计算机网络复习重点(重点终结版)

物理拓扑结构（physicaltopology）网状拓扑结构（meshtopology）各台设备之间都有一条专用的点到点的链路，每台设备必须有n-1个I/O端口星型拓扑结构（startopology）每台设备拥有一条仅与中央控制器连接的点到点专用链路总线拓扑结构（bustopology）环状拓扑结构（ringtopology）协议和标准:协议是用来管理数据通信的一组规则。协议规定了通信的内容、通信的方式和通信的时间。标准是由标准化组织协商达成一致的规则。因特网模型（Internetmodel）物理层;数据链路层;网络层;传输层;应用层AnexchangeusingtheInternetmodelPhysicallayerNote:Thephysicallayerisresponsiblefortransmittingindividualbitsfromonenodetothenext.DatalinklayerNote:Thedatalinklayerisresponsiblefortransmittingframesfromonenodetothenext.NetworklayerNote:Thenetworklayerisresponsibleforthedeliveryofpacketsfromtheoriginalsourcetothefinaldestination.TransportlayerNote:Thetransportlayerisresponsiblefordeliveryofamessagefromoneprocesstoanother.ApplicationlayerNote:Theapplicationlayerisresponsibleforprovidingservicestotheuser.Summaryofduties===========================================================PositionofthephysicallayerNote:Frequencyandperiodareinversesofeachother.Note:Adigitalsignalisacompositesignalwithaninfinitebandwidth.Thebitrateandthebandwidthareproportionaltoeachother.Theanalogbandwidthofamediumisexpressedinhertz;thedigitalbandwidth,inbitspersecond.Digitaltransmissionneedsalow-passchannel.Analogtransmissioncanuseaband-passchannel.noiselesschannelnoisychannelAttenuationDistortionNoise===========================================================Unipolar:Unipolarencodingusesonlyonevoltagelevel.Polar:Polarencodingusestwovoltagelevels(positiveandnegative).NRZ-L:InNRZ-Lthelevelofthesignalisdependentuponthestateofthebit.NRZ-I:InNRZ-Ithesignalisinvertedifa1isencountered.RZencodingAgoodencodeddigitalsignalmustcontainaprovisionforsynchronization.ManchesterencodingInManchesterencoding,thetransitionatthemiddleofthebitisusedforbothsynchronizationandbitrepresentation.DifferentialManchesterencodingIndifferentialManchesterencoding,thetransitionatthemiddleofthebitisusedonlyforsynchronization.Thebitrepresentationisdefinedbytheinversionornoninversionatthebeginningofthebit.BipolarInbipolarencoding,weusethreelevels:positive,zero,andnegative.BipolarAMIencodingFromanalogsignaltoPCMdigitalcodeAccordingtotheNyquisttheorem,thesamplingratemustbeatleast2timesthehighestfrequencyHowmanybitsshouldbesentforeachsample?1bitforthesignandXbitsforthevalue.===========================================================Bitrateisthenumberofbitspersecond.Baudrateisthenumberofsignalunitspersecond.Baudrateislessthanorequaltothebitrate.ASKRelationshipbetweenbaudrateandbandwidthinASKInASKthebaudrateandbitratearethesame.AnASKsignalrequiresaminimumbandwidthequaltoitsbaudrateFSKInFSKthebaudrateandbitratearethesame.BW=fc1-fc0+NbaudPSKThe4-PSKmethodThe8-PSKcharacteristicsRelationshipbetweenbaudrateandbandwidthinPSKAtelephonelinehasabandwidthofalmost2400Hzfordatatransmission.ThetotalbandwidthrequiredforAMcanbedeterminedfromthebandwidthoftheaudiosignal:BWt=2xBWm.ThetotalbandwidthrequiredforFMcanbedeterminedfromthebandwidthoftheaudiosignal:BWt=10xBWm.Thebandwidthofastereoaudiosignalisusually15KHz.Therefore,anFMstationneedsatleastabandwidthof150KHz.TheFCCrequirestheminimumbandwidthtobeatleast200KHz(0.2MHz).============================================================multiplexingFDMisananalogmultiplexingtechniquethatcombinessignals.FDMprocess&&FDMdemultiplexingexampleWDMWDMisananalogmultiplexingtechniquetocombineopticalsignals(光信号).TDMTDMisadigitalmultiplexingtechniquetocombinedata.InaTDM,thedatarateofthelinkisntimesfaster,andtheunitdurationisntimesshorter.Radiowavesareusedformulticastcommunications,suchasradioandtelevision,andpagingsystems.Microwavesareusedforunicastcommunicationsuchascellulartelephones,satellitenetworks,andwirelessLANs.Infraredsignalscanbeusedforshort-rangecommunicationinaclosedareausingline-of-sightpropagation.============================================================ADSLisanasymmetriccommunicationtechnologydesignedforresidentialusers;itisnotsuitableforbusinesses.Theexistinglocalloopscanhandlebandwidthsupto1.1MHz.语音通信带宽为4kDataLinkLayer奇偶性校验CRC校验(循环冗余码校验)校验和校验Simpleparitycheckcandetectallsingle-biterrors.Itcandetectbursterrorsonlyifthetotalnumberoferrorsineachdataunitisodd.Two-dimensionalparityBinarydivisioninaCRCgeneratorChecksumPositionsofredundancybitsinHammingcodeStop-and-WaitARQPiggybackingGo-Back-NARQGo-Back-NARQ:senderwindowsizeInGo-Back-NARQ,thesizeofthesenderwindowmustbelessthan2m;thesizeofthereceiverwindowisalways1.SelectiveRepeatARQInSelectiveRepeatARQ,thesizeofthesenderandreceiverwindowmustbeatmostone-halfof2m.HDLC高级数据链路控制HDLCframeBitstuffing(比特填充)istheprocessofaddingoneextra0whenevertherearefiveconsecutive1sinthedatasothatthereceiverdoesnotmistakethedataforaflag.Point-to-Point12.1PPPframeTransitionstatesMultiple-accessprotocolsFigure13.5CSMApersistentstrateg13.3ChannelizationFDMA:InFDMA,thebandwidthisdividedintochannels.TDMA:InTDMA,thebandwidthisjustonechannelthatistimeshared.CDMA:InCDMA,onechannelcarriesalltransmissionssimultaneously.Figure13.16CDMAmultiplexer=======================================================================================================================