工学通信网基础新第1章课件

CommunicationNetworks

AFirstCourse通信网络基础1Chapter1IntroductiontoCommunicationNetworksIntroductionto…..…..的入门（介绍，引言）Communication：单数：“通信，交流”这一行为复数：“通信”这一学科领域DefinitionofCommunicationNetworksEvolutionofCommunicationNetworksOrganizationofthebook21.1WhatarecommunicationnetworkCommunicationnetworksarearrangementsofhardware&softwarethatallowuserstoexchangeinformation.1.definition32.typesTelephonenetworkisdesignedforvoicetransmissionOfficecomputernetwork——personalcomputers/workstations——shareprograms，dataandsomeotherperipheralsInternet——coversmostoftheworld——exchangemessagesandcomputerfilesandsomevideoandaudiosignals43.notesAlthoughallthesesystemsarecommunicationnetworks,theyarequitedifferentintheinformationthattheytransmitandinthewaytheyareused.Nevertheless,theyoperateonsimilarprinciples.Eachsystemdescribedisdesignedtoexchangeinformation,whichmaybevoice,sounds,graphics,pictures,video,textordataamongusers.5DigitaltransmissionBeforetheinformationistransmitted,itisconvertedintobits(zerosorones).Thenthebitsaresenttoareceiveraselectricaloropticalsignals.Finally,theinformationisreconstructedfromthereceivedbits.Digitaltransmissionreducesthetransmissionerrors.（误码率低）…011001110001…6Lineconfiguration:therelationbetweencommunicationdevicesandlinkLink:thephysicalpaththatdatatransferfromonedevicetoanotherdeviceTopology7Point-to-pointLineConfigurationThetransmissionfromsomeuserAtoanotherBcantakeplaceoverapoint-to-pointcommunicationlink.（点对点的传输链路）ABCommunicationlink8Thephysicalmediumthatsupportsthiscommunicationmaybe

acable,copperwires,anopticalfiber,aradiolink,oralightwaveinfreespace.（电缆、铜线、光纤、无线电、空中的光波）9Point-to-PointLineConfiguration10TwotypesoflineconfigurationPoint-to-pointLineConfiguration:Adedicatedlinkcanbeusedtoconnecttwocomputerspermanently.Multipointlineconfiguration:Asharedlinkcanbeusedtoconnectionmorethantwocommunicationdevices11MultipointLineConfiguration12Figure1.2a:meshtopologymeshtopology:Eachnodeisconnectedbyadedicatedlinkwithothernodes13featureEachpairofdevicesisconnectedbyadedicatedlinkThissolutionisnotfeasibleforalargenumberofdevices，becauseofn(n-1)/2transmissionlinesarerequiredforinterconnectingndevicesinsolutionaThecostwouldbeprohibitiveResourceswasted14Figure1.2b:startopologystartopology:eachdeviceisonlyconnectedbyadedicatedlinkwithcentralcontrollersuchasahub.Eachpairofdevicesisnotconnected.15featureSomelinksareusedbydifferentpairsofdevices.Anetworkisorganizedsoastohavedifferentinformationflowssharecommunicationlinks.Solutionbachievessavingsinlinklength.NtransmissionlinesarerequiredforinterconnectingndevicesinsolutionbOneofthemainnetworkdesignproblemsistofindefficientwaysofsharingcommunicationlinks.16DefinitionAcommunicationnetworkisasetofnodesthatareinterconnectedtopermittheexchangeofinformation.（通信网络是用来交换信息的互联节点组）Nodes:Terminalentities:suchasphonesets,computers,printers,fileserversorvideomonitorsCommunicationdevices:switches,routers,repeaters(中继器）Terminalnodes（终端节点）:generateoruseinformationCommunicationnodes（通信节点）:transportinformation171.4EvolutionofCommunicationNetworks

