80211 IFS时间

Nowthatthe802.11gstandardhasbeenfinalized,comparisonswiththeotherstandardsinthe802.11familyareinevitable.Oneconclusionthatisfrequentlydrawnisthat802.11gofferssimilarspeedsto802.11a.Afterall,bothproductsareadvertisedashavingadatarateof54Mbps.ThisarticledevelopsasimplemodelforthemaximumTCPthroughputof802.11networkssothatacomparisoncanmovebeyondasimplecomparisonofnominalbitrates.Accordingtothemodel,802.11gissignificantlyfasterthan802.11b.Inanetworkconsistingonlyof802.11gclients,itisevenslightlyfasterthan802.11a.However,protectionmechanismsaddedto802.11gtoensurebackwardscompatibilitywithlegacy802.11bclientscancutthethroughputby50percentormore.TechnologyTransactionspersecondMbpsofTCPpayloadthroughputTransactionalspeedrelativeto802.11b11b,11Mbps4795.61.011a,54Mbps2,33627.34.911g,54Mbps/noprotection2,33627.34.911g,54Mbps/CTS-to-selfprotection1,11313.02.311g,54Mbps/RTS/CTSprotection7508.81.6TheTransactionalModelThebasictransactionalmodelassumedbythisarticleisamaximum-lengthframecontainingasingleTCPsegmentfollowedbyaTCPacknowledgement.Tocopewiththeinherentunreliabilityofradiowaves,the802.11MACrequirespositiveacknowledgementofeverytransmission.EachTCPpacketmustthereforebewrappedupinaframeexchange.Thecompletetransactionconsistsofthetwo802.11dataframescarryingTCPembeddedintheirrespectiveexchanges:TheTCPdatasegment:DistributedInterframeSpace(DIFS):thisinterframespaceindicatesthatanexchangehascompleted,anditissafetoaccessthemediumagain.ThedataframecontainingtheTCPsegment.AShortInterframeSpace(SIFS),whichisasmallgapbetweenthedataframeanditsacknowledgement.The802.11ACKframe.TheTCPACK:DIFSThedataframecontainingtheTCPACKSIFS802.11ACKframeForthismodel,Iwillassumethatthehostoperatingsystemlimitstheoutgoingframesizeto1,500bytes.802.11permitsmuchlargerframesizes,butthisflexibilityhasnottraditionallybeenusedbyclientproducts.MostaccesspointsconnecttoexistingnetworkswithEthernet,andthereforelimitthepayloadsizetothemaximumEthernetpayloadsize.(Infact,thissimpleprecautionisrequiredtoobtainWi-Ficertification.)EncapsulationInadditiontothepayloaddata,thereare36additionalbytesofdataaddedintheencapsulationprocess.The802.11MACheaderadds28bytesofdataforvariouscontrolandmanagementfunctions,errordetection,andaddressing.AfurthereightbytesareaddedbytheSNAPencapsulationheadertoidentifythenetworklayerprotocol.(Forfurtherencapsulationdetails,seeFigures3-9and3-13of802.11WirelessNetworks.)ThetotalsizeoftheMACpayloadfortheTCPdatasegmentis1,536bytes(12,288bits).FortheTCPACK,the40bytesofTCP/IPheaderinformationalsogetsafurther36bytesofSNAPheaderforatotalof76bytes(608bits).ThroughputThisarticlemeasuresthethroughputofthenetworkattheTCPpayloadlayer.Eachtransactiondeliversonedataframe.Thereare1,500bytesofEthernetpayload,but40bytesmustbesubtractedforheaders.Byaddingupthetotaltimerequiredforeachcomponentofthetransaction,atransactionratecanbederived.Forthethroughput,simplymultiplyby1,460bytes.WeaknessesoftheModelThetransactionalmodelisquitesimple,andneglectsseveralimportanteffects.Firstofall,itassumesasteadystreamofwell-orderedframeswithnocontentionforthemedium.802.11implementscollisionavoidanceandexponentialbackoff,soinreality,thetimebetweenframeexchangeswillbelongerthanoneDIFS.Exponentialbackoffinthepresenceofcontentionwillfurtherdecreasethroughput.Myinitialback-of-theenvelopeestimateisthatcontentionforthemediumwouldreducethemaximumthroughputfiguresaboveby25%to50%,dependingontheexactassumptionsmade.Furthermore,thereisalwaysthepossibilityofsimultaneoustransmission.802.11willorderlayertwoframesandpreventcollisions,buthigher-layerpacketsmaybetransmittedsimultaneously.802.11treatsTCPsegmentsandacknowledgementsasunrelatedhigher-layerframes,soacertainnumberofcollisionsareexpected.(Oneengineerhastoldmethatupto10%retransmissionsshouldbeexpectedforTCP/IPon802.11duetothiseffect.)Furthermore,themodelignoresthesophisticationintheTCPacknowledgementmodel.Toavoidconstrainingthroughput,TCPusesslidingwindowsandallowsmultipleoutstandingframestobetransmittedbeforeacknowledgement.Inpractice,TCPacknowledgementscanapplytomultiplesegments,sothismodeloverstatestheimpactofhigher-layerprotocolacknowledgements.TheBaseline:802.11bat11MbpsThebaselinespeedcomesfrom802.11b.Itisnotasfastasthenewerspecifications,butit'swhatmostofthe802.11cardsoutintheworlddo.I'lldothecalculationfor802.11bfirst,andthencompareeverythingelsetotheresultfor802.11b.Firstoff,thebasictimingnumbersfor802.11b:SIFS=10µsSlottime=20µsDIFS=2xSlottime+SIFS=50µs802.11brequiresthatapreamblebeprependedtoeveryframebeforeitistransmittedtotheair.Thatpreamblemaybeeitherthetraditionallongpreamble,whichrequires192µsfortransmission,oritmaybeanoptionalshortpreamblethatrequiresonly96µs.(Foradiagramofthephysicallayerheadersin802.11b,seeFigure10-26in802.11WirelessNetworks.)Supportofthelongpreambleismandatory,andisthedefaultsettingonmostdevices.Intheinterestofreducingthenumberofcalculations,thisarticlewillperformcalculationsusingonlythelongpreamble.802.11brunningatthemaxspeeddividesdataupinto8-bitsymbols.Thereare1,5368-bitbloc