福建工程学院期末计算机网络复习

Introduction

Aboveall,thetwomostimportanttechnologiesare:computerandnetwork.Computer:Computershavemadespectacular(引人注目的)progressinashorttime.Network(Communication):Wecancommunicatewitheverybodyintheworldandsharetheinformationthroughthenetwork.Introduction1)ComputerCenter(*):(1)Aroomwithalargecomputertowhichusersbringtheirworkforprocessing.(2)Inthismodel,asinglecomputerservingalloftheorganization’scomputationalneeds.

2)ComputerNetwork(*):(1)Alargenumberofseparatebutinterconnectedcomputersdothejob.(2)Acollectionofautonomous(自主的)

computersinterconnectedbyasingletechnology(definitionofthecomputernetwork).

Introduction2,Computernetworkanddistributedsystem*Adistributedsystemisacollection(一组)ofindependentcomputersappearstoitsusersasasinglecoherent(关联)system.Ithasasinglemodelorparadigm(范式，模式)thatitpresentstotheusers.Oftenalayerofsoftwareontopoftheoperatingsystem,calledmiddleware(中间件),isresponsibleforimplementing(执行)thismodel.

所谓分布式系统，是指一组相互独立的计算机，对于用户来说相当于一个单一的关联系统。对于用户而言，分布式系统有一个单一的模型或者范式。通常是在操作系统之上运行软件，也称为中间件，用来执行这个模型。

注：分布式系统，是利用网络将多个独立的计算机系统联合在一起，通过专用的软件将这一些系统抽象成一个单一的系统，说白了就是把很多台计算机“组装”成一台，但这“一台”的性能几乎是所有机子的性能总和，变成一台超级计算机。

1.1UsesofComputerNetworksBusinessApplications商业应用HomeApplicationsMobileUsersSocialIssues1.1.1BusinessApplications

Businessapplicationsofcomputersincluding:(1)monitorproduction(产品监控)(2)keeptrackofinventories(库存跟踪)(3)dothepayroll(整理工资清单)

1,firstgoal:resourcesharing(*)Thegoalistomakeallprograms(程序),equipment(设备),andespeciallydataavailabletoanyoneonthenetworkwithoutregardtothephysicallocationoftheresourceandtheuser.

2,secondgoal:communicationmedium

3,thirdgoal:doingbusinesselectronically(especiallyforsuppliersandcustomers)4,fourthgoal:doingbusinesswithconsumersovertheInternet1.1.1BusinessApplicationsAnetworkwithtwoclientsandoneserver.Theclient-servermodelinvolvesrequestsandreplies.1.1.2HomeApplications

Theproblemiswhydopeopleneedtobuycomputerathome?(1)Accesstoremoteinformation(远程信息接入)(2)Person-to-personcommunication（个人通信）(3)Interactiveentertainment（交互式娱乐）(4)Electroniccommerce（电子商务）1.1.2HomeApplicationsInpeer-to-peersystemtherearenofixed(固定的)clientsandservers.Everypersoncancommunicatewithoneormoreotherpeople.Individualsformaloosegroupcancommunicatewithothersinthegroup.1.1.2HomeApplications

Conclusion:Person-to-personcommunication

(1)E-mail(2)Instantmessaging

(3)Peer-to-peercommunication（对等系统）(4)Electronicgambling（电子赌博）(5)Internettocarrytelephonecalls,videophone,andInternetradio,tele-learning1.2NetworkHardwareLocalAreaNetworksMetropolitanAreaNetworksWideAreaNetworksWirelessNetworksHomeNetworksInternetworks1.2NetworkHardware

Thereisnogenerallyacceptedtaxonomy(分类学)

intowhichallcomputernetworkfit(适合),buttwodimensionsstandoutasimportant:transmissiontechnologyandscale（尺度）.

Typesoftransmissiontechnology（传输技术）:Broadcastlinks(广播链路)Point-to-pointlinks(点对点链路)

1,Broadcastlinks(1)Whatisbroadcastlinks?(*)Broadcastlinkshaveasinglecommunicationchannelthatissharedbyallthemachinesonthenetwork.1.2NetworkHardware

Exceptfortheabovesituation,broadcastnetworkalsosupporttwotypesofoperationsasbelow:

(1)broadcasting(广播)Whenapacketwiththecode(thecodeallowsthepossibilityofaddressingapackettoalldestinations)istransmitted,itisreceivedandprocessedbyeverymachineonthenetwork.

(2)multicasting(组播，或多播)(*)Apacketistransmittedtoasubset(子集)ofthemachines.

