IEEE 80211 Wireless LANs.docx
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IEEE80211WirelessLANs
IEEEP802.11
WirelessLANs
IEEE802.11QualityofService–WhitePaper
Date:
February15,2000
Author(s):
ArunAyyagari,YoramBernet,TimMoore
MicrosoftCorporation
OneMicrosoftWay,Redmond,WA98052-6399
Phone:
(425)936-8080
Fax:
(425)936-7329
e-Mail:
aruna@,yoramb@,timmoore@
Abstract
IEEE802.11provideshighbandwidthconnectivityinalocalareanetwork(LAN)environmentthatissuitableformostdataapplications.However,itdoesnotmeetthequalityofservice(QoS)requirementsforreal-timedatatrafficapplicationssuchasvoiceandvideotransmissions.TherehavebeenrecentproposalsonenhancementstotheIEEE802.11Mediumaccesscontrol(MAC)protocoltoprovideexpeditedaccesstothesharedwirelessnetworkforreal-timedatatransmissions.However,theseproposalswouldrequirechangestotheIEEE802.11standardthatmaypotentiallyimpactbackwardcompatibilityandfutureadoptionoftheIEEE802.11wirelessLAN.Therefore,inthiswhitepaperweproposethecombineduseofresourcebasedadmissionandtrafficflowcontrolandexpeditedtransmissionofhigherprioritydataoverthesharedwirelessnetworkasthemechanismstoprovideQoSguaranteeforreal-timeapplicationdatatrafficinIEEE802.11.OurunderlyingobjectivewastodefineaQoSapproachwithoutanychangestothefundamentalIEEE802.11MACprotocol.
Overview
IEEE802.11isasharedwirelesslocalareanetwork(LAN)standardusingthecarriersensemultipleaccess(CSMA)mediumaccesscontrol(MAC)protocolwithcollisionavoidance(CA)[1].Thestandardallowsforbothdirectsequence(DS)andfrequency-hopping(FH)spreadspectrumtransmissionsatthephysicallayer.Themaximumdatarateofferedinitiallybythestandardwas2Mb/s.However,ahigherspeedversionwithaphysicallayerdefinitionundertheIEEE802.11bspecificationallowsadatarateofupto11Mb/susingDSspreadspectrum.TheIEEEstandardscommitteehasalsodefinedphysicallayerspecificationsundertheIEEE802.11aspecificationbasedonorthogonalfrequency-divisionmultiplexing(OFDM)thatwillpermitdatatransferratesupto54Mb/s.WhileIEEE802.11provideshighbandwidthconnectivityinaLANenvironmentthatissuitableformostdataapplications,itdoesnotmeetQualityofService(QoS)requirementsforreal-timedatatrafficapplicationssuchasvoiceandvideotransmissions.
IEEE802.11wirelessstation(STA)isintendedtooperateineitherofthefollowingtwomodes:
∙Adhocnetworkmode:
STAswithinabasicserviceset(BSS)areabletocommunicatedirectlywithpeerSTAswithintheBSS.
∙Infrastructurenetworkmode:
STAscommunicateonlywithanaccesspoint(AP)intheBSS.AnAPprovidesconnectivitytodistributionsystem(DS)servicesinadditiontoperformingasanSTA.STAswithinaBSScommunicatewithotherstationswithinandoutsidetheBSSviatheAP.Therefore,allcommunicationfrom/toSTAsaredirectedto/fromtheAPintheBSS.
Weenvisionthatinmostwirelessenvironmentssuchascorporationsandpublicplaces,infrastructurenetworkmodewillbepredominantlyused.Usageoftheinfrastructurenetworkmodeisdrivenbytheneedtoprovideaccesscontrol,accounting,andcontentprovisioningmechanismsinwirelessenvironmentthatfollowtheclient/serverinteractionparadigmbetweenthewirelessstationandaremoteserver.Adhocnetworkmodewouldstillbeusedinareassuchashomenetworkingenvironments.Inthiswhitepaper,wewillfocusourdiscussionontheinfrastructurenetworkmode,howeveramanyoftheapproachesandconclusionspertaininginfrastructurenetworkmodewouldbeequallyapplicabletotheadhocnetworkmode.
Theneedforresourcebasedadmissionandtrafficflowcontrolisofgreaterimportanceinwirelessnetworksbecausebandwidthinwirelessnetworksismoreconstrainedcomparedtowirednetworks.SinceanAPandalltheSTAscommunicatingwiththeparticularAParegivenequalaccessprioritytothesharedwirelessnetworkinIEEE802.11infrastructurenetworkmode,theAPbecomesabottleneckfortypicalclient/serverinteractions.WhiletheSTA’swouldbeabletotransmitdatatotheAP,datadestinedtotheSTAswouldbequeuedattheAPsinceithastoservicealltheSTAswithinitsBSSwithaccessprioritysimilartotheSTAs.Thereforeinadditiontothelatencyandbandwidthrequirementsforapplicationleveldatatraffic,characteristicsofwirelesscommunicationandnetworkarchitectureevincestheneedforQoSmechanismsinIEEE802.11.ThiswhitepaperpresentanalternativeapproachtoprovideQoSguaranteestoapplicationleveldatatrafficinIEEE802.11wirelessLAN.
