计算机 毕业论文 外文文献翻译 中英文：ieee802.11 媒体接入控制

英文资料与中文翻译IEEE80211MEDIUMACCESSCONTROLTHEIEEE80211MACLAYERCOVERSTHREEFUNCTIONALAREASRELIABLEDATADELIVERY,MEDIUMACCESSCONTROL,ANDSECURITYTHISSECTIONCOVERSTHEFIRSTTWOTOPICSRELIABLEDATADELIVERYASWITHANYWIRELESSNETWORK,AWIRELESSLANUSINGTHEIEEE80211PHYSICALANDMACLAYERSISSUBJECTTOCONSIDERABLEUNRELIABILITYNOISE,INTERFERENCE,ANDOTHERPROPAGATIONEFFECTSRESULTINTHELOSSOFASIGNIFICANTNUMBEROFFRAMESEVENWITHERRORCORRECTIONCODES,ANUMBEROFMACFRAMESMAYNOTSUCCESSFULLYBERECEIVEDTHISSITUATIONCANBEDEALTWITHBYRELIABILITYMECHANISMSATAHIGHERLAYERSUCHASTCPHOWEVER,TIMERSUSEDFORRETRANSMISSIONATHIGHERLAYERSARETYPICALLYONTHEORDEROFSECONDSITISTHEREFOREMOREEFFICIENTTODEALWITHERRORSATTHEMACLEVELFORTHISPURPOSE,IEEE80211INCLUDESAFRAMEEXCHANGEPROTOCOLWHENASTATIONRECEIVESADATAFRAMEFROMANOTHERSTATIONITRETURNSANACKNOWLEDGMENTACKFRAMETOTHESOURCESTATIONTHISEXCHANGEISTREATEDASANATOMICUNIT,NOTTOBEINTERRUPTEDBYATRANSMISSIONFROMANYOTHERSTATIONIFTHESOURCEDOESNOTRECEIVEANACKWITHINASHORTPERIODOFTIME,EITHERBECAUSEITSDATAFRAMEWASDAMAGEDORBECAUSETHERETURNINGACKWASDAMAGED,THESOURCERETRANSMITSTHEFRAMETHUS,THEBASICDATATRANSFERMECHANISMINIEEE80211INVOLVESANEXCHANGEOFTWOFRAMESTOFURTHERENHANCERELIABILITY,AFOURFRAMEEXCHANGEMAYBEUSEDINTHISSCHEME,ASOURCEFIRSTISSUESAREQUESTTOSENDRTSFRAMETOTHEDESTINATIONTHEDESTINATIONTHENRESPONDSWITHACLEARTOSENDCTSAFTERRECEIVINGTHECTS,THESOURCETRANSMITSTHEDATAFRAME,ANDTHEDESTINATIONRESPONDSWITHANACKTHERTSALERTSALLSTATIONSTHATAREWITHINRECEPTIONRANGEOFTHESOURCETHATANEXCHANGEISUNDERWAYTHESESTATIONSREFRAINFROMTRANSMISSIONINORDERTOAVOIDACOLLISIONBETWEENTWOFRAMESTRANSMITTEDATTHESAMETIMESIMILARLY,THECTSALERTSALLSTATIONSTHATAREWITHINRECEPTIONRANGEOFTHEDESTINATIONTHATANEXCHANGEISUNDERWAYTHERTS/CTSPORTIONOFTHEEXCHANGEISAREQUIREDFUNCTIONOFTHEMACBUTMAYBEDISABLEDMEDIUMACCESSCONTROLTHE80211WORKINGGROUPCONSIDEREDTWOTYPESOFPROPOSALSFORAMACALGORITHMDISTRIBUTEDACCESSPROTOCOLS,WHICH,LIKEETHERNET,DISTRIBUTETHEDECISIONTOTRANSMITOVERALLTHENODESUSINGACARRIERSENSEMECHANISMANDCENTRALIZEDACCESSPROTOCOLS,WHICHINVOLVEREGULATIONOFTRANSMISSIONBYACENTRALIZEDDECISIONMAKERADISTRIBUTEDACCESSPROTOCOLMAKESSENSEFORANADHOCNETWORKOFPEERWORKSTATIONSTYPICALLYANIBSSANDMAYALSOBEATTRACTIVEINOTHERWIRELESSLANCONFIGURATIONSTHATCONSISTPRIMARILYOFBURSTTRAFFICACENTRALIZEDACCESSPROTOCOLISNATURALFORCONFIGURATIONSINWHICHAUMBEROFWIRELESSSTATIONSAREINTERCONNECTEDWITHEACHOTHERANDSOMESORTOFBASESTATIONTHATATTACHESTOABACKBONEWIREDLANITISESPECIALLYUSEFULIFSOMEOFTHEDATAISTIMESENSITIVEORHIGHPRIORITYTHEENDRESULTFOR80211ISAMACALGORITHMCALLEDDFWMACDISTRIBUTEDFOUNDATIONWIRELESSMACTHATPROVIDESADISTRIBUTEDACCESSCONTROLMECHANISMWITHANOPTIONALCENTRALIZEDCONTROLBUILTONTOPOFTHATFIGURE145ILLUSTRATESTHEARCHITECTURETHELOWERSUBLAYEROFTHEMACLAYERISTHEDISTRIBUTEDCOORDINATIONFUNCTIONDCFDCFUSESACONTENTIONALGORITHMTOPROVIDEACCESSTOALLTRAFFICORDINARYASYNCHRONOUSTRAFFICDIRECTLYUSESDCETHEPOINTCOORDINATIONFUNCTIONPCFISACENTRALIZEDMACALGORITHMUSEDTOPROVIDECONTENTIONFREESERVICEPCFISBUILTONTOPOFDCFANDEXPLOITSFEATURESOFDCFTOASSUREACCESSFORITSUSERSLETUSCONSIDERTHESETWOSUBLAYERSINTURN80211G24GHZDSSS,OFDMLOGICALLINKCONTROLLLCDISTRIBUTEDCOORDINATIONFUNCTIONDCFPOINTCOORDINATIONFUNCTIONPCF8021124GHZFHSS8021124GHZDSSS80211INFRARED80211A5GHZOFDM80211B24GHZDSSSCONTENTIONFREESERVICECONTENTIONSERVICEMACLAYERFIGURE145IEEE80211PROTOCOLARCHITECTUREDISTRIBUTEDCOORDINATIONFUNCTIONTHEDCFSUBLAYERMAKESUSEOFASIMPLECSMACARRIERSENSEMULTIPLEACCESSALGORITHM,WHICHFUNCTIONSASFOLLOWSIFASTATIONHASAMACFRAMETOTRANSMIT,ITLISTENSTOTHEMEDIUMIFTHEMEDIUMISIDLE,THESTATIONMAYTRANSMITOTHERWISETHESTATIONMUSTWAITUNTILTHECURRENTTRANSMISSIONISCOMPLETEBEFORETRANSMITTINGTHEDCFDOESNOTINCLUDEACOLLISIONDETECTIONFUNCTIONIECSMA/CDBECAUSECOLLISIONDETECTIONISNOTPRACTICALONAWIRELESSNETWORKTHEDYNAMICRANGEOFTHESIGNALSONTHEMEDIUMISVERYLARGE,SOTHATATRANSMITTINGSTATIONCANNOTEFFECTIVELYDISTINGUISHINCOMINGWEAKSIGNALSFROMNOISEANDTHEEFFECTSOFITSOWNTRANSMISSIONTOENSURETHESMOOTHANDFAIRFUNCTIONINGOFTHISALGORITHM,DCFINCLUDESASETOFDELAYSTHATAMOUNTSTOAPRIORITYSCHEMELETUSSTARTBYCONSIDERINGASINGLEDELAYKNOWNASANINTERFRAMESPACEIFSINFACT,THEREARETHREEDIFFERENTIFSVALUES,BUTTHEALGORITHMISBESTEXPLAINEDBYINITIALLYIGNORINGTHISDETAILUSINGANIFS,THERULESFORCSMAACCESSAREASFOLLOWSFIGURE146WAITEFORFRAMETOTRANSMITMEDIUMIDLEWAITIFSSTILLIDLETRANSMITFRAMEWAITUNTILCURRENTTRANSMISSIONENDSWAITIFSSTILLIDLEEXPONENTIALBACKOFFWHILEMEDIUMIDLETRANSMITFRAMENOYESNOYESYESNOFIGURE146IEEE80211MEDIUMACCESSCONTROLLOGIC1ASTATIONWITHAFRAMETOTRANSMITSENSESTHEMEDIUMIFTHEMEDIUMISIDLEITWAITSTOSEEIFTHEMEDIUMREMAINSIDLEFORATIMEEQUALTOIFSIFSO,THESTATIONMAYTRANSMITIMMEDIATELY2IFTHEMEDIUMISBUSYEITHERBECAUSETHESTATIONINITIALLYFINDSTHEMEDIUMBUSYORBECAUSETHEMEDIUMBECOMESBUSYDURINGTHEIFSIDLETIME,THESTATIONDEFERSTRANSMISSIONANDCONTINUESTOMONITORTHEMEDIUMUNTILTHECURRENTTRANSMISSIONISOVER3ONCETHECURRENTTRANSMISSIONISOVER,THESTATIONDELAYSANOTHERIFSIFTHEMEDIUMREMAINSIDLEFORTHISPERIOD,THENTHESTATIONBACKSOFFARANDOMAMOUNTOFTIMEANDAGAINSENSESTHEMEDIUMIFTHEMEDIUMISSTILLIDLE,THESTATIONMAYTRANSMITDURINGTHEBACKOFFTIME,IFTHEMEDIUMBECOMESBUSY,THEBACKOFFTIMERISHALTEDANDRESUMESWHENTHEMEDIUMBECOMESIDLE4IFTHETRANSMISSIONISUNSUCCESSFUL,WHICHISDETERMINEDBYTHEABSENCEOFANACKNOWLEDGEMENT,THENITISASSUMEDTHATACOLLISIONHASOCCURREDTOENSURETHATBACKOFFMAINTAINSSTABILITY,ATECHNIQUEKNOWNASBINARYEXPONENTIALBACKOFFISUSEDASTATIONWILLATTEMPTTOTRANSMITREPEATEDLYINTHEFACEOFREPEATEDCOLLISIONS,BUTAFTEREACHCOLLISION,THEMEANVALUEOFTHERANDOMDELAYISDOUBLEDUPTOSOMEMAXIMUMVALUETHEBINARYEXPONENTIALBACKOFFPROVIDESAMEANSOFHANDLINGAHEAVYLOADREPEATEDFAILEDATTEMPTSTOTRANSMITRESULTINLONGERANDLONGERBACKOFFTIMES,WHICHHELPSTOSMOOTHOUTTHELOADWITHOUTSUCHABACKOFF,THEFOLLOWINGSITUATIONCOULDOCCURTWOORMORESTATIONSATTEMPTTOTRANSMITATTHESAMETIME,CAUSINGACOLLISIONTHESESTATIONSTHENIMMEDIATELYATTEMPTTORETRANSMIT,CAUSINGANEWCOLLISIONTHEPRECEDINGSCHEMEISREFINEDFORDCFTOPROVIDEPRIORITYBASEDACCESSBYTHESIMPLEEXPEDIENTOFUSINGTHREEVALUESFORIFSSIFSSHORTIFSTHESHORTESTIFS,USEDFORALLIMMEDIATERESPONSEACTIONS，ASEXPLAINEDINTHEFOLLOWINGDISCUSSIONPIFSPOINTCOORDINATIONFUNCTIONIFSAMIDLENGTHIFS,USEDBYTHECENTRALIZEDCONTROLLERINTHEPCFSCHEMEWHENISSUINGPOLLSDIFSDISTRIBUTEDCOORDINATIONFUNCTIONIFSTHELONGESTIFS,USEDASAMINIMUMDELAYFORASYNCHRONOUSFRAMESCONTENDINGFORACCESSFIGURE147AILLUSTRATESTHEUSEOFTHESETIMEVALUESCONSIDERFIRSTTHESIFSANYSTATIONUSINGSIFSTODETERMINETRANSMISSIONOPPORTUNITYHAS,INEFFECT,THEHIGHESTPRIORITY,BECAUSEITWILLALWAYSGAINACCESSINPREFERENCETOASTATIONWAITINGANAMOUNTOFTIMEEQUALTOPIFSORDIFSTHESIFSISUSEDINTHEFOLLOWINGCIRCUMSTANCESACKNOWLEDGMENTACKWHENASTATIONRECEIVESAFRAMEADDRESSEDONLYTOITSELFNOTMULTICASTORBROADCASTITRESPONDSWITHANACKFRAMEAFTER,WAITINGON1YFORANSIFSGAPTHISHASTWODESIRABLEEFFECTSFIRST,BECAUSECOLLISIONDETECTIONISNOTUSED,THELIKELIHOODOFCOLLISIONSISGREATERTHANWITHCSMA/CD,ANDTHEMACLEVELACKPROVIDESFOREFFICIENTCOLLISIONRECOVERYSECOND,THESIFSCANBEUSEDTOPROVIDEEFFICIENTDELIVERYOFANLLCPROTOCOLDATAUNITPDUTHATREQUIRESMULTIPLEMACFRAMESINTHISCASE,THEFOLLOWINGSCENARIOOCCURSASTATIONWITHAMULTIFRAMELLCPDUTOTRANSMITSENDSOUTTHEMACFRAMESONEATATIMEEACHFRAMEISACKNOWLEDGEDAFTERSIFSBYTHERECIPIENTWHENTHESOURCERECEIVESANACK,ITIMMEDIATELYAFTERSIFSSENDSTHENEXTFRAMEINTHESEQUENCETHERESULTISTHATONCEASTATIONHASCONTENDEDFORTHECHANNEL,ITWILLMAINTAINCONTROLOFTHECHANNELUNTILITHASSENTALLOFTHEFRAGMENTSOFANLLCPDUCLEARTOSENDCTSASTATIONCANENSURETHATITSDATAFRAMEWILLGETTHROUGHBYFIRSTISSUINGASMALLREQUESTTOSENDRTSFRAMETHESTATIONTOWHICHTHISFRAMEISADDRESSEDSHOULDIMMEDIATELYRESPONDWITHACTSFRAMEIFITISREADYTORECEIVEALLOTHERSTATIONSRECEIVETHERTSANDDEFERUSINGTHEMEDIUMPOLLRESPONSETHISISEXPLAINEDINTHEFOLLOWINGDISCUSSIONOFPCFBUSYMEDIUMBACKOFFWINDOWNEXTFRAMTIMEDIFSSIFSPIFSDIFSCONTENTIONWINDOWSLOTTIMEDEFERACCESSSELECTSLOTUSINGBINARYEXPONENTIALBACKOFFIMMEDIATEACCESSWHENMEDIUMISFREELONGERTHANDIFSABASICACCESSMETHODPCFOPTIONALBUSYMEDIUMPCFOPTIONALSUPERFRAMEFIXEDNOMINALLENGTHSUPERFRAMEFIXEDNOMINALLENGTHFORESHORTENEDACTUALSUPERFRAMEPERIODCONTENTIONFREEPERIODVARIABLELENGTHPERSUPERFRAMECONTENTIONPERIODDCFPCFDEFERSCFBURSTASYNCHRONOUSTRAFFICDEFERSBPCFSUPERFRAMECONSTRUCTIONFIGURE147IEEE80211MACTIMINGTHENEXTLONGESTIFSINTERVALISTHEPIFSTHISISUSEDBYTHECENTRALIZEDCONTROLLERINISSUINGPOLLSANDTAKESPRECEDENCEOVERNORMALCONTENTIONTRAFFICHOWEVER,THOSEFRAMESTRANSMITTEDUSINGSIFSHAVEPRECEDENCEOVERAPCFPOLLFINALLY,THEDIFSINTERVALISUSEDFORALLORDINARYASYNCHRONOUSTRAFFICPOINTC00RDINATIONFUNCTIONPCFISANALTERNATIVEACCESSMETHODIMPLEMENTEDONTOPOFTHEDCETHEOPERATIONCONSISTSOFPOLLINGBYTHECENTRALIZEDPOLLINGMASTERPOINTCOORDINATORTHEPOINTCOORDINATORMAKESUSEOFPIFSWHENISSUINGPOLLSBECAUSEPIFSISSMALLERTHANDIFS,THEPOINTCOORDINATORCALLSEIZETHEMEDIUMANDLOCKOUTALLASYNCHRONOUSTRAFFICWHILEITISSUESPOLLSANDRECEIVESRESPONSESASANEXTREME,CONSIDERTHEFOLLOWINGPOSSIBLESCENARIOAWIRELESSNETWORKISCONFIGUREDSOTHATANUMBEROFSTATIONSWITHTIME,SENSITIVETRAFFICARECONTROLLEDBYTHEPOINTCOORDINATORWHILEREMAININGTRAFFICCONTENDSFORACCESSUSINGCSMATHEPOINTCOORDINATORCOULDISSUEPOLLSINAROUNDROBINFASHIONTOALLSTATIONSCONFIGUREDFORPOLLINGWHENAPOLLISISSUED,THEPOLLEDSTATIONMAYRESPONDUSINGSIFSIFTHEPOINTCOORDINATORRECEIVESARESPONSE,ITISSUESANOTHERPOLLUSINGPIFSIFNORESPONSEISRECEIVEDDURINGTHEEXPECTEDTURNAROUNDTIME,THECOORDINATORISSUESAPOLLIFTHEDISCIPLINEOFTHEPRECEDINGPARAGRAPHWEREIMPLEMENTED,THEPOINTCOORDINATORWOULDLOCKOUTALLASYNCHRONOUSTRAFFICBYREPEATEDLYISSUINGPOLLSTOPREVENTTHIS,ANINTERVALKNOWNASTHESUPERFRAMEISDEFINEDDURINGTHEFIRSTPARTOFTHISINTERVAL,THEPOINTCOORDINATORISSUESPOLLSINAROUND,ROBINFASHIONTOALLSTATIONSCONFIGUREDFORPOLLINGTHEPOINTCOORDINATORTHENIDLESFORTHEREMAINDEROFTHESUPERFRAME,ALLOWINGACONTENTIONPERIODFORASYNCHRONOUSACCESSFIGUREL47BILLUSTRATESTHEUSEOFTHESUPERFRAMEATTHEBEGINNINGOFASUPERFRAME,THEPOINTCOORDINATORMAYOPTIONALLYSEIZECONTROLANDISSUESPOLLSFORAGIVEPERIODOFTIMETHISINTERVALVARIESBECAUSEOFTHEVARIABLEFRAMESIZEISSUEDBYRESPONDINGSTATIONSTHEREMAINDEROFTHESUPERFRAMEISAVAILABLEFORCONTENTIONBASEDACCESSATTHEENDOFTHESUPERFRAMEINTERVAL,THEPOINTCOORDINATORCONTENDSFORACCESSTOTHEMEDIUMUSINGPIFSIFTHEMEDIUMISIDLETHEPOINTCOORDINATORGAINSIMMEDIATEACCESSANDAFULLSUPERFRAMEPERIODFOLLOWSHOWEVER,THEMEDIUMMAYBEBUSYATTHEENDOFASUPERFRAMEINTHISCASE,THEPOINTCOORDINATORMUSTWAITUNTILTHEMEDIUMISIDLETOGAINACCESSTHISRESULTINAFORESHORTENEDSUPERFRAMEPERIODFORTHENEXTCYCLEFCD/IADDRESSADDRESSADDRESSSCADDRESSDATAFCSOCTETS222666640TO2312HEADERFRAMEBODYTRAILERFCFRAMECONTROLSCSEQUENCECONTROLD/IDURATION/CONNECTIONIDFCSFRAMECHECKSEQUENCE（A）MACFRAMEPROICOLVERSIONTYPESUBTYPETODSFROMDSMFRTPMMDBITS2214WO1111111DSDISTRIBUTIONSYSTEMMDMOREDATAMFMOREFRAGMENTSWWIREDEQUIVALENTPRIVACYRTRETRYOORDERPMPOWERMANAGEMENTBFRAMECONTROLFILEDFIGURE148IEEE80211MACFRAMEFORMATMACFRAMEFIGURE148ASHOWSTHE80211FRAMEFORMATWHENNOSECURITYFEATURESAREUSEDTHISGENERALFORMATISUSEDFORALLDATAANDCONTROLFRAMES,BUTNOTALLFIELDSAREUSEDINALLCONTEXTSTHEFIELDSAREASFOLLOWSFRAMECONTROLINDICATESTHETYPEOFFRAMEANDPROVIDESCONTR01INFORMATION,ASEXPLAINEDPRESENTLYDURATION/CONNECTIONIDIFUSEDASADURATIONFIELD,INDICATESTHETIMEINMICROSECONDSTHECHANNELWILLBEALLOCATEDFORSUCCESSFULTRANSMISSIONOFAMACFRAMEINSOMECONTROLFRAMES,THISFIELDCONTAINSANASSOCIATION,ORCONNECTION,IDENTIFIERADDRESSESTHENUMBERANDMEANINGOFTHE48BITADDRESSFIELDSDEPENDONCONTEXTTHETRANSMITTERADDRESSANDRECEIVERADDRESSARETHEMACADDRESSESOFSTATIONSJOINEDTOTHEBSSTHATARETRANSMITTINGANDRECEIVINGFRAMESOVERTHEWIRELESSLANTHESERVICESETIDSSIDIDENTIFIESTHEWIRELESSLANOVERWHICHAFRAMEISTRANSMITTEDFORANIBSS,THESSIDISARANDOMNUMBERGENERATEDATTHETIMETHENETWORKISFORMEDFORAWIRELESSLANTHATISPARTOFALARGERCONFIGURATIONTHESSIDIDENTIFIESTHEBSSOVERWHICHTHEFRAMEISTRANSMITTEDSPECIFICALLY,THESSIDISTHEMACLEVELADDRESSOFTHEAPFORTHISBSSFIGURE144FINALLYTHESOURCEADDRESSANDDESTINATIONADDRESSARETHEMACADDRESSESOFSTATIONS,WIRELESSOROTHERWISE,THATARETHEULTIMATESOURCEANDDESTINATIONOFTHISFRAMETHESOURCEADDRESSMAYBEIDENTICALTOTHETRANSMITTERADDRESSANDTHEDESTINATIONADDRESSMAYBEIDENTICALTOTHERECEIVERADDRESSSEQUENCECONTROLCONTAINSA4BITFRAGMENTNUMBERSUBFIELDUSEDFORFRAGMENTATIONANDREASSEMBLY,ANDAL2BITSEQUENCENUMBERUSEDTONUMBERFRAMESSENTBETWEENAGIVENTRANSMITTERANDRECEIVERFRAMEBODYCONTAINSANMSDUORAFRAGMENTOFANMSDUTHEMSDUISALLCPROTOCOLDATAUNITORMACCONTROLINFORMATIONFRAMECHECKSEQUENCEA32BITCYCLICREDUNDANCYCHECKTHEFRAMECONTROLFILED,SHOWNINFIGURE148B,CONSISTSOFTHEFOLLOWINGFIELDSPROTOCOLVERSION80211VERSION,CURRENTVERSION0TYPEIDENTIFIESTHEFRAMEASCONTROL,MANAGEMENT,ORDATASUBTYPEFURTHERIDENTIFIESTHEFUNCTIONOFFRAMETABLE144DEFINESTHEVALIDCOMBINATIONSOFTYPEANDSUBTYPETODSTHEMACCOORDINATIONSETSTHISBITTO1INAFRAMEDESTINEDTOTHEDISTRIBUTIONSYSTEMFROMDSTHEMACCOORDINATIONSETSTHISBITTO1INAFRAMELEAVINGTHEDISTRIBUTIONSYSTEMMOREFRAGMENTSSETTO1IFMOREFRAGMENTSFOLLOWTHISONERETRYSETTO1IFTHISISARETRANSMISSIONOFAPREVIOUSFRAMETABLE144VALIDTYPEANDSUBTYPECOMBINATIONSTYPETYPEDESCRIPTIONSUBTYPESUBTYPEDESCRIPTIONVALUE00MANAGEMENT0000ASSOCIATIONREQUEST00MANAGEMENT0001ASSOCIATIONRESPONSE00MANAGEMENT0010REASSOCIATIONREQUEST00MANAGEMENT0011REASSOCIATIONRESPONSE00MANAGEMENT0100PROBEREQUEST00MANAGEMENT0101PROBERESPONSE00MANAGEMENT1000BEACON00MANAGEMENT1001ANNOUNCEMENTTRAFFICINDICATIONMESSAGE00MANAGEMENT1010DISSOCIATION00MANAGEMENT1011AUTHENTICATION00MANAGEMENT1100DEAUTHENTICATION01CONTROL1010POWERSAVEPOLL01CONTROL1011REQUESTTOSEND01CONTROL1100CLEARTOSEND01CONTROL1101ACKNOWLEDGMENT01CONTROL1110CONTENTIONFREECFEND01CONTROL1111CFENDCFACK10DATA0000DATA10DATA0001DATACFACK10DATA0010DATACFPOLL10DATA0011DATACFACKCFPOLL10DATA0100NULLFUNCTIONNODATA10DATA0101CFACKNODATA10DATA0110CFPOLLNODATA10DATA0111CFACKCFPOLLNODATAPOWERMANAGEMENTSETTOIFTHETRANSMITTINGSTATIONISINASLEEPMODEMOREDATAINDICATESTHATASTATIONHASADDITIONALDATATOSENDEACHBLOCKOFDATAMAYBESENTASONEFRAMEORAGROUPOFFRAGMENTSINMULTIPLEFRAMESWEPSETTO1IFTHEOPTIONALWIREDEQUIVALENTPROTOCOLISIMPLEMENTEDWEPISUSEDINTHEEXCHANGEOFENCRYPTIONKEYSFORSECUREDATAEXCHANGETHISBITALSOISSETIFTHENEWERWPASECURITYMECHANISMISEMPLOYED,ASDESCRIBEDINSECTION146ORDERSETTO1INANYDATAFRAMESENTUSINGTHESTRICTLYORDEREDSERVICE,WHICHTELLSTHERECEIVINGSTATIONTHATFRAMESMUSTBEPROCESSEDINORDERWENOWLOOKATTHEVARIOUSMACFRAMETYPESCONTROLFRAMESCONTROLFRAMESASSISTINTHERELIABLEDELIVERYOFDATAFRAMESTHEREARESIXCONTROLFRAMESUBTYPESPOWERSAVEPOLLPSPOLLTHISFRAMEISSENTBYANYSTATIONTOTHESTATIONTHATINCLUDESTHEAPACCESSPOINTITSPURPOSEISTOREQUESTTHATTHEAPTRANSMITAFRAMETHATHASBEENBUFFEREDFORTHISSTATIONWHILETHESTATIONWASINPOWERSAVINGMODEREQUESTTOSENDRTSTHISISTHEFIRSTFRAMEINTHEFOURWAYFRAMEEXCHANGEDISCUSSEDUNDERTHESUBSECTIONONRELIABLEDATADELIVERYATTHEBEGINNINGOFSECTION143THESTATIONSENDINGTHISMESSAGEISALERTINGAPOTENTIALDESTINATION,ANDALLOTHERSTATIONSWITHINRECEPTIONRANGE,THATITINTENDSTOSENDADATAFRAMETOTHATDESTINATIONCLEARTOSENDCTSTHISISTHESECONDFRAMEINTHEFOURWAYEXCHANGEITISSENTBYTHEDESTINATIONSTATIONTOTHESOURCESTATIONTOGRANTPERMISSIONTOSENDADATAFRAMEACKNOWLEDGMENTPROVIDESANACKNOWLEDGMENTFROMTHEDESTINATIONTOTHESOURCETHATTHEIMMEDIATELYPRECEDINGDATA,MANAGEMENT,ORPSPOLLFRAMEWASRECEIVEDCORRECTLYCONTENTIONFREECFENDANNOUNCESTHEENDOFACONTENTIONFREEPERIODTHATISPARTOFTHEPOINTCOORDINATIONFUNCTIONCFENDCFACKACKNOWLEDGESTHECFENDTHISFRAMEENDSTHECONTENTIONFREEPERIODANDRELEASESSTATIONSFROMTHERESTRICTIONSASSOCIATEDWITHTHATPERIODDATAFRAMESTHEREAREEIGHTDATAFRAMESUBTYPES,ORGANIZEDINTOTWOGROUPSTHEFIRSTFOURSUBTYPESDEFINEFRAMESTHATCARRYUPPERLEVELDATAFROMTHESOURCESTATIONTOTHEDESTINATIONSTATIONTHEFOURDATACARRYINGFRAMESAREASFOLLOWSDATATHISISTHESIMPLESTDATAFRAMEITMAYBEUSEDINBOTHACONTENTIONPERIODANDACONTENTIONFREEPERIODDATACFACKMAYONLYBESENTDURINGACONTENTIONFREEPERIODINADDITIONTOCARRYINGDATA,THISFRAMEACKNOWLEDGESPREVIOUSLYRECEIVEDDATADATACFPOLLUSEDBYAPOINTCOORDINATORTODELIVERDATATOAMOBILESTATIONANDALSOTOREQUESTTHATTHEMOBILESTATIONSENDADATAFRAMETHATITMAYHAVEBUFFEREDDATACFACKCFPOLLCOMBINESTHEFUNCTIONSOFTHEDATACFACKANDDATACFPOLLINTOASINGLEFRAMETHEREMAININGFOURSUBTYPESOFDATAFRAMESDONOTINFACTCARRYANYUSERDATATHENULLFUNCTIONDATAFRAMECARRIESNODATA,POLLS,ORACKNOWLEDGMENTSITISUSEDONLYTOCARRYTHEPOWERMANAGEMENTBITINTHEFRAMECONTROLFIELDTOTHEAP,TOINDICATETHATTHESTATIONISCHANGINGTOALOWPOWEROPERATINGSTATETHEREMAININGTHREEFRAMESCFACK,CFPOLL,CFACKCFPOLLHAVETHESAMEFUNCTIONALITYASTHECORRESPONDINGDATAFRAMESUBTYPESINTHEPRECEDINGLISTDATACFACK,DATACFPOLL,DATACFACKCFPOLLBUTWITHOTITTHEDATAMANAGEMENTFRAMESMANAGEMENTFRAMESAREUSEDTOMANAGECOMMUNICATIONSBETWEENSTATIONSANDAPSTHEFOLLOWINGSUBTYPESAREINCLUDEDASSOCIATIONREQUESTSENTBYASTATIONTOANAPTOREQUESTANASSOCIATION,WITHTHISBSSTHISFRAMEINCLUDESCAPABILITYINFORMATION,SUCHASWHETHERENCRYPTIONISTOBEUSEDANDWHETHERTHISSTATIONISPOLLABLEASSOCIATIONRESPONSERETURNEDBYTHEAPTOTHESTATIONTOINDICATEWHETHERITISACCEPTINGTHISASSOCIATIONREQUESTREASSOCIATIONREQUESTSENTBYASTATIONWHENITMOVESFROMONEBSSTOANOTHERANDNEEDSTOMAKEANASSOCIATIONWITHTIREAPINTHENEWBSSTHESTATIONUSESREASSOCIATIONRATHERTHANSIMPLYASSOCIATIONSOTHATTHENEWAPKNOWSTONEGOTIATEWITHTHEOLDAPFORTHEFORWARDINGOFDATAFRAMESREASSOCIATIONRESPONSERETURNEDBYTHEAPTOTHESTATIONTOINDICATEWHETHERITISACCEPTINGTHISREASSOCIATIONREQUESTPROBEREQUESTUSEDBYASTATIONTOOBTAININFORMATIONFROMANOTHERSTATIONORAPTHISFRAMEISUSEDTOLOCATEANIEEE80211BSSPROBERESPONSERESPONSETOAPROBEREQUESTBEACONTRANSMITTEDPERIODICALLYTOALLOWMOBILESTATIONSTOLOCATEANDIDENTIFYABSSANNOUNCEMENTTRAFFICINDICATIONMESSAGESENTBYAMOBILESTATIONTOALERTOTHERMOBILESTATIONSTHATMAYHAVEBEENINLOWPOWERMODETHATTHISSTATIONHASFRAMESBUFFEREDANDWAITINGTOBEDELIVEREDTOTHESTATIONADDRESSEDINTHISFRAMEDISSOCIATIONUSEDBYASTATIONTOTERMINATEANASSOCIATIONAUTHENTICATIONMULTIPLEAUTHENTICATIONFRAMESAREUSEDINANEXCHANGETOAUTHENTICATEONESTATIONTOANOTHERDEAUTHENTICATIONSENTBYASTATIONTOANOTHERSTATIONORAPTOINDICATETHATITISTERMINATINGSECURECOMMUNICATIONSIEEE80211媒体接入控制IEEE80211MAC层覆盖了三个功能区可靠的数据传送、接入控制以及安全。本节先讨论前两个内容。可靠的数据传送与所有的无线网络一样，一个使用IEEE80211物理层和MAC层的无线LAN被认为是不可靠的。噪声、干扰和其他传播效果导致极大量的帧丢失。即使带有纠错码，大量MAC帧也可能无法成功地被收到。可通过在诸如TCP的高层使用可靠性机制处理这种情况。然而，在更高层中用于重发的计时器一般是以秒为单位的。因此，在MAC层处理错误会更有效。为了达到这个目的，IEEE80211包括了帧交换协议。当一个站点收到从另一个站点发来的数据帧时，它向源站点返回一个确认ACKNOWLEDGMENT，ACK帧。此交换被作为一个原子单元处理，它不会被其他站点发出的传送打断。如果因为数据帧被损坏或因为返回的ACK被损坏，源站点在一个短的时间周期中没有收到ACK，它会重发该帧。这样，IEEE80211中的基本数据发送机制涉及两个帧的交换。为了更进一步地增强可靠性，可以使用四帧交换。在此模式中，源站点首先向目的站点发布一个请求发送REQUESTTOSEND，RTS帧。然后，目的站点用一个清除发送CLEARTOSEND，CTS帧响应。在收到CTS之后，源站点发送数据帧，而目的站点以一个ACK响应。RTS警告所有位于源站点接收范围内的站点一个交换正在进行。为了避免同时发送帧产生冲突，这些站点会抑制帧的发送。类似