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, …… 此处隐藏：21237字，全部文档内容请下载后查看。喜欢就下载吧 ……