英文資料與中文翻譯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媒體接入控制IEEE80211MAC層覆蓋了三個功能區(qū)可靠的數據傳送、接入控制以及安全。本節(jié)先討論前兩個內容?？煽康臄祿魉团c所有的無線網絡一樣，一個使用IEEE80211物理層和MAC層的無線LAN被認為是不可靠的。噪聲、干擾和其他傳播效果導致極大量的幀丟失。即使帶有糾錯碼，大量MAC幀也可能無法成功地被收到。可通過在諸如TCP的高層使用可靠性機制處理這種情況。然而，在更高層中用于重發(fā)的計時器一般是以秒為單位的。因此，在MAC層處理錯誤會更有效。為了達到這個目的，IEEE80211包括了幀交換協(xié)議。當一個站點收到從另一個站點發(fā)來的數據幀時，它向源站點返回一個確認ACKNOWLEDGMENT，ACK幀。此交換被作為一個原子單元處理，它不會被其他站點發(fā)出的傳送打斷。如果因為數據幀被損壞或因為返回的ACK被損壞，源站點在一個短的時間周期中沒有收到ACK，它會重發(fā)該幀。這樣，IEEE80211中的基本數據發(fā)送機制涉及兩個幀的交換。為了更進一步地增強可靠性，可以使用四幀交換。在此模式中，源站點首先向目的站點發(fā)布一個請求發(fā)送REQUESTTOSEND，RTS幀。然后，目的站點用一個清除發(fā)送CLEARTOSEND，CTS幀響應。在收到CTS之后，源站點發(fā)送數據幀，而目的站點以一個ACK響應。RTS警告所有位于源站點接收范圍內的站點一個交換正在進行。為了避免同時發(fā)送幀產生沖突，這些站點會抑制幀的發(fā)送。類似