福建工程學(xué)院期末計(jì)算機(jī)網(wǎng)絡(luò)復(fù)習(xí)

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(一組)ofindependentcomputersappearstoitsusersasasinglecoherent(關(guān)聯(lián))system.Ithasasinglemodelorparadigm(范式，模式)thatitpresentstotheusers.Oftenalayerofsoftwareontopoftheoperatingsystem,calledmiddleware(中間件),isresponsibleforimplementing(執(zhí)行)thismodel.

所謂分布式系統(tǒng)，是指一組相互獨(dú)立的計(jì)算機(jī)，對(duì)于用戶來說相當(dāng)于一個(gè)單一的關(guān)聯(lián)系統(tǒng)。對(duì)于用戶而言，分布式系統(tǒng)有一個(gè)單一的模型或者范式。通常是在操作系統(tǒng)之上運(yùn)行軟件，也稱為中間件，用來執(zhí)行這個(gè)模型。

注：分布式系統(tǒng)，是利用網(wǎng)絡(luò)將多個(gè)獨(dú)立的計(jì)算機(jī)系統(tǒng)聯(lián)合在一起，通過專用的軟件將這一些系統(tǒng)抽象成一個(gè)單一的系統(tǒng)，說白了就是把很多臺(tái)計(jì)算機(jī)“組裝”成一臺(tái)，但這“一臺(tái)”的性能幾乎是所有機(jī)子的性能總和，變成一臺(tái)超級(jí)計(jì)算機(jī)。

1.1UsesofComputerNetworksBusinessApplications商業(yè)應(yīng)用HomeApplicationsMobileUsersSocialIssues1.1.1BusinessApplications

Businessapplicationsofcomputersincluding:(1)monitorproduction(產(chǎn)品監(jiān)控)(2)keeptrackofinventories(庫(kù)存跟蹤)(3)dothepayroll(整理工資清單)

1,firstgoal:resourcesharing(*)Thegoalistomakeallprograms(程序),equipment(設(shè)備),andespeciallydataavailabletoanyoneonthenetworkwithoutregardtothephysicallocationoftheresourceandtheuser.

2,secondgoal:communicationmedium

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

Theproblemiswhydopeopleneedtobuycomputerathome?(1)Accesstoremoteinformation(遠(yuǎn)程信息接入)(2)Person-to-personcommunication（個(gè)人通信）(3)Interactiveentertainment（交互式娛樂）(4)Electroniccommerce（電子商務(wù)）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（對(duì)等系統(tǒng)）(4)Electronicgambling（電子賭博）(5)Internettocarrytelephonecalls,videophone,andInternetradio,tele-learning1.2NetworkHardwareLocalAreaNetworksMetropolitanAreaNetworksWideAreaNetworksWirelessNetworksHomeNetworksInternetworks1.2NetworkHardware

Thereisnogenerallyacceptedtaxonomy(分類學(xué))

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

Typesoftransmissiontechnology（傳輸技術(shù)）:Broadcastlinks(廣播鏈路)Point-to-pointlinks(點(diǎn)對(duì)點(diǎn)鏈路)

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

Exceptfortheabovesituation,broadcastnetworkalsosupporttwotypesofoperationsasbelow:

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

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

Buthow?

Onepossibleistoreserveonebittoindicatemulticasting.Theremainingn-1addressbitscanholdagroupnumber.Eachmachinecan“subscribe(申請(qǐng))”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廣播式(b)Ring環(huán)形

7,broadcastnetworkcanbefurtherdividedintotwocategories(dependingonhowthechannelisallocated):

(1)static

(2)dynamic1.2.2MetropolitanAreaNetworks

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

technology,suchasfiber-opticallinks,andprovidesup-link(級(jí)聯(lián))

servicestowideareanetworks(orWAN)andtheInternet.1.2.3WideAreaNetworks

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

forrunninguser(application)programs.

