83中无线局域网IEEE+802111仿真指南英文.docx

83中无线局域网IEEE+802111仿真指南英文.docx

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83中无线局域网IEEE+802111仿真指南英文

WirelessLAN（IEEE802.11）ModelGuide

Purpose

ThisdocumentdescribesWirelessLAN（WLAN）simulationmodelshippedaspartofstandardOPNETmodelrelease10.0AforITDecisionGuruandOPNETModelerwithradiomodule.

WirelessLANBackground

WirelessLANprotocolisbasedontheIEEE802.11standard.Thestandarddefinesamediumaccesscontrol（MAC）sublayerandthreephysical（PHY）layers.ThegoaloftheIEEE802.11protocolistodescribeawirelessLANthatdeliversservicescommonlyfoundinwirednetworks,e.g.,throughput,reliabledatadelivery,andcontinuousnetworkconnections.ThearchitectureoftheIEEE802.11WLANisdesignedtosupportanetworkwheremostdecisionmakingisdistributedacrossthemobilestations.Someofthecomponentsthatarethebasicbuildingblocksofthe802.11basednetworkaredescribedbelow:

1.Station:

InIEEE802.11networkastationisthecomponentthatconnectstothewirelessmedium.Thestationmaybemobile,portable,orstationary.Everystationsupportsallstationserviceswhichincludeauthentication,deauthentication,privacy,anddeliveryofthedata（MACservicedataunit）.

2.BasicServiceSet（BSS）:

TheIEEE802.11WLANarchitectureisbuiltaroundaBSS.ABSSisasetofstationsthatcommunicatewithoneanother.WhenallthestationsintheBSScancommunicatedirectlywitheachotherandthereisnoconnectiontoawirednetwork,theBSSiscalledanindependentBSS（IBSS）.IBSSisalsoknowasadhocnetworkwhichistypicallyashort-livednetworkwithsmallnumberofstationsindirectcommunicationrange.WhenaBSSincludesanaccesspoint（AP）,theBSSisnolongerindependentandiscalledaninfrastructureBSSorsimplyBSS.InaninfrastructureBSS,allmobilestationscommunicatewiththeAP.TheAPprovidesboththeconnectiontothewiredLAN,ifany,andthelocalrelayfunctionwithintheBSS.

3.ExtendedServiceSet（ESS）:

AnESSisasetofinfrastructureBSSs,wheretheAPscommunicateamongthemselvestoforwardtrafficfromoneBSStoanother.TheAPsperformthiscommunicationviadistributionsystem（DS）.TheDSisthebackboneoftheWLANandmaybeconstructedofeitherwiredorwirelessnetworks.

MediumAccessControl

TheIEEE802.11mediumaccesscontrol（MAC）suppliesthefunctionalityrequiredtoprovideareliabledeliverymechanismforuserdataoverwirelessmedia.ThefirstfunctionoftheWLANMACistoprovideareliabledatadeliveryservicetotheusers.ThisisachievedthroughaframeexchangeprotocolattheMAClevel.ThesecondfunctionoftheWLANMACistoprovidefairmechanismtocontrolaccessofsharedwirelessmedia.Itperformsthisfunctionthroughtwodifferentaccessmechanisms:

thecontention-basedmechanism,calledthedistributedcoordinationfunction（DCF）,andacentrallycontrolledaccessmechanism,calledthepointcoordinationfunction（PCF）.ThethirdfunctionoftheWLANMACistoprovideprotectiontothedataitdelivers.Thisisdonethroughaprivacyservice,calledWiredEquivalentPrivacy（WEP）whichencryptsthedatasentoverthewirelessmedium.

MACFrameExchange

TheminimalMACframeexchangeconsistsoftwoframe,adataframesentfromthesourcetothedestinationandanacknowledgement（Ack）framefromthedestinationtothesource.Ifthesourcedoesn’treceiveanacknowledgmentthenitattemptstoretransmitadataframeafterobservingappropriatedeference.Thereareretrylimitsassociatedwiththeretransmissionoftheframes.Theprotocolalsosuggestsanoptionaluseofrequesttosend（Rts）andcleartosend（Cts）frameexchangebetweensourceanddestinationstationsformediareservation.RtsistransmittedfromsourcetodestinationstationandCtsisaresponseinitiatedbythedestinationstationtothesourcestation.ThisinitialhandshakeisfollowedbytheminimalMACframeexchange.