Buthow?

Onepossibleistoreserveonebittoindicatemulticasting.Theremainingn-1addressbitscanholdagroupnumber.Eachmachinecan“subscribe(申请)”toanyorallofthegroups.Sowhenapacketissenttoacertaingroup,itisdeliveredtoallmachinessubscribingtothatgroup.1.2NetworkHardware

2,point-to-pointnetwork(unicasting)

(1)definition(*)point-to-pointnetworkconsistofmanyconnectionsbetweenindividualpairsofmachines,oftenwithonesenderandonereceiver.

(2)processApacketmayhavetofirstvisitoneormoreintermediatemachines,withdifferentlengths.Sofindinggoodonesisimportantinpoint-to-pointnetworks.1.2NetworkHardware

4,criterionforclassifyingnetworkwithscaleRighthere,weclassifynetworksystemsbytheirphysicalsize.

1.2.1LocalAreaNetworks

1,whatistheLocalAreaNetworks(*)LocalAreaNetworks,generallycalledLANs,areprivately-ownednetworkswithinasinglebuildingorcampusofuptoafewkilometersinsize.

2,application

Connectingpersonalcomputersandworkstationsincompanyofficesandfactoriestoshareresourcesandexchangeinformation.

5,speed(1)traditionalLANsrunatspeedsof10Mbpsto100Mbps.(2)NewerLANsisupto10Gbps.

1.2.1LocalAreaNetworks

6,Twobroadcastnetworks:(a)Bus广播式(b)Ring环形

7,broadcastnetworkcanbefurtherdividedintotwocategories(dependingonhowthechannelisallocated):

(1)static

(2)dynamic1.2.2MetropolitanAreaNetworks

1,whatisthemetropolitanareanetwork(*)Ametropolitanareanetwork(MAN)isalargecomputernetworkthatusuallyspans(延伸)acity.AMANusuallyinterconnectsanumberoflocalareanetworks(LANs)usingahigh-capacitybackbone(骨干网)

technology,suchasfiber-opticallinks,andprovidesup-link(级联)

servicestowideareanetworks(orWAN)andtheInternet.1.2.3WideAreaNetworks

1,whatiswideareanetwork(*)Awideareanetwork(WAN)spans(延伸)alargegeographical(地理)area,oftenacountryorcontinent(大陆).Itcontainsacollectionofmachinesintended(想让…做)

forrunninguser(application)programs.

2,hostsandcommunicationsubnet

host:(1)themachine(computer)inWAN.(2)itisownedbythecustomer.(3)forapplicationonly.

communicationsubnet(subnetforshort):

(1)connects(连接)hosts(2)ownedandoperatedbyatelephonecompanyorInternetserviceprovider(3)Thejobofthesubnetistocarrymessagefromhosttohost.Forcommunicationonly.(4)consistsoftwodistinctcomponents:transmissionlinesandswitchingelements(交换单元).*transmissionlinesmovebitsbetweenmachines.Theycanbemadeofcopperwire,opticalfiber,orevenradiolinks.*switchingelementsarespecializedcomputersthatconnectthreeormoretransmissionlines.Whendataarriveonanincomingline,theswitchingelementmustchooseanoutgoinglinetoforwardthem.1.2.3WideAreaNetworks

Inthiscase,thepacketisreceivedateachintermediate(中间的)routerinitsentirety(作为整体),storedthereuntiltherequiredoutputlineisfree,andthenforwarded.*Asubnetorganizedaccordingtoaboveprincipleiscalledstore-and-forward(存储转发)

orpacket-switched(包交换)subnet.*Whenthepacketsaresmallandallthesamesize,theyareoftencalledcells(信元).1.2.5HomeNetwork

Homenetworkingisonthehorizon(很快就会出现).Thefundamentalidea(基本的想法)

isthatinthefuturemosthomeswillbesetupfornetworking.1,whatishomenetworkInhomenetwork,everydeviceinthehomewillbecapableofcommunicatingwitheveryotherdevice,andallofthemwillbeaccessibleovertheInternet.2,devicesfornetwork(1)Computers(desktopPC,PDA,sharedperipherals(共享外设))(2)Entertainment(TV,DVD,VCR,camera,stereo(立体声),MP3)(3)Telecomm(telephone,cellphone,intercom(对讲机),fax)(4)Appliances(microwave,fridge,clock,furnace(火炉),airco(空调))(5)Telemetry(utilitymeter(电表),burglar(防盗)

alarm).1.2.6InternetworkwhatisinternetworkThefulfillment(完成)

oftheabovedesirerequiresthatdifferent,andfrequentlyincompatiblenetworks,beconnected,sometimesbymeansof(通过…方式)

machinescalledgateways(网关)tomaketheconnectionandprovidethenecessarytranslation,bothintermsofhardwareandsoftware.Acollectionofinterconnectednetworksiscalledaninternetworkorinternet.