2,hostsandcommunicationsubnet

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

communicationsubnet(subnetforshort):

(1)connects(連接)hosts(2)ownedandoperatedbyatelephonecompanyorInternetserviceprovider(3)Thejobofthesubnetistocarrymessagefromhosttohost.Forcommunicationonly.(4)consistsoftwodistinctcomponents:transmissionlinesandswitchingelements(交換單元).*transmissionlinesmovebitsbetweenmachines.Theycanbemadeofcopperwire,opticalfiber,orevenradiolinks.*switchingelementsarespecializedcomputersthatconnectthreeormoretransmissionlines.Whendataarriveonanincomingline,theswitchingelementmustchooseanoutgoinglinetoforwardthem.1.2.3WideAreaNetworks

Inthiscase,thepacketisreceivedateachintermediate(中間的)routerinitsentirety(作為整體),storedthereuntiltherequiredoutputlineisfree,andthenforwarded.*Asubnetorganizedaccordingtoaboveprincipleiscalledstore-and-forward(存儲(chǔ)轉(zhuǎn)發(fā))

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

Homenetworkingisonthehorizon(很快就會(huì)出現(xiàn)).Thefundamentalidea(基本的想法)

isthatinthefuturemosthomeswillbesetupfornetworking.1,whatishomenetworkInhomenetwork,everydeviceinthehomewillbecapableofcommunicatingwitheveryotherdevice,andallofthemwillbeaccessibleovertheInternet.2,devicesfornetwork(1)Computers(desktopPC,PDA,sharedperipherals(共享外設(shè)))(2)Entertainment(TV,DVD,VCR,camera,stereo(立體聲),MP3)(3)Telecomm(telephone,cellphone,intercom(對(duì)講機(jī)),fax)(4)Appliances(microwave,fridge,clock,furnace(火爐),airco(空調(diào)))(5)Telemetry(utilitymeter(電表),burglar(防盜)

alarm).1.2.6InternetworkwhatisinternetworkThefulfillment(完成)

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

machinescalledgateways(網(wǎng)關(guān))tomaketheconnectionandprovidethenecessarytranslation,bothintermsofhardwareandsoftware.Acollectionofinterconnectednetworksiscalledaninternetworkorinternet.

3,differencebetweeninternetandInternetThetermsofinternetworkandinternetwillbeusedinagenericsense(泛指的概念),incontrasttotheworldwideInternet,whichwewillalwayscapitalize.1.3NetworkSoftwareThefirstcomputernetworksweredesignedwithhardwareasthemainconcernandthesoftwareasanafterthought.Thisstrategynolongerworks(失效，失靈).Networksoftwareisnowhighlystructured(高度結(jié)構(gòu)化).

1.3.1ProtocolHierarchies(協(xié)議層次)

1,whatisthehierarchy

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

Thepurposeofeachlayeristooffercertainservicestohigherlayers,shielding(屏蔽)thoselayersfromthedetailsofhowtheofferedservicesareactuallyimplemented(執(zhí)行).Inasense,eachlayerisakindofvirtualmachine,offeringcertainservicestothelayeraboveit.

2,whatisprotocol

Aprotocolisanagreementbetweenthecommunicatingpartiesonhowcommunicationistoproceed.

Solayernononemachinecarriesonaconversationwithlayernonanothermachine.Therules(規(guī)則)

andconversation(會(huì)話)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(尋址編址機(jī)制)2.Errordetection（錯(cuò)誤控制）3.flowcontrol（流控制）Question1:Whatisflowcontrol?(*)Indatacommunications,flowcontrolistheprocessofmanagingtherateofdatatransmissionbetweentwonodestopreventafastsenderfromoutrunning(超過)aslowreceiver.Itprovidesamechanism(機(jī)制)

forthereceivertocontrolthetransmissionspeed,sothatthereceivingnodeisnotoverwhelmed(被壓倒)

withdatafromtransmittingnode.4.multiplexinganddemultiplexing（多路復(fù)用和多路解復(fù)用）5.routing（路由）4.multiplexinganddemultiplexing

