计算机网络复习课件chapter6

6:WirelessandMobileNetworks6-1Chapter6:WirelessandMobileNetworksBackground:

#wireless(mobile)phonesubscribersnowexceeds#wiredphonesubscribers!computernets:laptops,palmtops,PDAs,Internet-enabledphonepromiseanytimeuntetheredInternetaccesstwoimportant(butdifferent)challengeswireless:communicationoverwirelesslinkmobility:handlingthemobileuserwhochangespointofattachmenttonetwork6:WirelessandMobileNetworks6-2Chapter6outline6.1IntroductionWireless6.2Wirelesslinks,characteristicsCDMA6.3IEEE802.11wirelessLANs(“wi-fi”)6.4CellularInternetAccessarchitecturestandards(e.g.,GSM)Mobility6.5Principles:addressingandroutingtomobileusers6.6MobileIP6.7Handlingmobilityincellularnetworks6.8Mobilityandhigher-layerprotocols6.9Summary6:WirelessandMobileNetworks6-3Elementsofawirelessnetworknetworkinfrastructurewirelesshostslaptop,PDA,IPphonerunapplicationsmaybestationary(non-mobile)ormobilewirelessdoesnotalwaysmeanmobility6:WirelessandMobileNetworks6-4Elementsofawirelessnetworknetworkinfrastructure

basestationtypicallyconnectedtowirednetworkrelay-responsibleforsendingpacketsbetweenwirednetworkandwirelesshost(s)inits“area”e.g.,celltowers,802.11accesspoints6:WirelessandMobileNetworks6-5Elementsofawirelessnetworknetworkinfrastructure

wirelesslinktypicallyusedtoconnectmobile(s)tobasestationalsousedasbackbonelinkmultipleaccessprotocolcoordinateslinkaccessvariousdatarates,transmissiondistance6:WirelessandMobileNetworks6-6CharacteristicsofselectedwirelesslinkstandardsIndoor10-30mOutdoor50-200mMid-rangeoutdoor200m–4KmLong-rangeoutdoor5Km–20Km.056.384145-1154IS-95,CDMA,GSM2GUMTS/WCDMA,CDMA20003G802.15802.11b802.11a,gUMTS/WCDMA-HSPDA,CDMA2000-1xEVDO3Gcellularenhanced802.16(WiMAX)802.11a,gpoint-to-point200802.11nDatarate(Mbps)data6:WirelessandMobileNetworks6-7Elementsofawirelessnetworknetworkinfrastructure

infrastructuremodebasestationconnectsmobilesintowirednetworkhandoff:mobilechangesbasestationprovidingconnectionintowirednetwork6:WirelessandMobileNetworks6-8Elementsofawirelessnetworkadhocmodenobasestationsnodescanonlytransmittoothernodeswithinlinkcoveragenodesorganizethemselvesintoanetwork:routeamongthemselves6:WirelessandMobileNetworks6-9Wirelessnetworktaxonomysinglehopmultiplehopsinfrastructure(e.g.,APs)noinfrastructurehostconnectstobasestation(WiFi,WiMAX,cellular)whichconnectstolargerInternetnobasestation,noconnectiontolargerInternet(Bluetooth,adhocnets)hostmayhavetorelaythroughseveralwirelessnodestoconnecttolargerInternet:meshnetnobasestation,noconnectiontolargerInternet.MayhavetorelaytoreachotheragivenwirelessnodeMANET,VANET6:WirelessandMobileNetworks6-10WirelessLinkCharacteristics(1)Differencesfromwiredlink….decreasedsignalstrength:radiosignalattenuatesasitpropagatesthroughmatter(pathloss)interferencefromothersources:standardizedwirelessnetworkfrequencies(e.g.,2.4GHz)sharedbyotherdevices(e.g.,phone);devices(motors)interfereaswellmultipathpropagation:radiosignalreflectsoffobjectsground,arrivingaddestinationatslightlydifferenttimes….makecommunicationacross(evenapointtopoint)wirelesslinkmuchmore“difficult”6:WirelessandMobileNetworks6-11WirelessLinkCharacteristics(2)SNR:signal-to-noiseratiolargerSNR–easiertoextractsignalfromnoise(a“goodthing”)SNRversusBERtradeoffsgivenphysicallayer:increasepower->increaseSNR->decreaseBERgivenSNR:choosephysicallayerthatmeetsBERrequirement,givinghighestthroughputSNRmaychangewithmobility:dynamicallyadaptphysicallayer(modulationtechnique,rate)10203040QAM256(8Mbps)QAM16(4Mbps)BPSK(1Mbps)SNR(dB)BER10-110-210-310-510-610-710-46:WirelessandMobileNetworks6-12WirelessnetworkcharacteristicsMultiplewirelesssendersandreceiverscreateadditionalproblems(beyondmultipleaccess):ABCHiddenterminalproblemB,AheareachotherB,CheareachotherA,CcannotheareachothermeansA,CunawareoftheirinterferenceatBABCA’ssignalstrengthspaceC’ssignalstrengthSignalattenuation:B,AheareachotherB,CheareachotherA,CcannotheareachotherinterferingatB6:WirelessandMobileNetworks6-13WirelessnetworkcharacteristicsHiddenNodeProblemBCA6:WirelessandMobileNetworks6-14WirelessnetworkcharacteristicsExposedNodeProblemBCADX6:WirelessandMobileNetworks6-15Chapter6outline6.1Introduction

Wireless6.2Wirelesslinks,characteristicsCDMA6.3IEEE802.11wirelessLANs(“wi-fi”)6.4cellularInternetaccessarchitecturestandards(e.g.,GSM)Mobility6.5Principles:addressingandroutingtomobileusers6.6MobileIP6.7Handlingmobilityincellularnetworks6.8Mobilityandhigher-layerprotocols6.9Summary6:WirelessandMobileNetworks6-16IEEE802.11WirelessLAN802.11b2.4-5GHzunlicensedspectrumupto11Mbpsdirectsequencespreadspectrum(DSSS)inphysicallayerallhostsusesamechippingcode802.11a

5-6GHzrangeupto54Mbps802.11g

2.4-5GHzrangeupto54Mbps802.11n:multipleantennae2.4-5GHzrangeupto200MbpsalluseCSMA/CAformultipleaccessallhavebase-stationandad-hocnetworkversions6:WirelessandMobileNetworks6-17802.11LANarchitecturewirelesshostcommunicateswithbasestationbasestation=accesspoint(AP)BasicServiceSet(BSS)(aka“cell”)ininfrastructuremodecontains:wirelesshostsaccesspoint(AP):basestationadhocmode:hostsonlyBSS1BSS2Internethub,switchorrouterAPAP6:WirelessandMobileNetworks6-18802.11:Channels,association802.11b:2.4GHz-2.485GHzspectrumdividedinto11channelsatdifferentfrequenciesAPadminchoosesfrequencyforAPinterferencepossible:channelcanbesameasthatchosenbyneighboringAP!host:mustassociatewithanAPscanschannels,listeningforbeaconframescontainingAP’sname(SSID)andMACaddressselectsAPtoassociatewithmayperformauthentication[Chapter8]willtypicallyrunDHCPtogetIPaddressinAP’ssubnet6:WirelessandMobileNetworks6-19802.11:passive/activescanningAP2AP1H1BBS2BBS112234ActiveScanning:ProbeRequestframebroadcastfromH1ProbesresponseframesentfromAPsAssociationRequestframesent:H1toselectedAPAssociationResponseframesent:H1toselectedAPAP2AP1H1BBS2BBS11231PassiveScanning:

beaconframessentfromAPsassociationRequestframesent:H1toselectedAPassociationResponseframesent:H1toselectedAP6:WirelessandMobileNetworks6-20IEEE802.11:multipleaccessavoidcollisions:2+nodestransmittingatsametime802.11:CSMA-sensebeforetransmittingdon’tcollidewithongoingtransmissionbyothernode802.11:nocollisiondetection!difficulttoreceive(sensecollisions)whentransmittingduetoweakreceivedsignals(fading)can’tsenseallcollisionsinanycase:hiddenterminal,fadinggoal:avoidcollisions:CSMA/C(ollision)A(voidance)ABCABCA’ssignalstrengthspaceC’ssignalstrength6:WirelessandMobileNetworks6-21IEEE802.11MACProtocol:CSMA/CA802.11sender1ifsensechannelidleforDIFS

