80211 ifs时间doc.docx
- 文档编号:12902264
- 上传时间:2023-06-09
- 格式:DOCX
- 页数:14
- 大小:284.72KB
80211 ifs时间doc.docx
《80211 ifs时间doc.docx》由会员分享,可在线阅读,更多相关《80211 ifs时间doc.docx(14页珍藏版)》请在冰点文库上搜索。
80211ifs时间doc
Nowthatthe802.11gstandardhasbeenfinalized,comparisonswiththeotherstandardsinthe802.11familyareinevitable.Oneconclusionthatisfrequentlydrawnisthat802.11gofferssimilarspeedsto802.11a.Afterall,bothproductsareadvertisedashavingadatarateof54Mbps.
ThisarticledevelopsasimplemodelforthemaximumTCPthroughputof802.11networkssothatacomparisoncanmovebeyondasimplecomparisonofnominalbitrates.Accordingtothemodel,802.11gissignificantlyfasterthan802.11b.Inanetworkconsistingonlyof802.11gclients,itisevenslightlyfasterthan802.11a.However,"protection"mechanismsaddedto802.11gtoensurebackwardscompatibilitywithlegacy802.11bclientscancutthethroughputby50percentormore.
Technology
Transactionspersecond
MbpsofTCPpayloadthroughput
Transactionalspeedrelativeto802.11b
11b,11Mbps
479
5.6
1.0
11a,54Mbps
2,336
27.3
4.9
11g,54Mbps/noprotection
2,336
27.3
4.9
11g,54Mbps/CTS-to-selfprotection
1,113
13.0
2.3
11g,54Mbps/RTS/CTSprotection
750
8.8
1.6
TheTransactionalModel
Thebasictransactionalmodelassumedbythisarticleisamaximum-lengthframecontainingasingleTCPsegmentfollowedbyaTCPacknowledgement.Tocopewiththeinherentunreliabilityofradiowaves,the802.11MACrequirespositiveacknowledgementofeverytransmission.EachTCPpacketmustthereforebewrappedupinaframeexchange.Thecompletetransactionconsistsofthetwo802.11dataframescarryingTCPembeddedintheirrespectiveexchanges:
TheTCPdatasegment:
∙DistributedInterframeSpace(DIFS):
thisinterframespaceindicatesthatanexchangehascompleted,anditissafetoaccessthemediumagain.
∙ThedataframecontainingtheTCPsegment.
∙AShortInterframeSpace(SIFS),whichisasmallgapbetweenthedataframeanditsacknowledgement.
∙The802.11ACKframe.
TheTCPACK:
∙DIFS
∙ThedataframecontainingtheTCPACK
∙SIFS
∙802.11ACKframe
Forthismodel,Iwillassumethatthehostoperatingsystemlimitstheoutgoingframesizeto1,500bytes.802.11permitsmuchlargerframesizes,butthisflexibilityhasnottraditionallybeenusedbyclientproducts.MostaccesspointsconnecttoexistingnetworkswithEthernet,andthereforelimitthepayloadsizetothemaximumEthernetpayloadsize.(Infact,thissimpleprecautionisrequiredtoobtainWi-Ficertification.)
Encapsulation
Inadditiontothepayloaddata,thereare36additionalbytesofdataaddedintheencapsulationprocess.The802.11MACheaderadds28bytesofdataforvariouscontrolandmanagementfunctions,errordetection,andaddressing.AfurthereightbytesareaddedbytheSNAPencapsulationheadertoidentifythenetworklayerprotocol.(Forfurtherencapsulationdetails,seeFigures3-9and3-13of802.11WirelessNetworks.)ThetotalsizeoftheMACpayloadfortheTCPdatasegmentis1,536bytes(12,288bits).FortheTCPACK,the40bytesofTCP/IPheaderinformationalsogetsafurther36bytesofSNAPheaderforatotalof76bytes(608bits).
Throughput
ThisarticlemeasuresthethroughputofthenetworkattheTCPpayloadlayer.Eachtransactiondeliversonedataframe.Thereare1,500bytesofEthernetpayload,but40bytesmustbesubtractedforheaders.Byaddingupthetotaltimerequiredforeachcomponentofthetransaction,atransactionratecanbederived.Forthethroughput,simplymultiplyby1,460bytes.
WeaknessesoftheModel
Thetransactionalmodelisquitesimple,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,TCPuses"slidingwindows"andallowsmultipleoutstandingframestobetransmittedbeforeacknowledgement.Inpractice,TCPacknowledgementscanapplytomultiplesegments,sothismodeloverstatestheimpactofhigher-layerprotocolacknowledgements.
TheBaseline:
802.11bat11Mbps
Thebaselinespeedcomesfrom802.11b.Itisnotasfastasthenewerspecifications,butit'swhatmostofthe802.11cardsoutintheworlddo.I'lldothecalculationfor802.11bfirst,andthencompareeverythingelsetotheresultfor802.11b.