BasicAccessMechanism

Thebasicaccessmechanismiscarriersensemultipleaccesswithcollisionavoidance（CSMA/CA）withbinaryexponentialbackoff.Inthistypeofaccessmechanism,astationwilllistentothemediumbeforebeginningatransmission.Ifthemediumisalreadycarryingatransmission,thestationthatislisteningwillnotbeginitsowntransmission.ThisistheCSMAportionoftheaccessmechanism.Iftwoormorestationsbegantheirtransmissionatthesametimethentherewillbeacollisionwhichmaycauseoneormoreframestobecorrupted.Inordertoavoidcollision,astationlistenstothemediumbeforebeginningitsowntransmission.Ifitdetectsanexistingtransmissionthenitwillenterintoadeferralperioddeterminedbybinaryexponentialbackoffalgorithm.Thebinaryexponentialbackoffmechanismchoosesarandomnumberwhichrepresentstheamountoftimethatmustelapsewhilethemediumisidlei.e.thelisteningstationmayattempttobeginitstransmissionagain.TheIEEE802.11MACusescollisionavoidanceratherthancollisiondetectioninordertobeabletosimultaneouslytransmitandreceive.Forthisreason,theIEEE802.11MACimplementsanetworkallocationvector（NAV）.TheNAVisavaluethatindicatestoastationtheamountoftimethatremainsbeforethemediumwillbecomeavailable.TheNAViskeptcurrentthroughdurationvaluesthataretransmittedinallframes.Bycombiningthevirtualcarriersensingmechanism（usingNAV）withthephysicalcarriersensingmechanism,theMACimplementsthecollisionavoidanceportionoftheCSMA/CAaccessmechanism.Distributedcoordinationfunction（DCF）isabasicaccessmechanismdescribedintheprotocol.Itusesphysicalandvirtualcarriersensemechanisms.IfbothmechanismsindicatethatmediuminnotinuseforanintervalofDIFS（distributedinterframespace）thenthestationwillbegintotransmittheframe.However,ifthemediumisnotbusythenthebackoffalgorithmisapplied.Thetransmissionisconsideredtobeunsuccessfulifthereisnoacknowledgementreceivedfromthedestination.Thismayresultintheretransmissionoftheframe.

CentrallyControlledAccessMechanism

Thecentrallycontrolledmechanismusesapollandresponseprotocoltoeliminatethepossibilityofconnectionforthemedium.Thisaccessmechanismiscalledthepointcoordinationfunction（PCF）.Apointcoordinator（PC）controlsthePCF.ThePCisalwayslocatedinanAP.Generally,thePCFoperatesbystationsrequestingthatthePCregisterthemonapollinglist,andthePCthenregularlypollsthestationsfortrafficwhilealsodeliveringtraffictothestations.ThePCFisanoptionalpartoftheIEEE802.11standard.ItisbuiltovertheDCFandbothoperatesimultaneously.

ModelScopeandLimitations

ThissectiondiscussestheimplementationchoicesmadeindevelopingthesimulationmodelfortheWLANMACprotocol.Certainpartsoftheprotocolhavebeensimplified,omittedordeferredforlaterdevelopmentinviewofthefactthatitisintendedprimarilyforDCFbasedMACperformanceestimation.

WirelessLANModelApplications

SomeofthestudiesthatyoucanperformusingtheWirelessLANprotocolmodelarelistedbelow:

1.StudythepossibilityofhavingWLANasanalternativetolocalareanetworkaccesstechnology（e.g.comparedtoethernet）.

2.Analyzenetworkperformancebyvaryingthecapacity（ornumberofnodes）inthenetwork.

3.PerformanceevaluationofprotocolspecificfeatureslikefragmentationandreassemblyorRTS/CTSframeexchange.

4.SettingupindependentandinfrastructureBSSnetworkandevaluatetheirperformances.

5.ResearchnewmediaaccesscontrolmechanismtoreducebandwidthoverheadresultingfromRTS/CTSexchange.ThissectiondiscussesthekeyelementsoftheWLANsimulationmodelsuiteincludingmodelattributesandstatistics.Theideaistoshowthathowthemodelsimulatestheprotocolbehavior,sosomeoftheimplementationspecificdetailsarenotdiscussed.Asmentionedbeforetherearetwotypesofwirelessnetworkconfiguration:

InfrastructureBSSandIndependentBSS.ThecurrentWLANmodelsupportsboththeconfigurationsandtheirsetupisasfollows:

1.InfrastructureBSS

WorkstationscancommunicateoutsideoftheBSSusingAccessPoint（AP）whichisconnectedtothedistributionsystem.Also,thisAPactsasarelaywithinaBSS.

2.IndependentBSS

Thisisanadhocnetworkofseveralstations.Theworkstationscanhaveapeer-to-peerconnectionwithotherstationsintheBSSbutthecommunicationwillbelimitedtotheBSS.

“wireless_lan”ObjectPalette

Followingarethemodelswhicharecontainedinthepalette:

1.WirelessWorkstation（fixedandmobile）

2.WirelessServer（fixedandserver）

3.Wirelessterminalstation（WLANMACwithoutIP）

4.WirelessRouter（withonewirelessinterfaceandoneormorewirelineinterfacelikeethernet,FDDI,tokenring,ATMetc.）

BothmodelerandITDGhaveseveralwirelessnodesdefinedintheobjectpalette.Moremodelscanbecreatedandaddedtothispaletteasneeded.

NodeArchitecture

NodemodelsofsomecommonlyusedWirelessLANobjectsareasfollows:

1.WirelessWorkstation

TheWirelessLANmodelingoftheMACandthephysicallayeriscomprisedofthewireless_lan_macprocess,transmitter,receiver,andthechannelstreams（asshownbythedottedboxinthefigureabove）.TheARP（addressresolutionprotocol）isaninterfacebetweentheMACandthehigherlayers.

2.WirelessRouter（orAccessPoint）

TheillustratedaccesspointcontainsonewirelessinterfaceandoneEthernetinterface.ItcanbeusedtoconnectaBSSwithawirelinedistributionsystem.Notethatthewirelessinterfacereceivesthedataframeandsendsittothehigherlayerforaddressresolutionifthedestinationstationdoesnotbelong

tothesameBSS.

3.BSS:

WirelessLAN-basednetworkisconfiguredusingOPNETsubnetobject.EachOPNETsubnetcanbeconsidered