计算机 毕业论文 外文文献翻译 中英文IEEE80211 媒体接入控制.docx
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计算机 毕业论文 外文文献翻译 中英文IEEE80211 媒体接入控制.docx
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计算机毕业论文外文文献翻译中英文IEEE80211媒体接入控制
英文资料与中文翻译
IEEE802.11MEDIUMACCESSCONTROL
TheIEEE802.11MAClayercoversthreefunctionalareas:
reliabledatadelivery,mediumaccesscontrol,andsecurity.Thissectioncoversthefirsttwotopics.
ReliableDataDelivery
Aswithanywirelessnetwork,awirelessLANusingtheIEEE802.11physicalandMAClayersissubjecttoconsiderableunreliability.Noise,interference,andotherpropagationeffectsresultinthelossofasignificantnumberofframes.Evenwitherror-correctioncodes,anumberofMACframesmaynotsuccessfullybereceived.Thissituationcanbedealtwithbyreliabilitymechanismsatahigherlayer.suchasTCP.However,timersusedforretransmissionathigherlayersaretypicallyontheorderofseconds.ItisthereforemoreefficienttodealwitherrorsattheMAClevel.Forthispurpose,IEEE802.11includesaframeexchangeprotocol.Whenastationreceivesadataframefromanotherstation.Itreturnsanacknowledgment(ACK)frametothesourcestation.Thisexchangeistreatedasanatomicunit,nottobeinterruptedbyatransmissionfromanyotherstation.IfthesourcedoesnotreceiveanACKwithinashortperiodoftime,eitherbecauseitsdataframewasdamagedorbecausethereturningACKwasdamaged,thesourceretransmitstheframe.
Thus,thebasicdatatransfermechanisminIEEE802.11involvesanexchangeoftwoframes.Tofurtherenhancereliability,afour-frameexchangemaybeused.Inthisscheme,asourcefirstissuesarequesttosend(RTS)frametothedestination.Thedestinationthenrespondswithacleartosend(CTS).AfterreceivingtheCTS,thesourcetransmitsthedataframe,andthedestinationrespondswithanACK.TheRTSalertsallstationsthatarewithinreceptionrangeofthesourcethatanexchangeisunderway;thesestationsrefrainfromtransmissioninordertoavoidacollisionbetweentwoframestransmittedatthesametime.Similarly,theCTSalertsallstationsthatarewithinreceptionrangeofthedestinationthatanexchangeisunderway.TheRTS/CTSportionoftheexchangeisarequiredfunctionoftheMACbutmaybedisabled.
MediumAccessControl
The802.11workinggroupconsideredtwotypesofproposalsforaMACalgorithm:
distributedaccessprotocols,which,likeEthernet,distributethedecisiontotransmitoverallthenodesusingacarrier-sensemechanism;andcentralizedaccessprotocols,whichinvolveregulationoftransmissionbyacentralizeddecisionmaker.Adistributedaccessprotocolmakessenseforanadhocnetworkofpeerworkstations(typicallyanIBSS)andmayalsobeattractiveinotherwirelessLANconfigurationsthatconsistprimarilyofbursttraffic.AcentralizedaccessprotocolisnaturalforconfigurationsinwhichaumberofwirelessstationsareinterconnectedwitheachotherandsomesortofbasestationthatattachestoabackbonewiredLAN:
itisespeciallyusefulifsomeofthedataistimesensitiveorhighpriority.
Theendresultfor802.11isaMACalgorithmcalledDFWMAC(distributedfoundationwirelessMAC)thatprovidesadistributedaccesscontrolmechanismwithanoptionalcentralizedcontrolbuiltontopofthat.Figure14.5illustratesthearchitecture.Thelowersub-layeroftheMAClayeristhedistributedcoordinationfunction(DCF).DCFusesacontentionalgorithmtoprovideaccesstoalltraffic.OrdinaryasynchronoustrafficdirectlyusesDCE.Thepointcoordinationfunction(PCF)isacentralizedMACalgorithmusedtoprovidecontention-freeservice.PCFisbuiltontopofDCFandexploitsfeaturesofDCFtoassureaccessforitsusers.Letusconsiderthesetwosub-layersinturn.
Figure14.5IEEE802.11ProtocolArchitecture
DistributedCoordinationFunction
TheDCFsub-layermakesuseofasimpleCSMA(carriersensemultipleaccess)algorithm,whichfunctionsasfollows.IfastationhasaMACframetotransmit,itlistenstothemedium.Ifthemediumisidle,thestationmaytransmit;otherwisethestationmustwaituntilthecurrenttransmissioniscompletebeforetransmitting.TheDCFdoesnotincludeacollisiondetectionfunction(i.e.CSMA/CD)becausecollisiondetectionisnotpracticalonawirelessnetwork.Thedynamicrangeofthesignalsonthemediumisverylarge,sothatatransmittingstationcannoteffectivelydistinguishincomingweaksignalsfromnoiseandtheeffectsofitsowntransmission.
Toensurethesmoothandfairfunctioningofthisalgorithm,DCFincludesasetofdelaysthatamountstoapriorityscheme.Letusstartbyconsideringasingledelayknownasaninter-framespace(IFS).Infact,therearethreedifferentIFSvalues,butthealgorithmisbestexplainedbyinitiallyignoringthisdetail.UsinganIFS,therulesforCSMAaccessareasfollows(Figure14.6):
Figure14.6IEEE802.11MediumAccessControlLogic
1.Astationwithaframetotransmitsensesthemedium.Ifthemediumisi
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