3,differencebetweeninternetandInternetThetermsofinternetworkandinternetwillbeusedinagenericsense(泛指的概念),incontrasttotheworldwideInternet,whichwewillalwayscapitalize.1.3NetworkSoftwareThefirstcomputernetworksweredesignedwithhardwareasthemainconcernandthesoftwareasanafterthought.Thisstrategynolongerworks(失效，失灵).Networksoftwareisnowhighlystructured(高度结构化).

1.3.1ProtocolHierarchies(协议层次)

1,whatisthehierarchy

Toreducetheirdesigncomplexity,mostnetworksareorganizedasastackoflayersorlevels,eachonebuiltupontheonebelowit.Thenumberoflayers,thenameofeachlayer,thecontentsofeachlayer,andthefunctionofeachlayerdifferfromnetworktonetwork.1.3.1ProtocolHierarchies

Thepurposeofeachlayeristooffercertainservicestohigherlayers,shielding(屏蔽)thoselayersfromthedetailsofhowtheofferedservicesareactuallyimplemented(执行).Inasense,eachlayerisakindofvirtualmachine,offeringcertainservicestothelayeraboveit.

2,whatisprotocol

Aprotocolisanagreementbetweenthecommunicatingpartiesonhowcommunicationistoproceed.

Solayernononemachinecarriesonaconversationwithlayernonanothermachine.Therules(规则)

andconversation(会话)arecollectivelyknownasthelayernprotocol.

1.3.1ProtocolHierarchiesLayers,protocols,andinterfaces.3,afive-layernetwork

Howdotheprotocollayersprocess?Twoanswersbelow,choosetherightone.(a)Dataaredirectlytransferredfromlayernononemachinetolayernonanothermachine.(b)Eachlayerpassesdataandcontrolinformationtothelayerimmediatelybelowit,untilthelowestlayerisreached.Obviously,thesecondoneistherightanswer.1.3.2DesignIssuesfortheLayersAddressingErrorControlFlowControlMultiplexingRouting1.3.2DesignIssuesfortheLayers

1.Addressing(寻址编址机制)2.Errordetection（错误控制）3.flowcontrol（流控制）Question1:Whatisflowcontrol?(*)Indatacommunications,flowcontrolistheprocessofmanagingtherateofdatatransmissionbetweentwonodestopreventafastsenderfromoutrunning(超过)aslowreceiver.Itprovidesamechanism(机制)

forthereceivertocontrolthetransmissionspeed,sothatthereceivingnodeisnotoverwhelmed(被压倒)

withdatafromtransmittingnode.4.multiplexinganddemultiplexing（多路复用和多路解复用）5.routing（路由）4.multiplexinganddemultiplexing

Question:Whendomultiplexinganddemultiplexinghappen?Asweknow,whenwesurfingontheInternet,computermayhavemultiplecommunicationprocessesatthesametime.Whenitisinconvenientandexpensivetosetupaseparateconnectionforeachpairofcommunicationprocesses,theunderlying(下层的)layersmaydecidetousethesameconnectionformultiple,unrelatedconversation.Especiallyinthephysicallayer.5.routing

Routingorrouteingistheprocessofselectingpathsinanetworkalongwhichtosendnetworktraffic.

1.3.3Connection-OrientedandConnectionlessServices

Layerscanoffertwodifferenttypesofservicetothelayersabovethem:connection-orientedandconnectionless.（%）

1.connection-oriented

Questions:whatisconnection-orientedservice?Itismodeledafterthetelephonesystem.Touseaconnection-orientednetworkservice,thereceiveuserfirstestablishesaconnection,usestheconnection,andthenreleasetheconnection.Itactslikeatube(隧道，管道).

