IEEE 80211 Tutorial

IEEE802.11Tutorial1IEEE802.11TutorialbyJimZyrenandAlPetrickApprovaloftheIEEE802.11standardforwirelesslocalareanetworking(WLAN)andrapidprogressmadetowardhigherdatarateshaveputthepromiseoftrulymobilecomputingwithinreach.WhilewiredLANshavebeenamainstreamtechnologyforatleastfifteenyears,WLANsareunchartedterritoryformostnetworkingprofessionals.Someobviousquestionscometomindwhenconsideringwirelessnetworking:•HowcanWLANsbeintegratedwithwirednetworkinfrastructure?•Whatistheunderlyingradiotechnology?•Howismultipleaccesshandled?•Whataboutnetworksecurity?IEEE802.11islimitedinscopetothePhysical(PHY)layerandMediumAccessControl(MAC)sublayer,withMACoriginstoIEEE802.3Ethernetstandard.ThefollowingoverviewexplainsmajordifferencesbetweenwiredandwirelessLANsandshouldanswersomeofthequestionsfacingMISprofessionalsevaluatingWLANtechnology.NetworkTopologyWLANscanbeusedeithertoreplacewiredLANs,orasanextensionofthewiredLANinfrastructure.Thebasictopologyofan802.11networkisshowninFigure1.ABasicServiceSet(BSS)consistsoftwoormorewirelessnodes,orstations(STAs),whichhaverecognizedeachotherandhaveestablishedcommunications.Inthemostbasicform,stationscommunicatedirectlywitheachotheronapeer-to-peerlevelsharingagivencellcoveragearea.Thistypeofnetworkisoftenformedonatemporarybasis,andiscommonlyreferredtoasanadhocnetwork,orIndependentBasicServiceSet(IBSS).StationAH3StationStationAH1AH2AdHocNetworkFigure1Peer-to-PeerCommunicationsinAdHocNetworkInmostinstances,theBSScontainsanAccessPoint(AP).ThemainfunctionofanAPistoformabridgebetweenwirelessandwiredLANs.TheAPisanalogoustoabasestationusedincellularphonenetworks.WhenanAPispresent,stationsdonotcommunicateonapeer-to-peerIEEE802.11Tutorial2basis.AllcommunicationsbetweenstationsorbetweenastationandawirednetworkclientgothroughtheAP.AP’sarenotmobile,andformpartofthewirednetworkinfrastructure.ABSSinthisconfigurationissaidtobeoperatingintheinfrastructuremode.StationStationStationStationA1A2B1B2BSS-ABSS-BAAPAPBServerDISTRIBUTIONSYSTEMFigure2ESSProvidesCampus-WideCoverageTheExtendedServiceSet(ESS)showninFigure2consistsofaseriesofoverlappingBSSs(eachcontaininganAP)connectedtogetherbymeansofaDistributionSystem(DS).AlthoughtheDScouldbeanytypeofnetwork,itisalmostinvariablyanEthernetLAN.MobilenodescanroambetweenAPsandseamlesscampus-widecoverageispossible.RadioTechnologyIEEE802.11providesfortwovariationsofthePHY.Theseincludetwo(2)RFtechnologiesnamelyDirectSequenceSpreadSpectrum(DSSS),andFrequencyHoppedSpreadSpectrum(FHSS).TheDSSSandFHSSPHYoptionsweredesignedspecificallytoconformtoFCCregulations(FCC15.247)foroperationinthe2.4GHzISMband,whichhasworldwideallocationforunlicensedoperation.RegionAllocatedSpectrumUS2.4000–2.4835GHzEurope2.4000–2.4835GHzJapan2.471-2.497GHzFrance2.4465-2.4835GHzSpain2.445-2.475GHzTable1GlobalSpectrumAllocationat2.4GHzBothFHSSandDSSSPHYscurrentlysupport1and2Mbps.However,all11MbpsradiosareDSSS.OperatingprinciplesofDSSSradiosaredescribedinthefollowingparagraphs.IEEE802.11Tutorial311chips1bitperiod11chips1bitDataPRNOut11BitBarkerCode(PRN):101110100001000101111011101000Figure3DigitalModulationofDatawithPRNSequenceDSSSsystemsusetechnologysimilartoGPSsatellitesandsometypesofcellphones.EachinformationbitiscombinedviaanXORfunctionwithalongerPseudo-randomNumerical(PN)sequenceasshowninFigure3.TheresultisahighspeeddigitalstreamwhichisthenmodulatedontoacarrierfrequencyusingDifferentialPhaseShiftKeying(DPSK).BARKERCODE:+1-1+1+1-1+1+1+1-1-1-1SSRxDATAFROMADCs2xCHIPCLOCKPARALLELPNREGISTERLOADA/DSAMPLECLOCKZZ-1-111RR11ZZ-1-122ZZ-1-133RR22ZZ-1-144ZZ-1-155RR33ZZ-1-166RRNNZZ-1-12N2NN=16CHIPPERIODSYMBOLPERIODCORRELATIONSCOREFigure4MatchedFilterCorrelatorUsedforReceptionofDSSSSignalWhenreceivingtheDSSSsignal,amatchedfiltercorrelatorisusedasshowninFigure4.ThecorrelatorremovesthePNsequenceandrecoverstheoriginaldatastream.Tatthehigherdataratesof5.5and11Mbps,DSSSreceiversemploydifferentPNcodesandabankofcorrelatorstorecoverthetransmitteddatastream.ThehighratemodulationmethodiscalledComplimentaryIEEE802.11Tutorial4CodeKeying(CCK).TheeffectsofusingPNcodestogeneratethespreadspectrumsignalareshowninFigure5.ffXORFigure5aEffectofPNSequenceonTransmitSpectrumfCorrelatorfFigure5bReceivedSignalisCorrelatedwithPNtoRecoverDataandRejectInterferenceAsshowninFigure5a,thePNsequencespreadsthetransmittedbandwidthoftheresultingsignal(thustheterm,“spreadspectrum”)andreducespeakpower.Notehowever,thattotalpowerisunchanged.Uponreception,thesignaliscorrelatedwiththesamePNsequencetorejectnarrowbandinterferenceandrecovertheoriginalbinarydata(Fig.5b).Regardlessofwhetherthedatarateis1,2,5.5,or11Mbps,thechannelbandwidthisabout20MHzforDSSSsystems.Therefore,theISMbandwillaccommodateuptothreenon-overlappingchannels2.4000GHz2.4835GHzCh.1Ch.6Ch.11Figure6ThreeNon-OverlappingDSSSChannelsintheISMBandMultipleAccessThebasicaccessmethodfor802.11istheDistributedCoordinationFunction(DCF)whichusesCarrierSenseMultipleAccess/CollisionAvoidance(CSMA/CA).Thisrequireseachstationtolistenforotherusers.Ifthechannelisidle,thestationmaytransmit.Howeverifitisbusy,eachst