Weexaminethemainstepsintheevolutionofthetelephoneandcomputernetworks我们将介绍给用户提供网络服务的通信网络发展的主要过程。这个过程有很多技术上的创新，我们所介绍的技术创新是在通信网络发展过程中最具有决定性意义的。181.4.1TelephoneNetworkTelegraph(1830s)Telephone(1876)Firsttelephonesystems(1880s)Switch:humanoperatorsElectromagneticswitchescomputerizedswitches数字传输、CCS（公共信道信令，信令和语音传输相分离）、业务集成、光纤链路以及电路交换19DigitalTransmissionandCCSCCS:commonchannelsignaling公共信道信令Signaling≠information信令是和通信有关的一系列控制信号。信令不同于用户信息，用户信息在网络中只是“透明”（即未经任何变化）传输，而信令却在网络的每一节点被分析处理，并导致一系列的控制操作。20DigitalTransmissionDigitaltelephonetransmissiontransmitsthevoicesignalsasbitstreams.Suchtransmissionshavelownoiselevels.（信噪比高）Facilitatetheswitchingofsignalsandthesimultaneoustransmissionofmanysignalsonthesameline.（交换和复用更方便）21switchingClassificationCircuitswitching电路交换Messageswitching报文交换Packetswitching：分组交换Datagrampacketswitching数据报分组交换Virtualcircuitswitching虚电路交换22multiplexingDefinition:permitstwoormorechannelsshareonecommunicationlink.允许多个通道共享一条通信链路的数字传输技术。MUXDEMUXOnelinkThreechannel23

Typesofmultiplexing

TDM(Time-DivisionMultiplexing)FDM(Frequency-DivisionMultiplexing)CDM(Code-DivisionMultiplexing)WDM(Wave-DivisionMultiplexing)24Multiplexing,FrequencyDomain25Demultiplexing,FrequencyDomain26TDM27CCS（公共信道信令）Commonchannelsignalingtransmitsconnectioncontrolinformationamongtelephoneswitchingequipment.Thiscontrolinformationpermitsefficientimplementationsofmanyservices,suchascallforwarding,creditcardcalls,and800numbers.（呼叫转移、信用卡呼叫、800免费电话）28IntegrationofServicesDigitaltransmissionandcommonchannelsignalingformthebasisoftheISDNs(综合业务数字网）TDM(Time-DivisionMultiplexing:时分多路复用）2Bchannels:transmitvoiceorcomputerdatadigitaltransmission用于传送用户数据。1Dchannel:transmitsignaling(controlinformation)CCS用于传送控制信息。64kbps:8000valuespersecond,8bits/valueB2(64kbps)B1(64kbps)D(16kbps)TDM29综合业务数字网(ISDN)就是在一个统一的网络系统内传送和处理各种类型的数据，向用户提供多种业务服务，如电话、传真、视频以及数据通信业务等。30SynchronousNetwork(同步网络）ThetelephonenetworksareimplementingatransmissiontechnologycalledtheSONETintheUnitedStates,compatiblewithandsimilartotheSDHusedinEuropeandJapan.SONET:synchronousopticalnetwork同步光网络是一种北美光纤网的标准，定义了支持数据传输率为52Mbps整数倍的点到点专用服务。