Approach
AQoSguaranteeforreal-timeapplicationdatatrafficiscurrentlynotavailableinIEEE802.11.TherehavebeenrecentproposalsfromSharewave(Whitecapprotocol)[2]andLucent(Black-Burstprotocol)[3]onenhancementstotheIEEE802.11MACprotocoltoprovideexpeditedaccesstothesharedwirelessnetworkforreal-timedatatransmissions.However,boththeseproposalswouldrequirechangestotheIEEE802.11standardthatmaypotentiallyimpactbackwardcompatibilityandfutureadoptionoftheIEEE802.11wirelessLAN.Therefore,webelievethatitwouldbeprudenttoavoidorattheleastlimitthechangestotheIEEEMAC802.11protocolbyadoptingQoSapproachesthatdonotwarrantmajorchanges.
WeproposethecombineduseofresourcebasedadmissionandtrafficflowcontrolandexpeditedtransmissionofhigherprioritydataoverthesharedwirelessnetworkasthemechanismstoprovideQoSguaranteeforreal-timeapplicationdatatrafficinIEEE802.11.TheresourcebasedadmissionandtrafficflowcontrolmechanismmanagesthenumberandtrafficflowcharacteristicsofpeerapplicationsessionstoensurethattheirQoSrequirementsaresatisfied.TheexpeditedtransmissionofhigherpriorityframesbytheSTAsoverthesharedwirelessnetworkensuresthatthelatencyrequirementsofreal-timeapplicationsaresatisfied.Thisalsoresultsinsmallerlatencyvariabilityforreal-timeapplicationsthatleadstoabetterutilizationofthesharedwirelessnetworkbyaccommodatingagreaternumberofreal-timepeerapplicationsessions.
ResourceBasedAdmissionandTrafficFlowControl
InordertomaintainQoSguarantees,theresourcebasedadmissionandtrafficflowcontrolwouldassistinthemanagementofapplicationdatatraffictransmissionsoverthesharedwirelessnetwork.Intheproposedapproachresourcereservationprotocol(RSVP)signaling[4-5]andthesubnetbandwidthmanager(SBM)[6]wouldbeusedtoestablishend-to-end,per-conversation,QoStopeerapplicationsbetweentheIEEE802.11STAandaremotestation.WhileRSVPisalayer-3protocol,SBMisalayer-2protocolthatextendsthefunctionalityofRSVPtothesharedwirelessnetwork(betweentheSTAsandAP)andbeyond.Inadditiontotheadmissionandtrafficflowcontrol,theRSVPsignalingdescribingthetrafficcharacteristicsissenttothenetworkdevicesforaggregatetraffichandlingmechanismssuitableforIEEE802.11sharedwirelessnetwork.
Figure1illustratesatypicalIEEE802.11architecturethatconsistsofanumberofBSSsthatareinterconnectedviatheDSformingtheextendedserviceset(ESS).AlltheSTAsintheESSaremembersofasinglesubnet.Inatypicaldeployment,theAPswithintheESSareinterconnectedvialayer-2switchednetwork,forexample,usingapoint-to-pointfull-duplexfastEthernetconnectionbetweenagivenAPandalayer-2switchingdeviceatthecoreofthenetworkintheDS.ResourcebasedadmissionandtrafficflowcontrolwithintheESSmaybeaggregatedacrosstheentireDSforallBSSsorspecifictoeachBSSresultinginSBMprotocolbeingenabledatvariousdensitieswithintheIEEE802network[7].Also,eachSTAwithintheBSSisexpectedtohavefullSBMclientfunctionalityinordertomaintaincontroloftheIEEE802.11sharedwirelessnetworkresources.
AggregationofresourcebasedadmissionandtrafficflowcontrolacrosstheentireDSresultsinthelowestdensitywhereasingledesignatedSBM(DSBM)inthecoreofthelayer-2networkactsastheadmissioncontrolagentfortheentirelayer-2subnet.InthisscenariotheSBMclientateachSTAwithintheBSSdetectsthepresenceofaDSBMintheDS(corenetwork)androutesRSVPsignalingrequestsviatheDSBM.Inaddition,theDSBMmayalsobeconfiguredtoadvertiseaNonResvSendLimitonthemanagedsubnettolimitthemaximumrateatwhichSTAsmaytransmitintheabsenceofapprovedreservation.
Attheotherextreme,eachAPisanadmissioncontrolagent(DSBM)fortheSTAswithinaBSS.Inthiscase,theDSBMelectionprotocolwillresultinthedivisionoftheESSsuchthateachBSSbecomesamanagedsegmentmanagedbyanAP(DSBM).ThehigherdensityofDSBMsresultsinbettermanagementofthesharedwirelessnetworkresourceswithineachBSS.However,thiswouldrequirethateachAPshouldincludetheSBMserverfunctionalityontheportthatinterfaceswiththeSTAswithinitsBSS.SimilartothesingleDSBMscenario,allSTAswithintheBSSdetectsthepresenceoftheDSBMintheAPandrouteRSVPsignalingrequestsviatheDSBM.SinceallthetrafficfromSTAswouldnormallyberoutedviatheAPthisdoesnotdeviatefromthenormaldatatrafficpath.Also,theDSBMco-locatedwiththeAPmayalsobeconfiguredtoadvertiseaNonResvSendLimitonthemanagedsubnettolimitthemaximumrateatwhichSTAsmaytransmitintheabsenceofapprovedreservation.
RelativetowiredLANs,bandwidthismoreseverelyconstrainedinIEEE802l.11sharedwirelessLAN.Therefore,inordertoguaranteeQoSbetweentheSTAandaremotestation,resourcebasedadmissionandtrafficflowcontro