本科毕业设计的英文资料与中文翻译

天津工程师范学院2007届本科生毕业设计1英文资料与中文翻译IEEE802.11MEDIUMACCESSCONTROLTheIEEE802.11MAClayercoversthreefunctionalareas：reliabledatadelivery,mediumaccesscontrol,andsecurity.Thissectioncoversthefirsttwotopics.ReliableDataDeliveryAswithanywirelessnetwork,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.天津工程师范学院2007届本科生毕业设计2MediumAccessControlThe802.11workinggroupconsideredtwotypesofproposalsforaMACalgorithm: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.802.11g2.4-GHzDSSS,OFDMLogicallinkcontrol(LLC)Distributedcoordinationfunction(DCF)Pointcoordinationfunction(PCF)802.112.4-GHzFHSS802.112.4-GHzDSSS802.11Infrared802.11a5-GHzOFDM802.11b2.4-GHzDSSSContention-freeserviceContentionserviceMAClayerFigure14.5IEEE802.11ProtocolArchitecture天津工程师范学院2007届本科生毕业设计3DistributedCoordinationFunctionTheDCFsub-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):WaiteforframetotransmitMediumidle?WaitIFSStillidle?TransmitframeWaituntilcurrenttransmissionendsWaitIFSStillidle?ExponentialbackoffwhilemediumidleTransmitframeNoYesNoYesYesNoFigure14.6IEEE802.11MediumAccessControlLogic天津工程师范学院2007届本科生毕业设计41.Astationwithaframetotransmitsensesthemedium.Ifthemediumisidle.ItwaitstoseeifthemediumremainsidleforatimeequaltoIFS.Ifso,thestationmaytransmitimmediately.2.Ifthemediumisbusy(eitherbecausethestationinitiallyfindsthemediumbusyorbecausethemediumbecomesbusyduringtheIFSidletime),thestationdeferstransmissionandcontinuestomonitorthemediumuntilthecurrenttransmissionisover.3.Oncethecurrenttransmissionisover,thestationde