SDH:synchronousdigitalhierarchy同步数字体系实际上，SONET(SDH)是个TDM(Time-DivisionMultiplexing,时分复用）系统。31InaSONETnetwork:Thetransmittersofthetelephonecompanyaresynchronizedtoacommonclock.取得同步复用的方法是不同输入端被公共主时钟控制以达到同步要求。Equipmentcombinesasetofsynchronizedsignalsfromdifferenttransmittersbyinterleavingtheirbytes.来自不同输入端的一组同步信号通过字节交错的多路复用器（即一个输入端的单个字节跟来自另一个输入端的单个字节交错在一起）组合起来。32源1源2源N同步字节流字节交替多路复用Transmittingrate=n*52Mbps公共时钟33Suchastraightforwardinterleavingisnotpossiblewithtransmittersthatarenotsynchronized.34Circuitswitching（电路交换）UsedbythetelephonenetworktodeliverinformationThecomputersetsupacircuitbetweenthetwotelephoneterminalsforthedurationoftheconversationandreleasesthecircuitwhenthecallterminates.ConnectionsetupTransmitinformation(callmaintenance)ConnectionteardownCircuit:constant-ratelogicchannelAdurablecircuit(64kbps)TelephoneterminalTelephoneterminal35电路交换的三个阶段源节点向网络发出连接或呼叫请求,网络根据路由为数据流选择要经过的每条链路,然后发出呼叫请求被许可的通知(如果网络不能完成分配工作,呼叫将被拒绝).现在要进行数据传输,这个阶段称为呼叫保持阶段.在数据传输完成后,需要将连接拆除.36FeaturesThemechanismissimple.(equipment,operatingprinciples)Theutilizationfactoroflinkislow.Forthedurationoftheconversation,theseresourcesareallocatedtothatconversationandcannotbesharedbyanyothercommunication.电话交换在两个要求通信的节点之间建立一条专用的通信链路，当数据传送结束，才可把链路提供给别的用户使用，故链路利用率不高。37Thetransmissionrateisfixable.在一个电话交换网络中，连接以固定数据速率传输。这样被连接的两台设备中每一台必须以相同的数据率传送和接收，限制了互连多种不同主机和终端时网络的利用率Minimizesthedelayoftheinformationtransfer.38从资源分配角度来看，电路交换是预先分配链路带宽的。用户在开始通话之前，先要通过拨号申请建立一条从发送端到接收端的物理通路。只有在此物理通路建立之后，双方才能通话。在通话过程中，用户始终占有从发送端到接收端的固定传输带宽。39课堂练习：翻译第4页1.3WhatShouldYouLearnaboutCommunicationNetworks?40Datatransmissionparalleltransmissionserialtransmissionasynchronouslinksynchronouslink1.4.2ComputerNetworks41paralleltransmissionparalleltransmissionisfasterthanserialtransmissionuneconomical42

serialtransmission43Datatransmittedinthiswayissentandreceivedbitbybit.SuchatransmissionismoreeconomicalthanparalleltransmissionAserialtoparallelconversiontakesplaceattheinterfaceofthesenderTwotypesofserialtransmission:asynchronouslinkandsynchronouslink44