then

transmitentireframe(noCD)2if

sensechannelbusythen

startrandombackofftimetimercountsdownwhilechannelidletransmitwhentimerexpiresifnoACK,increaserandombackoffinterval,repeat2802.11receiver-ifframereceivedOK

returnACKafterSIFS(ACKneededduetohiddenterminalproblem)senderreceiverDIFSdataSIFSACK6:WirelessandMobileNetworks6-22CSMA/CAinIEEE802.11Physicalcarriersense,andVirtualcarriersenseusingNetworkAllocationVector(NAV)NAVisupdatedbasedonoverheardRTS/CTS/DATA/ACKpackets,eachofwhichspecifieddurationofapendingtransmissionBackoffintervalsusedtoreducecollisionprobability6:WirelessandMobileNetworks6-23BackoffIntervalWhentransmittingapacket,chooseabackoffintervalintherange[0,cw]cwiscontentionwindowCountdownthebackoffintervalwhenmediumisidleCount-downissuspendedifmediumesbusyWhenbackoffintervalreaches0,transmitRTS6:WirelessandMobileNetworks6-24DCFExample1datawaitB1=5B2=15B1=25B2=20datawaitB1andB2arebackoffintervalsatnodes1and2cw=31B2=106:WirelessandMobileNetworks6-25DIFSDIFSDIFSDCFExample26:WirelessandMobileNetworks6-26BackoffIntervalThetimespentcountingdownbackoffintervalsisapartofMACoverheadChoosingalargecw

leadstolargebackoffintervalsandcanresultinlargeroverheadChoosingasmallcwleadstoalargernumberofcollisions(whentwonodescountdownto0simultaneously)6:WirelessandMobileNetworks6-27Sincethenumberofnodesattemptingtotransmitsimultaneouslymaychangewithtime,somemechanismtomanagecontentionisneededIEEE802.11DCF:contentionwindowcwischosendynamicallydependingoncollisionoccurrenceBackoffInterval6:WirelessandMobileNetworks6-28BinaryExponentialBackoffinDCFWhenanodefailstoreceiveCTSinresponsetoitsRTS,itincreasesthecontentionwindowcwisdoubled(uptoanupperbound)Whenanodesuccessfullycompletesadatatransfer,itrestorescwtoCwmincwfollowsasawtoothcurve6:WirelessandMobileNetworks6-29Avoidingcollisions(more)idea:

allowsenderto“reserve”channelratherthanrandomaccessofdataframes:avoidcollisionsoflongdataframessenderfirsttransmitssmallrequest-to-send(RTS)packetstoBSusingCSMARTSsmaystillcollidewitheachother(butthey’reshort)BSbroadcastsclear-to-sendCTSinresponsetoRTSCTSheardbyallnodessendertransmitsdataframeotherstationsdefertransmissionsavoiddataframecollisionscompletelyusingsmallreservationpackets!6:WirelessandMobileNetworks6-30CollisionAvoidance:RTS-CTSexchangeAPABtimeRTS(A)RTS(B)RTS(A)CTS(A)CTS(A)DATA(A)ACK(A)ACK(A)reservationcollisiondefer6:WirelessandMobileNetworks6-31RTS/CTS/DATA/ACK

6:WirelessandMobileNetworks6-32framecontroldurationaddress1address2address4address3payloadCRC22666260-23124seqcontrol802.11frame:addressingAddress2:MACaddressofwirelesshostorAPtransmittingthisframeAddress1:MACaddressofwirelesshostorAPtoreceivethisframeAddress3:MACaddressofrouterinterfacetowhichAPisattachedAddress4:usedonlyinadhocmode6:WirelessandMobileNetworks6-33InternetrouterAPH1R1APMACaddrH1