Question:Whendomultiplexinganddemultiplexinghappen?Asweknow,whenwesurfingontheInternet,computermayhavemultiplecommunicationprocessesatthesametime.Whenitisinconvenientandexpensivetosetupaseparateconnectionforeachpairofcommunicationprocesses,theunderlying(下層的)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(參數(shù))tobeused.1.4ReferenceModelsTheOSIReferenceModelTheTCP/IPReferenceModelAComparisonofOSIandTCP/IPACritiqueoftheOSIModelandProtocolsACritiqueoftheTCP/IPReferenceModelDifferencesbetweentwomodels:TheprotocolsassociatedwiththeOSImodelarerarely(很少，幾乎不)usedanymore,butthemodelitselfisactuallyquitegeneralandstillvalid(有效的).TheTCP/IPmodelisnotofmuchusebutprotocolsarewidelyused.1.4.1TheOSIReferenceModels（%）

3.Principalsforsetting7layers

(1)Alayershouldbecreatedwhereadifferentabstraction(抽象，可引申為分工)isneeded.(2)Eachlayershouldperformawell-defined(明確的)

function.(3)Thefunctionofeachlayershouldbechosenwithaneyetoward(著眼于)

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

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

Thephysicallayerisconcernedwithtransmittingrawbitsoveracommunicationchannel.（%）Thedesignissuesherelargelydealwithmechanical,electrical,andtiminginterfaces,andphysicaltransmissionmedium,whichliesbelowthephysicallayer.（定時(shí)接口以及物理層下的物理傳輸介質(zhì)）2.DataLinkLayer(dataframes,acknowledgementframe)

Thedatalinklayerisconcernedwithtransformingarawtransmissionfacilityintoalinethatappearsfreeofundetectedtransmissionerrorstothenetworklayer,aswellashowtokeepafasttransmitterfromdrowningaslowreceiverindata.Broadcastnetworkshaveanadditionalissueinthedatalinklayer:howtocontroltothesharedchannel.3.TheNetworkLayerThenetworklayercontrolstheoperationofthesubnet.Akeyissueisdetermininghowpacketsareroutedfromsourcestodestination.Thecontrolofthecongestion(bottleneck)alsobelongstothenetworklayer.Aswellasthequalityofservicesprovidedbynetwork.（服務(wù)質(zhì)量，比如延時(shí)、抖動(dòng)）Inbroadcastnetworks,theroutingproblemissimple,sothenetworklayerisoftenthinorevennonexistent.4.TheTransportLayerThebasicofthetransportlayeristoacceptdatafromabove,splititupintosmallerunitsifneedbe,passthesetothenetworklayer,andensurethatthepiecesallarrivecorrectlyattheotherend.Thetransportlayeralsodetermineswhattypeofservicetoprovidetothesessionlayer,andultimately,theusersofthenetwork.5.TheSessionLayerThesessionlayerallowsusersondifferentmachinestoestablishsessionsbetweenthem.Sessionoffervariousservices,includingdialogcontrol,tokenmanagementandsynchronization.6.ThePresentationLayerThepresentationlayerisconcernedwiththesyntax(語(yǔ)法)andsemantics(語(yǔ)義)

oftheinformationtransmitted.7.TheApplicationLayerTheapplicationlayercontainavarietyofprotocolsthatarecommonlyneededbyusers.Forexample,HTTP.1.4.2TheTCP/IPReferenceModels1.BornoftheTCP/IPreferencemodelTheARPANETwasaresearchnetworksponsored(贊助)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(關(guān)鍵)thatholdsthewholearchitecturetogether.Itsjobistopermithoststoinjectpacketsintoanynetworkandhavethemtravelindependentlytodestination.Atthesametime,theinternetlayerdefinesanofficialpacketformatandprotocolcalledIP(InternetProtocol).(2)ThetransportlayerThetransportlayerisdesignedtoallowpeerentitiesonthesourceanddestinationhoststocarryonaconversation,justasintheOSItransportlayer.Therearetwoimportantprotocolshere:TCPandUDP.