1.Asynchronouslink异步通信Theneedforstandardizedconnectionsofacomputertosuchperipheralsastapedrives,keyboards,printers,diskdrives,andterminalsledtothepublicationofthespecificationsoftheconnectionRS-232-CbytheEIAin1969.Thisconnectionallowstransmissionratesofupto38,400bitsto15meters.TheRS-232-Cconnectionstilliswidelyusedandisknownastheserialport.451.4.2ComputerNetworks

1.Asynchronouslink异步通信Serialport:串行端口。计算机处理和输出的是并行数据。首先必须把并行数据转化为串行数据，然后才能发送出去；或者把接收的串行数据转化为并行数据再输入计算机中，担任对数据进行转化和控制的是串行通信接口。其标准为RS-232-C。ParalleldataserialdataEIA:ElectronicsIndustriesAssociation美国电子工业协会当数据传输率为38,400bps时，双绞线长度可达15米。RS-232-C46DTE-DCEinterfaceDataterminalequipment(DTE):suchastapedrivers,printers,diskdrivers,andterminals数据终端设备Datacircuit-terminatingequipment(DCE):suchasmodems—数据电路终接设备DTEDCEnetworkDCEDTE47在数据通信中一般涉及到四个基本单元:两端各有一个DTE和DCE。DTE产生数据,并连同必要的控制字符一起传送给DCE。DCE将信号转化成适合于传输介质的形式并将它发送到网络中。当信号到达另一端时,将发生相反的过程。DTE:可作为二进制数字数据源点或终点的任何设备。DCE:通过网络,能够传送和接收模拟或数字数据的任何功能单元都称为DCE。两个DTE设备不需要互相协调,但是每个DTE设备必须与它所连接的DCE设备协调工作。48DTE-DCEinterfacestandardspecifiestheelectricalandmechanicalcharacteristicsoftheinterconnections.RS-232-CDTEDCEnetworkDCEDTEDTE-DCEinterfacestandard4950Asynchronoustransmissionsendsandreceivescharacterbycharacter.Onecharacterissentatatime,andtwocharactersmustbeseparatedbyaminimumtimeinterval.Acharacterisrepresentedbyagroupof7or8bits,dependingonthecodebeingused.Thistypeoftransmissioniscalledasynchronousbecausesuccessivecharacterscanbetransmittedatarbitrarytimes,otherthanfortheminimumseparation.异步通信发送和接收数据是一个字符一个字符进行。每次只发送一个字符，计算机设备将每个字符进行7或8比特编码.另外,每发送完一个字节,可能还要跟上一段可变长的时间间隙。这段间隔或者通过信道空闲状态代表,或者通过附加的停止位流代表。51Asynchronous:thereceiverstartsitsclockwhenitdetectsthestartofthecharacter.Usingthisclock,thereceiverreadoffthesuccessivebits.当接收端检测到一个起始位后，它启动时钟以定时接收后续的字符。在接收完n个比特后，接收方就等待停止位到达。当检测到停止位时，接收方在下一个起始位到达前忽略接收的所有信号1|11111011|0startbitendbitdatasenderreceiver1|11111011|01|00111011011|0idletimepertransmitteddata5254AsynchronousTransmission53ModemsModems:permitthetransmissionofbitsovertelephonelines.Modulation:convertsbitsintosoundsinthefrequencyrangetransmittedbytelephonelines.bitssoundsdigital-to-analogconversion调制是把数字信号变换为模拟信号的过程。(D/A)Demodulation:convertssuchsoundsbackintobitssoundsbitsanalog-to-digitalconversion解调是从模拟信号恢复数字信号的过程。

