網(wǎng)絡(luò)工程專業(yè)外文翻譯-計(jì)算機(jī)網(wǎng)絡(luò)

外文原文：COMPUTERNETWORKSDATECOMMUNICATIONSOneofthefunctionsofthehostcomputeristostoretheapplicationsprogramsforthevarioussecondaries.Theendequipmentwhicheithergeneratesthedigitalinformationfortransmissionorusesthereceiveddigitaldatacanbecomputer,printers,keyboards,CRTs,andsoon.Thisequipmentgenerallymanipulatesdigitalinformationinternallyinwordunits—allthebitsthatmakeupawordinaparticularpieceofequipmentaretransferredinparallel.Digitaldata,whentransmitted,areinserialform.Paralleltransmissionofan8-bitwordrequireeightpairsoftransmissionlines—notatallcost-effective.Dataterminal(DTE)isageneralphraseencompassingallofthecircuitrynecessarytoperformparallel-to-serialandserial-to-parallelconversionsfortransmissionandreceptionrespectivelyandfordatalinkmanagement.TheUART(UniversalAsynchronousReceiver/Transmitter)andUSART(UniversalAsynchronous/AsynchronousReceiver/Transmitter)arethedevicesthatperformtheparallel-to-serialandserial-to-parallelconversions.TheprimaryDTEincludesalinecontrolunit(LCUorLinCo)whichcontrolstheflowofinformationinamultipointdatalinksystem.Astationcontroller(STACO)isthecorrespondingthatbelongedtothesubscriberinadatalinksystem.BetweentheDTEs,startingwiththemodems,wascommunicationsequipmentownedandmaintainedbyTelcoproperty.Datacommunicationsequipment(DCE)acceptstheserialdatastreamfromtheDTEandconvertsittosomeformofanalogsignalsuitablefortransmissiononvoice-gradelined.Atthereceiveend,theDCEperformsthereversefunctionofconvertingthereceivedanalogsignaltoaserialdigitaldatastream.ThesimplestformofDCEisamodem(modulator/demodulator)ordataset.Atthetransmitend,themodemcanbeconsideredaformofdigital-to-analogconverter,whileatthereceiveend,itcanconsideredaformofanalog-to-digitalconverter.Themostcommonofmodulationbymodemsarefrequencyshiftkeying(FSK),phaseshiftkeying(PSK),andquadratureamplitudemodulation(QAM).Thisisatypicallydatatransmissionmodeusingtheanalogtelephonelines.Ifyoutransmitdatabydigitalchannel(sometimesitiscalled“DigitalT-carrier”),apulseCodeModulation(PCM)equipmentmustbeused.Amicrowavetransmissionsystemcanalsobeusedforthedatacommunication.Finally,youcanusethesatellitecommunicationsystemfordatatransmission.IfthecablesandsignallevelsusedtointerconnecttheDTEandDCEwereleftunregulated,thevariationsgeneratedwouldprobablybeproportionaltothenumberofmanufacturers.ElectronicsindustriesAssociation(EIA),anorganizationofmanufactureswithestablishingtheDTEandmodem.Thisisa25-pincablewhosepinshavedesignatedfunctionsandspecifiedsignallevels.TheRS-232Cisanticipatedtobereplacedbyanupdatestandard.ARCHITECTUREOFCOMPUTERNETWORKSComputernetworkisacomplexconsistingoftwoormoreconnedcomputingunits,itisusedforthepurposeofdatacommunicationandresourceresourcesharing.Designofanetworkanditslogicalstructureshouldcomplywithasetofdesignprinciples,includingtheorganizationoffunctionsandthedescriptionofdataformatsandprocedure.Thisisthenetworkarchitectureandsocalledasasetoflayersandprotocols,becausethearchitectureisalayer-based.Inthenexttwosectionswewilldiscusstwoimportantnetworkarchitectures,theOSIreferencemodelandtheTCP/IPreferencemodel.TheOSIReferenceModelTheOSImodelisshowninFig.14-2(minusthephysicalmedium).ThismodelisbasedonaproposaldevelopedbytheInternationalStandardsOrganizations(OSI)asthefirststeptowardinternationalstandardizationoftheprotocolsusedinthevariouslayers.ThemodeliscalledtheISOOSI(OpenSystemInterconnection)ReferenceModelbecauseitdealswithconnectingopensystems--thatis,systemsthatareopenforcommunicationwithothersystems,WewillusuallyjustcallittheOSImodelforshort.TheOSImodelhassevenhassevenlayers.NotethattheOSImodelitselfisnotanetworkarchitecturebecauseitdoesnotspecifytheexactservicesandprotocolstobeusedineachlayer.Itjusttellswhateachlayershoulddo.However,However,ISOhasalsoproducedstandardsforallthelayers,althoughthesearenotpartofthereferencemodelitself.Eachonecustomers.CCITTRecommendationX.25controlstheaccessfromapacketmodeDTE,suchasaterminaldeviceorcomputersystemcapableofformingpackets,totheDCEatapacketmode.CCITTRecommendationX.28controlstheinterfacebetweennon-packetmodedevicesthatcannotinterfacebetweenthePADandthehostcomputer.CCITTRecommendationX.3specifiestheparametersettingsonthePADandX.75specifiestheinterfacebetweenpacketnetwork.LOCALAREANETWORKLocalareadatanetwork,normallyreferredtosimplyaslocalareanetworkorLANs,areusedtointerconnectdistributedcommunitiesofcomputer-basedDTEslocatedwithinabuildingorlocalizedgroupofbuilding.Forexample,aLANmaybeusedtointerconnectworkstationsdistributedaroundofficeswithinasinglebuildingoragroupofbuildingssuchasauniversitycampus.Alternatively,itmaybecomplex.Sincealltheequipmentislocatedwithinasingleestablishment,however,LANsarenormallyinstalledandmaintainedbytheorganization.Hencetheyarealsoreferredtoasprivatedatanetworks.ThemaindifferencebetweenacommunicationpathestablishedusingaLANandaconnectionmadethroughapublicdatanetworkisthataLANnormallyoffersmuchhigherdatetransmissionratesbecauseoftherelativelyshortphysicalseparationsinvolved.InthecontextoftheISOReferenceModelforOSI,however,thisdifferencemanifestsitselfonlyatthelowernetworkdependentlayers.Inmanyinstancesthehigherprotocollayersinthereferencemodelarethesameforbothtypesofnetwork.BeforedescribingthestructureandoperationofthedifferenttypesofLAN,itisperhapshelpfultofirstidentifysomeoftheselectionissuesthatmustbeconsidered.Itshouldbestressedthatthisisonlyasummary;therearealsomanypossiblelinksbetweenthetipsofthebranchesassociatedwiththefigure.TopologyMostwideareanetworks,suchasthePSTN,useamesh(sometimesreferredtoasanetwork)topology.WithLANs,however,thelimitedphysicalseparationofthesubscriberDTEsallowssimplertopologiestobeused.Thefourtopologiesincommonusearestar,bus,ringandhub.ThemostwidespreadtopologyforLANsdesignedtofunctionasdatacommunicationsubnetworksfortheinterconnectionoflocalcomputer-basedequipmentisthehubtopology,whichisavariationavariationofthebusandring.Sometimesitiscalledhub/treetopology.2.TransmissionmediaTwistedpair,coaxialcableandopticalfiberarethethreemaintypeoftransmissionmediumusedforLANs.MediumaccesscontrolmethodsTwotechniqueshavebeenadoptedforuseofthemediumaccesscontrolintheLANs.Theyarecarrier-sense-multiple-accesswithcollisiondetection(CSMA/CD),forbusnetworktopologies,andcontroltoken,forusewitheitherbusorringnetworks.CSMA/CDisusedtocontrolmultiple-accessnetworks.Eachonthenetwork“l(fā)istens”beforeattemptingtosendamessage,waitingforthe“traffic”toclear.Iftwostationstrytosenttheirmessagesatexactlythesametime,a“collision”isdetected,andbothstationsarerequiredto“stepback”andtrylater.Controltokenisanotherwayofcontrollingaccesstoasharedtransmissionmediumthatisbytheuseofacontrol(permission)token.ThistokenispassedformoneDTEtoanotheraccordingtoadefinedsetofrulesunderstoodandadheredtobyallDTEsconnectedtothemedium.ADTEmayonlytransmitaframewhenitisinpossessionofthetokenand,afterithastransmittedtheframe,itpassedthetokenontoallowanotherDTEtoaccessthetransmissionmedium.中文譯文：計(jì)算機(jī)網(wǎng)絡(luò)14.1數(shù)據(jù)通信圖14-1是數(shù)據(jù)通信鏈路的簡化框圖。雖然圖中只畫了一個(gè)從設(shè)備，但它代表了多點(diǎn)系統(tǒng)中的典型從設(shè)備。主計(jì)算機(jī)的功能之一是存儲供各種設(shè)備使用的應(yīng)用程序。端設(shè)備可以是計(jì)算機(jī)、打印機(jī)、鍵盤、CRT等，它們可以產(chǎn)生要發(fā)送的數(shù)字信息，也可使用所接收的數(shù)字?jǐn)?shù)據(jù)。這種設(shè)備通常在內(nèi)部以字為單位處理數(shù)字信息——在一個(gè)實(shí)際設(shè)備中構(gòu)成一個(gè)字的所有位是并行傳遞的。數(shù)字?jǐn)?shù)據(jù)在發(fā)送一個(gè)8對傳輸線，這是不經(jīng)濟(jì)的。那些在發(fā)送、接收數(shù)據(jù)分別完成并-串和串-并轉(zhuǎn)換并進(jìn)行數(shù)據(jù)鏈路管理的所有電路，一般稱為數(shù)據(jù)終端設(shè)備（DTE）。UART（通用異步接收/發(fā)送器）和USART（通用同步/異步接收/發(fā)送器）是實(shí)現(xiàn)并-串和串-并轉(zhuǎn)換的設(shè)備。主DTE包括控制多點(diǎn)數(shù)據(jù)鏈路系統(tǒng)中信息流的線路控制部件（LCU或LinCo）。從設(shè)備一方的相應(yīng)設(shè)備是工作站控制器（STACO）。如果DTE有與LCU相關(guān)的軟件，則稱為前端處理機(jī)（FEP）。同時(shí)，DTE也是數(shù)據(jù)鏈路系統(tǒng)中屬于用戶的最后一個(gè)設(shè)備。在兩個(gè)DTE之間，先是調(diào)制解調(diào)器，接著是屬于并有電話公司維護(hù)的通信設(shè)備。最近的調(diào)整結(jié)果是調(diào)制解調(diào)器已不屬于電話公司獨(dú)家管理的設(shè)備了。數(shù)據(jù)通信設(shè)備（DCE）從DTE接收串行數(shù)據(jù)流，并將其轉(zhuǎn)化成適合在話音線路上發(fā)送的某種模擬信號形式。在接收端，DCE完成相反的功能，把接收到的模擬信號轉(zhuǎn)換成串行數(shù)字?jǐn)?shù)據(jù)流。最簡單的DCE是調(diào)制解調(diào)器或數(shù)傳機(jī)。在發(fā)送端，調(diào)制解調(diào)器可以視為一種數(shù)/模轉(zhuǎn)換器，而在接收端則可視為一種模/數(shù)轉(zhuǎn)換器。調(diào)制解調(diào)器最常用的幾種調(diào)制方式是移頻鍵控（FSK）、移相鍵控（PSK）和正交調(diào)幅（QAM）。這是一種典型的使用模擬電話線路傳輸數(shù)據(jù)的方式。如果使用數(shù)字通信（有時(shí)稱為T-載波）傳輸數(shù)據(jù)，則必須使用脈碼調(diào)制（PCM）設(shè)備。也可以用微波傳輸系統(tǒng)進(jìn)行的數(shù)據(jù)通信，當(dāng)然還可以用衛(wèi)星通信系統(tǒng)去傳輸數(shù)據(jù)。