1)TCP---TransmissionControlProtocolTCPisareliableconnection-orientedprotocolthatallowsabytesstreamoriginatingononemachinetobedeliveredwithouterroronanyothermachineintheinternet.2)UDP---UserDatagramProtocolUDPisanunreliable,connectionlessprotocolforapplication.Itisalsowidelyusedforclient-servetype,request-replyqueriesandapplicationsinwhichpromptdelivery(限時(shí)傳輸)ismoreimportantthanaccuratedelivery.(3)TheapplicationlayerTheapplicationlayercontainsallthehigher-levelprotocols.

(4)TheHost-to-NetworkLayer（主機(jī)至網(wǎng)絡(luò)層）ProtocolsandnetworksintheTCP/IPmodelinitially.HybridModel（%）Thehybridreferencemodeltobeusedinthisbook.

Insummary,despitethesedifferences,theOSImodelhasproventobeexceptionally(特別地)

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

Thephysicallayerdefinesthemechanical(機(jī)械),electrical(電氣),andtiminginterfaces(定時(shí)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(惡化)rapidly.Andthereisalwaysrandomnoisepresentduetothemotionofthemoleculesinthesystem.Theamountofthermalnoisepresentismeasuredbytheratioofthesignalpowertothenoisepower,calledthesignal-to-noiseratio.Ifwedenote(以…為符號(hào))

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(物理介質(zhì))

canbeusedfortheactualtransmission.Eachonehasitsownniche(合適的位置)

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

Oneofthemostcommonwaystotransportdatafromonecomputertoanotheristowritethemontomagnetictapeorremovablemedia(可移動(dòng)存儲(chǔ)媒體)(e.g.,recordableDVDs),physicallytransportthetapeordiskstothedestinationmachine,andreadthembackagain.（%）2.2.2TwistedPair

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

Atwistedpairconsistsoftwoinsulatedcopperwires,typicallyabout1mmthick.Thewiresaretwistedtogetherinahelical(螺旋狀的)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(到來)

ofInternetovercable.Thisdistinctionisbasedonhistorical,ratherthantechnicalfactors.

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

conductor,oftenasaclosely-woven(編織的)braidedmesh(編織網(wǎng)).Theouterconductoriscoveredinaprotectiveplasticsheath(起到保護(hù)作用的外套).2.2.3CoaxialCable4.thebandwidthofcoaxialcable

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

Asystemwithfiberforthelong-haul(遠(yuǎn)距離)runsandcoaxialcabletothehousesiscalledanHFC(HybridFiberCoax)system.（%）

Theelectro-opticalconvertersthatinterfacebetweentheopticalandelectricalpartsofthesystemarecalledfibernodes.Becausethebandwidthoffiberissomuchmorethanofcoax,afibernodecanfeedmultiplecoaxialcables.

2.7.3SpectrumAllocation2.spectrumallocation（%）

CabletelevisionchannelsinNorthAmericannormallyoccupythe54-550MHzregion(exceptforFMradiofrom88to108MHz).Thesechannelsare6MHzwide,includingguardbands(保護(hù)頻帶).

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

3.1DataLinkLayerDesignIssuesFourparts:ServicesProvidedtotheNetworkLayer（提供給網(wǎng)絡(luò)層的服務(wù)）Framing（幀定界）ErrorControl（差錯(cuò)控制）FlowControl（流量控制）Chapter3TheDataLinkLayer3.1DataLinkLayerDesignIssuesToaccomplishthesegoals,thedatalinklayertakesthepacketsit

getsfromthenetworklayerandencapsulates(封裝)themintoframesfortransmission.

Eachframecontainsaframeheader(幀首部),apayloadfield(負(fù)載場(chǎng))

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

FunctionsoftheDataLinkLayer:（%）(1)Provideserviceinterface（服務(wù)接口）tothenetworklayer(2)Dealingwithtransmissionerrors(3)Regulating（控制）dataflowsothatslowreceiversnotswampedbyfa