(A/D)54Figure:Along-distanceconnectionComputerAModemModemComputerBTelephonelineAlong-distanceconnectionToimplementalong-distanceconnectionbetweentwocomputers,onecanconnectthecomputerAtoamodem,themodemtoatelephoneline,andtheotherendofthelinetoanothermodemthatisconnectedtothecomputerB.55Summary:通过调制解调器，可以实现两个计算机之间的远程通信，上图是实现两个计算机通过电话网进行通信的联接框图。在这个框图中，通过联接到微机的串行通信口，调制解调器和电话网构成了两个计算机之间的传输信道。InformationsourceTransmitter(modulator)TransmissionMedium(TelephoneNetwork)Receiver(demodulator)Destination(PC)Communicationsystem56Synchronouslink:同步链路Computerswerefirstinterconnectedwithpoint-to-pointlinksinthemid-1960s.TheneedforfastconnectionswithautomaticerrorcontrolledtothedevelopmentofasetofprocedurescalleddatalinkprotocolsknownundertheabbreviationsSDLC,LAPA,LAPB,andHDLC.20世纪70年代的同步传输标准增加了传输速率和传输链路的有效距离。这个标准称为同步数据链路控制（SDLC)协议。很多协议是基于SDLC的，包括HDLC、LAPB、LAPD。57Packet（数据包、分组）packet:agroupofbitsFormat:atypicalpacketconsistsofaheader,userinformation,andatrailerHeader:containstheaddressesofthedestinationandofthesourceofthepacket.Data:userinformationTrailer:errorcontrolbitsPacket:一组同时发送的数据位，包括地址、数据和控制位，是信息的基本单元。HeaderDataTrailer58processThetransmitterencodesthepacketbitsinasignalthatcontainsthetiminginformation.Thereceiverextractsthattiminginformationtorecoverthebitsaccurately.Thissynchronizationofthereceiverthroughthetiminginformationcontainedinthesignalenablesthetransmissionoflongpackets.SDLCH|10100011|11111011|11110110|CRC59WhysynchronoustransmissionsfasterthanasynchronoustransmissionsThereisaminimumseparationrequirementbetweensuccessivepacketsinsteadofbetweensuccessivecharacters,aswasthecaseforasynchronoustransmissionsTheamountofinsertedidletimepertransmittedbitissmallerforsynchronoustransmissionsthanforasynchronoustransmissions60在同步传输中，比特流被组装成更长的“packet”，一“packet”包含有许多个字节。与异步方式不同的是，引入“packet”内的字节与字节之间没有间隔，需要接收方在解码时将比特流分解成字节。即数据被当作不间断的0和1比特流传输，而由接收方来将比特流分割成重建信息所需的一个个字节。同步传输比异步传输快：同步传输每比特插入空闲时间要比异步传输小，即同步传输避免RS-232-C(串行传输)由于连续字符的空白间隔而造成的时间浪费。SDLCH|10100011|11111011|11110110|CRC61两种synchronous的理解InSONET:refertothefactthatthetransmittersaresynchronizedtoacommonclocktoenablethebyte-interleavedmultiplexing.所有的输入端被主公共时钟同步，以使用字节交错的多路复用。（TDM）62Insynchronouslink:refertothesynchronizationofthereceiverbythetiminginformationcontainedinthesignalthatcarriesthebitstream.在同步链路中，同步是指接收者通过数据中有关时间信息的自同步码进行同步。（归零码含有同步信息）63Store-and-ForwardTransmission(存储转发传输）IfcomputerBisconnectedtocomputerAandCbytwopoint-to-pointlinks,itispossibletosendmessagesfromAtoCbysendingthemfromAtoB,thenfromBtoCMessageSwitching：报文交换ABC64MessageSwitching：报文交换65Astore-and-forwardtransmissionfromAtoCviaBismoreefficientifthetransmissionfromBtoCcanstartbeforethatfromAtoBiscompleted.ABCmessage66packetswitching:分组交换Todothis,onemustdecomposethemessageintorelativelysmallpackets.ThisdecompositionallowsthetransmissionfromBtoCtostartassoonasBhasreceivedonepacketfromAinsteadofhavingtowaituntilBhasreceivedacompletemessagefromA.ABCpacket67exampleAwantstosendalongmessagetoCandthatthetransmissionofthatmessageonadirectlinktakes1minute.IfAsendsthecompletemessagetoB(whichtakes1minute)beforeBsendsittoC(whichtakesanotherminute),thenittakes2minutesbeforethemessageisreceivedbyCABCmessageAB1’BC1’2minutes68Ifthemessageisdecomposedinto60packetsthattake1secondeachtobetransmittedonadirectlink.Then60packetscanbesentfromAtoCin61seconds12601”AB2”ABC3”ABC1’1”ABC126012112369IfthereareNintermediatenodes(中间结点),thetransmissionoftheundividedmessagetakesN+1minuteswhilethatofthe60packetstakesonly1minuteandNseconds.Thetransmissionofmessagesassmallpacketsiscalledstore-and-forwardpacket-switching.Pipelininggain（流水线增益，管道增益）70Summary:存储转发传输将待发送的信息分割成分组或包（信息的逻辑单位），在包中附加包含对方节点地址等有关信息的头和尾，形成一份份分组，然后以包为单位在网络上从一个节点传送到另一个节点，在每个节点接收包后进行暂时存储，然后根据包的目的节点地址，寻找一个空闲通路，将此信息转发给下一个节点，直到到达目的地址为止。由于这种交换方式的节点可把待传送的信息存储起来，等到链路空闲时再转发出来，故称为存储转发。71Advantages(comparisonofcircuitswitching)Thelinkisnotreservedbyanyonecommunication.Instead,thelinkissharesbymanycommunicationandeachusesthelinkonlywhenithasapackettosendonthatlinkTheutilizationfactoroflinkishigh.链路利用率高，一个节点与另一个节点之间的连接链路可以为多个包传输分时使用。传完一个包后，链路可以为别的包（可能属于另一个报文或对话）占用。Ifthereceiverisbusy,thepacketcanstoreinsomenodeandwaitforafreelink.它不要求发送者与接收者同时处于工作状态，在接收者“忙”时，网络节点可先将信息存储在中间某个节点等待链路的空闲。在电路交换方式，当网络信息量增大，只好拒绝一些传输请求。72Apacketcanbesenttomorereceiveratonetime.一个包可以同时发给多个接收者，这在电路交换难以实现。Apacket-switchingenablehigh/lowtransmissionrateconversion.可以实现高/低速网络的转换。两个不同数据率的站点之间能交换分组，因为每一个站点以它自己的数据率连接到这个节点上。731.4.6Datagrampacketswitching