Theadvantagesinclude:(1)Inmostcasestheorderispreserved(保留)sothatthebitsarriveintheordertheyweresent.(2)Whenaconnectionisestablished,thesender,receiverandsubnetconductanegotiationaboutparameters(参数)tobeused.1.4ReferenceModelsTheOSIReferenceModelTheTCP/IPReferenceModelAComparisonofOSIandTCP/IPACritiqueoftheOSIModelandProtocolsACritiqueoftheTCP/IPReferenceModelDifferencesbetweentwomodels:TheprotocolsassociatedwiththeOSImodelarerarely(很少，几乎不)usedanymore,butthemodelitselfisactuallyquitegeneralandstillvalid(有效的).TheTCP/IPmodelisnotofmuchusebutprotocolsarewidelyused.1.4.1TheOSIReferenceModels（%）

3.Principalsforsetting7layers

(1)Alayershouldbecreatedwhereadifferentabstraction(抽象，可引申为分工)isneeded.(2)Eachlayershouldperformawell-defined(明确的)

function.(3)Thefunctionofeachlayershouldbechosenwithaneyetoward(着眼于)

defininginternationallystandardizedprotocols.(4)Thelayerboundariesshouldbechosentominimizetheinformationflowacrosstheinterfaces.(5)Thenumberoflayersshouldbelargeenoughthatdistinctfunctionsneednotbethrown(放到)

inthesamelayeroutofnecessityandsmallenoughthatthearchitecturedoesnotbecomeunwieldy(难以控制的).1.4.1TheOSIReferenceModelsTheOSIreferencemodel.1.4.1TheOSIReferenceModels1.ThePhysicalLayer

Thephysicallayerisconcernedwithtransmittingrawbitsoveracommunicationchannel.（%）Thedesignissuesherelargelydealwithmechanical,electrical,andtiminginterfaces,andphysicaltransmissionmedium,whichliesbelowthephysicallayer.（定时接口以及物理层下的物理传输介质）2.DataLinkLayer(dataframes,acknowledgementframe)

Thedatalinklayerisconcernedwithtransformingarawtransmissionfacilityintoalinethatappearsfreeofundetectedtransmissionerrorstothenetworklayer,aswellashowtokeepafasttransmitterfromdrowningaslowreceiverindata.Broadcastnetworkshaveanadditionalissueinthedatalinklayer:howtocontroltothesharedchannel.3.TheNetworkLayerThenetworklayercontrolstheoperationofthesubnet.Akeyissueisdetermininghowpacketsareroutedfromsourcestodestination.Thecontrolofthecongestion(bottleneck)alsobelongstothenetworklayer.Aswellasthequalityofservicesprovidedbynetwork.（服务质量，比如延时、抖动）Inbroadcastnetworks,theroutingproblemissimple,sothenetworklayerisoftenthinorevennonexistent.4.TheTransportLayerThebasicofthetransportlayeristoacceptdatafromabove,splititupintosmallerunitsifneedbe,passthesetothenetworklayer,andensurethatthepiecesallarrivecorrectlyattheotherend.Thetransportlayeralsodetermineswhattypeofservicetoprovidetothesessionlayer,andultimately,theusersofthenetwork.5.TheSessionLayerThesessionlayerallowsusersondifferentmachinestoestablishsessionsbetweenthem.Sessionoffervariousservices,includingdialogcontrol,tokenmanagementandsynchronization.6.ThePresentationLayerThepresentationlayerisconcernedwiththesyntax(语法)andsemantics(语义)

oftheinformationtransmitted.7.TheApplicationLayerTheapplicationlayercontainavarietyofprotocolsthatarecommonlyneededbyusers.Forexample,HTTP.1.4.2TheTCP/IPReferenceModels1.BornoftheTCP/IPreferencemodelTheARPANETwasaresearchnetworksponsored(赞助)bytheDoD.Iteventuallyconnectedhundredsofuniversitiesandgovernmentinstallations,usingleased(租用的)

telephonelines.Whensatelliteandradionetworkswareaddedlater,theexistingprotocolshadtroubleininterworkingwiththem,soanewreferencearchitecturewasneeded.Thustheabilitytoconnectmultiplenetworksinaseamlesswaywasoneofthemajordesigngoalsfromtheverybeginning.ThisarchitecturelaterbecameknownastheTCP/IPReferenceModel.Itwasdefinedin1974,andrevisedin1985and19TheTCP/IPReferenceModelsTheTCP/IPreferencemodel.1.4.2TheTCP/IPReferenceModels2.fourlayersofthemodel(1)TheinternetlayerTheinternetlayer,isthelinchpin(关键)thatholdsthewholearchitecturetogether.Itsjobistopermithoststoinjectpacketsintoanynetworkandhavethemtravelindependentlytodestination.Atthesametime,theinternetlayerdefinesanofficialpacketformatandprotocolcalledIP(InternetProtocol).(2)ThetransportlayerThetransportlayerisdesignedtoallowpeerentitiesonthesourceanddestinationhoststocarryonaconversation,justasintheOSItransportlayer.Therearetwoimportantprotocolshere:TCPandUDP.

