoso.20secondsintothesimulation,anadditional ow( ow16)at1.8Mbpsandpacketsizeof1000B(CWmin=15)startstransmission.Theresultisthatall owsexperiencedqualitydegradationduetochanneloverload.Weidenti edtworegionsforinvestigation,the owphasefrom5to20secondswhenall owsaretransmittingbelowtheirestimatedachievablethroughputlimits,andthesaturationphasefrom20secondsonwardswhenthenew owcausesthelimittobeexceeded.

350000flow 1 1800Kbpsflow 2 1800Kbpsflow 6 1800Kbps300000

flow 7 500Kbpsflow 8 500Kbpsflow 12 80Kbpsflow 13 80Kbps

250000)

ces/set200000

yb( tuphgu150000

orhFlow Phase

Saturation Phase

t100000

50000

5101520

25303540

time (sec)

Fig.5:Throughputperformancewithadmissioncontrolenabled100(t=0to20s)anddisabled(t=20to40s)

High Priorityflow 2 90AC[3]

flow 3 flow 4 flow 5 80Medium Priorityflow 6 flow 7 AC[2]

flow 8 )

70%flow 10 flow 9 ( flow 11 no60Low Priorityflow 12 ituAC[1]flow 13 biflow 14 rts50flow 15

id evitalu40muc3020100

00.020.04

0.060.080.10.12

latency (sec)

Fig.6:Mediumaccessdelaysduring owphase

Abstract — This paper proposes an admission control algorithm that enables the upcoming IEEE 802.11e contention based Enhanced Distributed Channel Access (EDCA) to provide quantitative bandwidth guarantees for Wireless Local Area Networks (WLANs), rather

100flow 2 90

flow 3 High priority

flow 4 flow 5 80flow 6 flow 7 flow 8 )

70%flow 10 flow 9 ( flow 11 no60Medium and low priorities

flow 12 ituflow 13 bflow 14 irtsflow 15 id50flow 16

evitalu40muc3020100

00.20.4

0.60.811.21.4

latency (sec)

Fig.7:Mediumaccessdelaysduringsaturationphase

ThroughputperformanceisshowninFig.5duringthe35secondperiod.Throughputisstableforthe rst15secondsindicatingthemediumiscapableofhandlingthetraf c.However,wecanseethatthebandwidth http://paringthisgraphwithFig.2,duringthesaturationphase(from13secondsonwardsinFig.2),the owsthatremainbelowtheirachievablethroughputareunaffected.ThesituationisdifferentinFig.5sinceall owsaretransmittingjustbelowtheirachievablethroughput.Theeffectofoverloadingextendstoall ows,asthethroughputofall owsstartsto uctuate.Theimplicationofthiseffectcanbeappliedtoprotectimportant owsbyassigninghigherachievablethroughputtothese owsthantheofferedtraf c,thusprovidingabuffertomaintainQoStosomeextents.Thedrawbackislowerchannelutilizationandhighercollisionratesbecauseofsmallercontentionwindowsizeandhighertransmissionprobability.Fig.6showsthemediumaccessdelaydistributionduringthe owphase.Flows11to15havetheworstdelayperformancebutthemaximumdelayremainsbelow100ms.Fig.7showsthedelaydistri-butionduringthesaturationphase.Thedelayperformancedeterioratesrapidlyasadditionaltraf cisintroducedtothenetwork.Delayisgreaterthan0.6sforatleast10%ofthetraf cinthemediumandlowpriorityACs.

VI.RELATEDWORK

TheVirtualMACalgorithmin[5]passivelymonitorstheradiochannelandestimateslocallyachievableservicelevels.Themaincriteriaforthedecisionofadmissioncontrolisbasedondelayandcollisionestimates.Itdoesnotprovideanyachievablethroughputinformation,whichisalsousefultomultimediaapplications.AnotherdrawbackisthatthevirtualMACalgorithmcomputationscomplicatethemobilestation.TheAssuredRateMACExtensionproposedin[6]extendstheassuredrateserviceinDifferentiatedServicesto802.11WLANs.Thisschemeachievesthedesiredthroughputbydynamicallyadjustingthecontentionwindowsizebasedonobservationoftheestimatedsendinganddesiredrates.Inthecasewhenthesendingrateisbelowthedesiredrate,thecon-tentionwindowsizeisreducedtoobtainahigherthroughput.Alongwithothersimilarproposals[7],[8]for802.11MACtosupportQoS,theseschemesonlyproviderelativepriorityandbandwidthofcurrent owsarenotprotected.

VII.CONCLUSIONANDFUTUREWORK

Thispaperhighlightsthedif cultiesofmaintainingQoSincontentionbased802.11WLANs.ByconsideringtheinteractionofframetransmissionsintheMAClayer,weareabletopredicttheachievablethroughputforeach ow.Thisinformationprovidesausefulguidingpointtoef cientlymanagebandwidthprovisioninEDCAbyeliminatingmuchoftheguesswork.SimulationresultsshowedourproposedadmissioncontrolcaneffectivelymaintaintheQoSofadmitted owsbypreventinganynew owsoverloadingthechannel.Throughputanddelayperformanceremainedatacceptablelevelsforreal-timetraf caslongasofferedtraf cisbelowtheestimatedlimits.

TheCWminparameteraffectsboththedelayandbandwidththata owexperiences.Inorderfora owtoreceivelowdelay,itneedstohavealowCWmin,whichgivesitahighthrough-put.However,applicationslikevoicerequirelowerdelayandlowerthroughputthanapplicationslikevideo.OnesolutionistoassignlargerTXOPdurationtoavoice owtoallowittotransmitmultiplepacketsineachsuccessfulcontention.However,thedelayjitterwillincreaseasthe rstpacketinapacketburstwillexperienceahigherdelaycomparedtothesubsequentpackets.Theextentofdelayjitterrequiresfurtherstudy.However,simulationresultsdemonstratethatevenifTXOPsarenotused,delaycanremainatsatisfactorylevel.Thecurrentdraftof802.11e[1]speci esadirectlinkprotocoltoallowdirectcommunicationsbetweennon-APstationsininfrastructuremode,uponregistrationwiththeAP.Ourproposedadmissioncontrolalgorithmcanbemodi edforusedunderthisscheme,withtheworkofcollisionmonitoringbeingshiftedtothenon-APstationsandpassingthecollisioninformationtotheAPusingasignalingprotocol.Pleasereferto[9]fordetails.Futurestudieswillalsoincludevariouschan-nelerrormodelstostudytheeffectivenessoftheadmissioncontrolalgorithm.

ACKNOWLEDGEMENTS

ThisworkissupportedbytheAustralianResearchCouncilandindustrypartnerSingTelOptusPtyLtd.

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