(数据报分组交换）TheInternettransportspacketsoneatatime.Eachpacketismarkedwithitsdestinationaddress.Whenanodegetsapacket,itreadsitsdestinationaddressandfiguresoutwheretosendthepacketnext.Ifconditions(congestion,alinkmayfail)changeinthenetwork,successivepacketsthatbelongtothesamecommunicationbetweentwocomputersmayfollowdifferentpathsinthenetworks.74Datagrampacketswitching(数据报分组交换）在包交换方式中，当为包在网络中寻找路径时，并将其交付到预定的目的地。当代网络广泛使用以下两种方式：数据报和虚电路。Packetswitchingdatagrampacketswitchingsection2.1.3Internetvirtualcircuitswitchingsection2.3ATM75DatagrampacketswitchingEachpacketismarkedwithitsdestinationaddress(header)whenanodegetsapacket,itreadsitsdestinationaddressandfiguresoutwheretosendthepacketnext.在数据报方式，每一个节点对每一个数据报分别进行路由选择，这时要考虑到从相邻节点接收有关通信量、链路故障等信息。76Ifconditions(congestion,alinkmayfail)changeinthenetwork,successivepacketsthatbelongtothesamecommunicationbetweentwocomputersmayfollowdifferentpathsinthenetworks.如果网络的条件发生变化（拥塞、链路故障），在两台计算机之间属于同一条通信的连续数据报可能沿着不同的路径到达目的地。77HostA123333123121212Figure:Datagrampacketswitching（数据报分组交换）HostBCommunicationsubnetwork(通信子网）：Transportorforwarddata78Figure:Datagrampacketswitching79Theheadermayalsoindicateasequencenumberthatthedestinationusestoverifythatallthepacketswerereceivedortoreorderthem.因此每一个带有相同目的地址的数据包，并非全部沿着相同的路径，它们可以在网络目的节点不按顺序到达，目的节点或网络出口点负责对数据包进行重新排序。也有这种可能，数据包在网络上被破坏，那么由目的节点或网络出口点来检测数据报丢失，并决定如何恢复它。80advantageMorereliable:如果一个节点出故障了，随后的数据包可以寻找到一个替换路径绕过这个节点moreflexible:如果网络的一部分发生拥塞了，进入网络的数据包可以选择能避开拥塞的路由。Becauseitcanadapttochangingconditionsduringagivenconnection81disadvantageMaycauseindefinitedelaytime不确定的时延一个给定的源和目的节点之间的各个数据包可以沿不同的路径，也可以在沿途的节点中经历不同的时延，所以数据包的总时延可能变化很大。这种被称为“抖动”（jitter)的现象对于一些应用来说是不希望的。（例如在包括电话、语音和实时图象的应用中）82TheFutureofNetworksProtocol:协议CableTV：有线电视Video-on-demand：视频点播Hybridoffiberandcoaxialcabletechnology：光纤电缆混合技术LANswitches：局域网交换机ThroughputofLANs：局域网的数据吞吐量Ethernet：以太网Tokenring：令牌环网GigabitEthernet：千兆位以太网Backbone：主（骨）干网83ALOHA:TheBirthofMultipleAccessMultipleAccess:多点接入技术即通过一个公共信道将所有的用户连接起来，以便减少链路资源的消耗。TwoormorenodesshareasinglecommunicationchannelAnearlyexampleofamultiple-accessnetworkisALOHA.ALOHA网络是70年代由夏威夷大学发明的，目的是为了解决地面无线电广播信道的争用问题。84Figure1.5ThecomputersoftheALOHAnetworktransmitonthesameradiochannelwhenevertheyhaveapackettotransmit.Multiple-accessnetworkschannel85Whentwoormorecomputerstrytotransmitsimultaneously,theinformationisgarbled.(混淆）Thecomputerstransmittheirpackets,thencheckwhetherthetransmissionsweregarbled;ifso,theywaitforarandomtimebeforetryingagain.86信道在不进行复用的情况下，只允许一个用户使用。当两个或更多的用户同时在一个信道发送信息时，就会发生冲突(collision,碰撞）。凡是发生了冲突的用户所发送的信息都是失败的。因此，在多路访问技术中必须找出解决冲突的方法。ALOHA网络是让各站等待一段随机的时间，然后再进行重发。如果再发生冲突，则再等待一段随机时间，直到重发成功。

在ALOHA系统中，网络负载一定不能过重。当通信量不大时，这种网络比较有效，且控制简单。87AsynchronousTransferModeATM:异步传输模式采用信元交换技术的ATM使得宽带ISDN(B-ISDN)网络的实现成为可能。Cell:AnATMnetworktransportsinformationinfixed-sizepacketsof53bytes(calledcells)ATM网络上传输的信息单位