2023年全國碩士研究生考試考研英語一試題真題（含答案詳解+作文范文）_第1頁
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1、英文資料與中文翻譯英文資料與中文翻譯IEEE802.11MEDIUMACCESSCONTROLTheIEEE802.11MAClayercoversthreefunctionalareas：reliabledatadeliverymediumaccesscontrolsecurity.Thissectioncoversthefirsttwotopics.ReliableDataDeliveryAswithanywirelesswkawi

2、relessLANusingtheIEEE802.11physicalMAClayersissubjecttoconsiderableunreliability.Noiseinterferenceotherpropagationeffectsresultinthelossofasignificantnumberofframes.EvenwitherrcrectioncodesanumberofMACframesmaynotsuccess

3、fullybereceived.Thissituationcanbedealtwithbyreliabilitymechanismsatahigherlayer.suchasTCP.Howevertimersusedfretransmissionathigherlayersaretypicallyonthederofseconds.ItistherefemeefficienttodealwitherrsattheMAClevel.Fth

4、ispurposeIEEE802.11includesaframeexchangeprotocol.Whenastationreceivesadataframefromanotherstation.Itreturnsanacknowledgment(ACK)frametothesourcestation.Thisexchangeistreatedasanatomicunitnottobeinterruptedbyatransmissio

5、nfromanyotherstation.IfthesourcedoesnotreceiveanACKwithinashtperiodoftimeeitherbecauseitsdataframewasdamagedbecausethereturningACKwasdamagedthesourceretransmitstheframe.ThusthebasicdatatransfermechanisminIEEE802.11involv

6、esanexchangeoftwoframes.Tofurtherenhancereliabilityafourframeexchangemaybeused.Inthisschemeasourcefirstissuesarequesttosend(RTS)frametothedestination.Thedestinationthenrespondswithacleartosend(CTS).AfterreceivingtheCTSth

7、esourcetransmitsthedataframethedestinationrespondswithanACK.TheRTSalertsallstationsthatarewithinreceptionrangeofthesourcethatanexchangeisunderwaythesestationsrefrainfromtransmissionindertoavoidacollisionbetweentwoframest

8、ransmittedatthesametime.SimilarlytheCTSalertsallstationsthatarewithinreceptionrangeofthedestinationthatanexchangeisunderway.TheRTSCTSptionoftheexchangeisarequiredfunctionoftheMACbutmaybedisabled.MediumAccessControlalgith

9、mwhichfunctionsasfollows.IfastationhasaMACframetotransmititlistenstothemedium.Ifthemediumisidlethestationmaytransmitotherwisethestationmustwaituntilthecurrenttransmissioniscompletebefetransmitting.TheDCFdoesnotincludeaco

10、llisiondetectionfunction(i.e.CSMACD)becausecollisiondetectionisnotpracticalonawirelesswk.Thedynamicrangeofthesignalsonthemediumisverylargesothatatransmittingstationcannoteffectivelydistinguishincomingweaksignalsfromnoise

11、theeffectsofitsowntransmission.ToensurethesmoothfairfunctioningofthisalgithmDCFincludesasetofdelaysthatamountstoapriityscheme.Letusstartbyconsideringasingledelayknownasaninterframespace(IFS).InfacttherearethreedifferentI

12、FSvaluesbutthealgithmisbestexplainedbyinitiallyigningthisdetail.UsinganIFStherulesfCSMAaccessareasfollows(Figure14.6):WaitefframetotransmitMediumidleWaitIFSStillidleTransmitframeWaituntilcurrenttransmissionendsWaitIFSSti

13、llidleExponentialbackoffwhilemediumidleTransmitframeNoYesNoYesYesNoFigure14.6IEEE802.11MediumAccessControlLogic1.Astationwithaframetotransmitsensesthemedium.Ifthemediumisidle.Itwaitstoseeifthemediumremainsidlefatimeequal

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