如果鏈接DTE和DCE的電纜和信號電平都是未調(diào)整的，則所產(chǎn)生的偏差就是可能與生產(chǎn)廠家的數(shù)量成正比。電子工業(yè)協(xié)會（EIA）（由關(guān)心建立行業(yè)標(biāo)準(zhǔn)的廠家組成的組織）同意將RS-232C作為DTE和調(diào)制解調(diào)器間的標(biāo)準(zhǔn)接口。它是一個(gè)25針電纜，每一針都規(guī)定了功能及信號電平，預(yù)計(jì)RS-232C將被修訂的標(biāo)準(zhǔn)所取代。14.2計(jì)算機(jī)網(wǎng)絡(luò)的體系結(jié)構(gòu)計(jì)算機(jī)網(wǎng)絡(luò)是由兩個(gè)或多個(gè)計(jì)算機(jī)設(shè)備互連而成的一種復(fù)合系統(tǒng)，用于數(shù)據(jù)通信和資源共享。網(wǎng)絡(luò)設(shè)計(jì)及邏輯結(jié)構(gòu)應(yīng)該遵循一套設(shè)計(jì)原則，包括功能的組織與數(shù)據(jù)格式和過程的描述，這就是網(wǎng)絡(luò)的體系結(jié)構(gòu)。由于這種體系結(jié)構(gòu)是分層次的，故又稱為一組層次和協(xié)議。下面兩節(jié)將討論兩個(gè)重要的網(wǎng)絡(luò)體系結(jié)構(gòu)，即OSI參考模型和TCP/IP參考模型。OSI參考模型OSI模型如圖14-2所示（物理介質(zhì)未畫出）。這一模型是基于國際標(biāo)準(zhǔn)化組織（ISO）提出的一個(gè)建議，作為用于各層次的國際標(biāo)準(zhǔn)協(xié)議的第一步。該模型也稱為ISO開放系統(tǒng)互連參考模型，因?yàn)樗婕伴_放系統(tǒng)的互連，即于其他系統(tǒng)的通信是開放的，通常簡稱為OSI模型。OSI模型有7層。應(yīng)該注意的是，OSI模型本身并不是一種網(wǎng)絡(luò)體系結(jié)構(gòu)，因?yàn)樗痪唧w規(guī)定每一層所使用的確切的服務(wù)和協(xié)議。它只是說明每一層應(yīng)該做什么。然而，ISO還是對各層制定出了標(biāo)準(zhǔn)，盡管這些標(biāo)準(zhǔn)不是參考模型本身的一部分。每一層都作為一個(gè)單獨(dú)的國際標(biāo)準(zhǔn)來頒布。TCP/IP參考模型TCP/IP參考模型是美國國防部大約于1978年設(shè)計(jì)的一種早期的運(yùn)輸協(xié)議。該協(xié)議通常被視為OSI無連接工作方式的源頭。TCP/IP仍在廣泛的使用，并且被稱為互聯(lián)網(wǎng)事實(shí)上的行業(yè)標(biāo)準(zhǔn)。TCP/IP有兩部分：TCP和IP。TCP表明是處于運(yùn)輸層，IP意為是網(wǎng)絡(luò)層。（1）運(yùn)輸層有兩個(gè)端到端協(xié)議，其中之一是傳輸協(xié)議（TCP），另一個(gè)是用戶數(shù)據(jù)報(bào)協(xié)議（UDP）。TCP是一種面向連接的協(xié)議，它可以在互聯(lián)網(wǎng)上無差錯(cuò)地將字節(jié)流從一臺機(jī)器發(fā)送出去。UDP是一種不可靠的無連接協(xié)議，用于不希望那種TCP流控制序列，而希望只提供自身功能的場合。（2）網(wǎng)絡(luò)層定義了正式的分組格式及稱為互聯(lián)網(wǎng)協(xié)議的IP。該網(wǎng)絡(luò)層的任務(wù)是將IP分組發(fā)送到指定的地方。圖14-3所示為TCP/IP參考模型。在運(yùn)輸層上面是應(yīng)用層。應(yīng)用層包括所有高層協(xié)議。早期的協(xié)議包括虛擬終IELNET、文件傳送協(xié)議FTP、電子郵件（SMTP）和域名服務(wù)DNS。14.3廣域網(wǎng)廣域網(wǎng)跨越地域很大，通常是一個(gè)國家或一個(gè)大陸。這種網(wǎng)絡(luò)中匯集了運(yùn)行各種用戶程序（即應(yīng)用程序）的計(jì)算機(jī)。本文還是沿用傳統(tǒng)的稱呼把它們叫做主機(jī)，在一些文獻(xiàn)里有時(shí)也稱其為端系統(tǒng)。這些主機(jī)是通過通信子網(wǎng)（有時(shí)簡稱子網(wǎng)）連接起來的。子網(wǎng)的任務(wù)是把報(bào)文從一個(gè)主機(jī)傳送到另一個(gè)主機(jī)，就像電話系統(tǒng)把話音從講話人傳到收聽者一樣。把網(wǎng)絡(luò)（子網(wǎng)）的純通信任務(wù)與主機(jī)的應(yīng)用任務(wù)分開以后，整個(gè)網(wǎng)絡(luò)的設(shè)計(jì)工作就得到很大的簡化。圖14-4展示了主機(jī)與通信子網(wǎng)之間的關(guān)系。廣域網(wǎng)的分類方法很多，其中一種方法是按傳輸設(shè)備中的信息流分類。如果使用這種方法，可將廣域網(wǎng)分成3種基本類型：電路交換、專線和分組交換。電路交換網(wǎng)電路交換網(wǎng)是最常用的并且是幾乎所有用戶都要用的網(wǎng)絡(luò)類型。然而，公用電話交換網(wǎng)不限于電話公司。任何單位只要購買了合適的交換設(shè)備，就可以建立自己的內(nèi)部電路交換網(wǎng)，而且如果需要，還可配置一至多個(gè)與公用交換網(wǎng)互連的接口，在公用網(wǎng)和他們的專用內(nèi)部網(wǎng)之間話音和數(shù)據(jù)傳輸。專線網(wǎng)這是一種通過租用線連接的專用網(wǎng)。租用線是由租用客戶專用的一條通信線，不須進(jìn)行交換機(jī)間的交換。租用或私人線路是用戶專用的。專線的優(yōu)點(diǎn)是終端或計(jì)算機(jī)一直連在租用線上，服務(wù)的響應(yīng)時(shí)間極短。分組交換網(wǎng)分組網(wǎng)絡(luò)是用分組裝拆設(shè)備、分組路由的設(shè)備，以及將分組從源傳到目的地的設(shè)備構(gòu)成的。某些類型的數(shù)據(jù)終端設(shè)備DTE可以生成他們自己的分組，而其他一些DTE則要使用分組裝拆器（PAD）把其協(xié)議轉(zhuǎn)化成分組。分組通過分組交換機(jī)在網(wǎng)上傳遞。當(dāng)分組在網(wǎng)上傳輸時(shí)，分組