1)TCP---TransmissionControlProtocolTCPisareliableconnection-orientedprotocolthatallowsabytesstreamoriginatingononemachinetobedeliveredwithouterroronanyothermachineintheinternet.2)UDP---UserDatagramProtocolUDPisanunreliable,connectionlessprotocolforapplication.Itisalsowidelyusedforclient-servetype,request-replyqueriesandapplicationsinwhichpromptdelivery(限时传输)ismoreimportantthanaccuratedelivery.(3)TheapplicationlayerTheapplicationlayercontainsallthehigher-levelprotocols.

(4)TheHost-to-NetworkLayer（主机至网络层）ProtocolsandnetworksintheTCP/IPmodelinitially.HybridModel（%）Thehybridreferencemodeltobeusedinthisbook.

Insummary,despitethesedifferences,theOSImodelhasproventobeexceptionally(特别地)

usefulfordiscussingcomputernetworks.Incontrast,theOSIprotocolshavenotbecomepopular.ThereverseistrueofTCP/IP.InthisbookwewilluseamodifiedOSImodelbutconcentrateprimarilyontheTCP/IPandrelatedprotocols.Chapter2ThePhysicalLayer

Thephysicallayerdefinesthemechanical(机械),electrical(电气),andtiminginterfaces(定时接口)

tothenetwork.Contentsofthechapter:1.theoreticalanalysisofdatatransmission.2.threekindsoftransmissionmedia:guided(copperwireandfiberoptics),wireless(terrestrialradio),andsatellite.3.threeexamplesofcommunicationsystems.Chapter2

ThePhysicalLayer2.1.2Bandwidth-LimitedSignalsbandwidth(*)Therangeoffrequenciestransmittedwithoutbeingstronglyattenuatediscalledthebandwidth.

Inpractice,thecutoffisnotreallysharp,sooftenthequotedbandwidthisfrom0tothefrequencyatwhichhalfthepowergetsthrough.Thebandwidthisaphysicalpropertyofthetransmissionmediumandusuallydependsontheconstruction,thickness,andlengthofthemedium.Insomecasesafilterisintroducedintocircuittolimittheamountofbandwidthavailabletoeachcustomer.Forexample,atelephonewiremayhaveabandwidthof1MHzforshortdistance,buttelephonecompaniesaddafilterrestrictingeachcustomertoabout3100Hz.2.1.3TheMaximumDataRateofaChannelIfthesignalconsistsofVdiscretelevels,Nyquist’stheoremstates:Forexample,anoiseless3-KHzchannelcannottransmitbinary(two-level)signalatarateexceeding6000bps.Sofarwehaveconsideredonlynoiselesschannels.Ifrandomnoiseispresent,thesituationdeteriorates(恶化)rapidly.Andthereisalwaysrandomnoisepresentduetothemotionofthemoleculesinthesystem.Theamountofthermalnoisepresentismeasuredbytheratioofthesignalpowertothenoisepower,calledthesignal-to-noiseratio.Ifwedenote(以…为符号)

thesignalpowerbySandthenoisepowerbyN,thesignal-to-noiseratioisS/N.

Usually,theratioitselfisnotquoted(引用);instead,thequantity10log10S/Nisgiven.Theseunitsarecalleddecibels(dB).2.1.3TheMaximumDataRateofaChannelShannon’smajorresultisthatthemaximumdatarateofanoisychannelwhosebandwidthisHHz,andwhosesignal-to-noiseratioisS/N,isgivenbyForexample,achannelof3000-Hzbandwidthwithasignaltothermalnoiseratioof30dBcannevertransmitmuchmorethan30,000bps,nomatterhowmanyorhowfewsignallevelsareusedandnomatterhowoftenorhowinfrequentlysamplesaretaken.Shannon’sresultwasderivedfrominformation-theoryargumentsandappliestoanychannelsubjecttothermalnoise.2.2GuidedTransmissionData

Thepurposeofthephysicallayeristotransportarawbitstream(原始比特流)fromonemachinetoanother.Variousphysicalmedia(物理介质)

canbeusedfortheactualtransmission.Eachonehasitsownniche(合适的位置)

intermsofbandwidth,delay,cost,andeaseofinstallationandmaintenance.2.2.1MagneticMedia(磁性媒体)