Firstoff,thebasictimingnumbersfor802.11b:
∙SIFS=10µs
∙Slottime=20µs
∙DIFS=2xSlottime+SIFS=50µs
802.11brequiresthatapreamblebeprependedtoeveryframebeforeitistransmittedtotheair.Thatpreamblemaybeeitherthetraditional"long"preamble,whichrequires192µsfortransmission,oritmaybeanoptional"short"preamblethatrequiresonly96µs.(Foradiagramofthephysicallayerheadersin802.11b,seeFigure10-26in802.11WirelessNetworks.)Supportofthelongpreambleismandatory,andisthedefaultsettingonmostdevices.Intheinterestofreducingthenumberofcalculations,thisarticlewillperformcalculationsusingonlythelongpreamble.
802.11brunningatthemaxspeeddividesdataupinto8-bitsymbols.Thereare1,5368-bitblocksintheTCPdatasegment,and768-bitblocksintheTCPacknowledgement.TheACKdoesnothaveSNAPheaders,andisonly14byteslong.
EncodingtheMACframesiseasy.802.11bdividesuptheMACframeintoaseriesof8-bit"symbols,"andthentransmits1.375millionsymbolspersecond.Soadduptheindividualcomponentsofthetransactiontogetthetotalduration:
TCPdata
TCPACK
DIFS
50µs
50µs
802.11Data
192µs+1536/1.375Msps
=192µs+1,118µs
=1,310µs
192µs+76/1.375Msps
=192µs+56µs
=248µs
SIFS
10µs
10µs
802.11ACK
192µs+14/1.375Msps
=192µs+11µs
=203µs
=203µs
Frameexchangetotal
1,573µs
511µs
TransactionTotal
2,084µs
Eachtransactionrequires2,084µs.Atthatduration,479exchangescancompletepersecond.WithaTCPpayloadof1,460bytesperexchange,thethroughputis5.7Mbps.
802.11a
802.11aisfasterthan802.11bfortworeasons:
timingrelationshipsbetweenframesintheexchangesaretighter,andtheencodingusedby802.11adoesnotrequiresuchlongpreamblesforsynchronization.
Herearethebasictimingnumbersfor802.11a:
∙SIFS=16µs
∙Slottime=9µs
∙DIFS=2xSlottime+SIFS=34µs
Like802.11b,802.11adividesdataupintoaseriesofsymbolsfortransmission.However,theencodingusedby802.11ausesmuchlargersymbols.At54Mbps,eachsymbolencodes216bits(每个符号的时间为4µs).(Forafulllistingofencodingblocksizes,seeTable11-3in802.11WirelessNetworks.)TheOFDMencodingusedby802.11aaddssixbitsforencodingpurposestotheendoftheframe,soourmaximum-lengthframeof1,536bytesbecomesaseriesof12,288databitsplusthesixtailbits.Thetotalbitstringof12,294bitscanbeencodedin57symbols.The608bit(40+36=76Byte)TCPACKplustailbitsrequiresthreesymbols,andthe802.11ACKalsorequiresjustonesymbol.
Eachframeispreparedfortransmissionintheairwitha20µsheadertosynchronizethereceiver.Followingthe20µsheaderisaseriesofsymbols,eachrequiring4µsfortransmission.
TCPdata
TCPACK
DIFS
34µs
34µs
802.11Data
20µs+57*4µs/symbol
=20µs+228µs
=248µs
20µs+3*4µs/symbol
=20+12µs
=32µs
SIFS
16µs
16µs
802.11ACK
20µs+1*4µs/symbol
=20µs+4µs
=24µs
=24µs
Frameexchangetotal
322µs
106µs
TransactionTotal
428µs
At428µspertransaction,itispossibletocomplete2,336exchangespersecond.Thatcorrespondstoathroughputof27.3Mbps.(注:
428us*2336=1s)
802.11g
Performingthesamecalculationsfor802.11gisabittricky.802.11goperatesinthesamefrequencybandas802.11b,andisrequiredtoremainbackwards-compatible.Theencodingusedby802.11gwillnotberecognizedby802.11bstations,so"protection"mechanismsaredefinedtolimitthecross-talkinmixedb/genvironments.Essentially,theprotectionmechanismsrequirethat802.11gstationsoperatingathighratespre-reservetheradiomediumbyusingslower,802.11b-compatiblereservationmechanisms.
802.11gSIFS=10µs
802.11gshortslottime=9µs(802.11g-onlymodewithnolegacystations)
802.11glongslottime=20µs(mixedmoderequiresslowslottime)
802.11gusesmanyofthesametimingparametersas802.11a.However,itissaddledwithbackwardscompatibilityrequirements.Itinheritstheshort10µsSIFStimefrom802.11b,butthehigh-ratecodingin802.11gneedsadditionaltime.Therefore,802.11gaddsa6µs"signalextension"timeattheendofeveryframe.
802.11g-onlyBSS
Whenno802.11bstationsarepresent,noprotectionisrequired.Thissituationisextremelyunlikely,giventhehugeinstalledbaseof802.11bcards.
Asalways,startwiththebasictimingparameters:
∙802.11gSIFS=10µs
∙802.11gfastslottime=9µs(canonlybeusedwhenno802.11bstationsarepresent)
∙802.11gD
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- 80211 ifs时间doc ifs 时间 doc