Oneofthemostcommonwaystotransportdatafromonecomputertoanotheristowritethemontomagnetictapeorremovablemedia(可移动存储媒体)(e.g.,recordableDVDs),physicallytransportthetapeordiskstothedestinationmachine,andreadthembackagain.（%）2.2.2TwistedPair

theadvantageofmagnetictape:bandwidthcharacteristics.thedisadvantageofit:delaycharacteristics.1.whatistwistedpair?(*)

Atwistedpairconsistsoftwoinsulatedcopperwires,typicallyabout1mmthick.Thewiresaretwistedtogetherinahelical(螺旋状的)form,justlikeaDNAmolecule.2.whyweneedtwisting?Twistingisdonebecausetwoparallelwiresconstituteafineantenna.Whenthewiresaretwisted,thewavesfromdifferenttwistscancelout,sothewireradiates(幅射)lesseffectively.

2.2.3CoaxialCable

1.advantageofcoaxialcableCoaxialcablehasbettershielding(屏蔽性)thantwistedpairs,soitcanspanlongerdistancesathigherspeeds.2.twokindsofcoaxialcable50-ohmcable,iscommonlyusedwhenitisintendedfordigitaltransmissionfromstart.75-ohmcable,iscommonlyusedforanalogtransmissionandcabletelevisionbutisbecomingmoreimportantwiththeadvent(到来)

ofInternetovercable.Thisdistinctionisbasedonhistorical,ratherthantechnicalfactors.

2.2.3CoaxialCableAcoaxialcable.3.thestructureofcoaxialcableAcoaxialcableconsistsofastiff(硬的)copperwireasthecore,surroundedbyaninsulatingmaterial.Theinsulatorisencasedbyacylindrical(圆柱形的)

conductor,oftenasaclosely-woven(编织的)braidedmesh(编织网).Theouterconductoriscoveredinaprotectiveplasticsheath(起到保护作用的外套).2.2.3CoaxialCable4.thebandwidthofcoaxialcable

Theconstructionandshieldingofthecoaxialcablegiveitagoodcombinationofhighbandwidthandexcellentofnoiseimmunity.Moderncableshaveabandwidthofcloseto1GHz.（%）5.applicationofcoaxialcableCoaxialcablesusedtobewidelyusedwithinthetelephonesystemforlong-distancelinesbuthavenowlargelybeenreplacedbyfiberoptics.CoaxisstillwidelyusedforcabletelevisionandMANs.2.7.2InternetoverCable1.whatisHFC(*)

Asystemwithfiberforthelong-haul(远距离)runsandcoaxialcabletothehousesiscalledanHFC(HybridFiberCoax)system.（%）

Theelectro-opticalconvertersthatinterfacebetweentheopticalandelectricalpartsofthesystemarecalledfibernodes.Becausethebandwidthoffiberissomuchmorethanofcoax,afibernodecanfeedmultiplecoaxialcables.

2.7.3SpectrumAllocation2.spectrumallocation（%）

CabletelevisionchannelsinNorthAmericannormallyoccupythe54-550MHzregion(exceptforFMradiofrom88to108MHz).Thesechannelsare6MHzwide,includingguardbands(保护频带).

（%）Thelowpartofthebandisnotused.Moderncablescanalsooperatewellabove550MHz,oftento750MHzormore.Thesolutionchosenwastointroduceupstreamchannelsinthe5-42MHzbandandusethefrequenciesatthehighendforthedownstream.2.7.3SpectrumAllocationFrequencyallocationinatypicalcableTVsystemusedforInternetaccess

3.1DataLinkLayerDesignIssuesFourparts:ServicesProvidedtotheNetworkLayer（提供给网络层的服务）Framing（帧定界）ErrorControl（差错控制）FlowControl（流量控制）Chapter3TheDataLinkLayer3.1DataLinkLayerDesignIssuesToaccomplishthesegoals,thedatalinklayertakesthepacketsit

getsfromthenetworklayerandencapsulates(封装)themintoframesfortransmission.

Eachframecontainsaframeheader(帧首部),apayloadfield(负载场)

forholdingthepacket,andframetrailer(帧尾部),asshowninFig.3-1.Relationshipbetweenpacketsandframes.3.1DataLinkLayerDesignIssues

FunctionsoftheDataLinkLayer:（%）(1)Provideserviceinterface（服务接口）tothenetworklayer(2)Dealingwithtransmissionerrors(3)Regulating（控制）dataflowsothatslowreceiversnotswampedbyfa