通信英语1、通信专业基础知识2、基础英语知识专业英语学习的要素：通信英语学习的三个要点：1、常见语法现象2、构词方法3、翻译方法和技巧重要提示：1、网上作业提交2、综合练习3、课后练习ThePrincipleofPCM专业背景介绍专业背景介绍PCMisdependentonthreeseparateoperations,sampling,quantizing,andcoding.dependentadj.依靠，依赖依靠，依赖onseparateadj.不同的不同的operationn.操作，运算，手术操作，运算，手术samplev.取样取样quantizev.量化量化codev.编码编码Manydifferentschemesforperformingthesethreefunctionshaveevolvedduringrecentyears,andweshalldescribethemainones.schemen.方案，计划方案，计划performv.执行，完成执行，完成functionn.功能功能evolvev.发展，演化发展，演化recentadj.近来近来describev.描述描述Inthesedescriptionsweshallseehowaspeechchanneloftelephonequalitymaybeconveyedasaseriesofamplitudevalues,eachvaluebeingrepresented,thatis,coded,asasequenceof8binarydigits.descriptionn.描述描述channeln.信道，话路信道，话路conveyv.转化，传达转化，传达amplituden.幅度representv.表示，代表sequencen.序列binaryadj.二进制的Furthermore,weshallprovethataminimumtheoreticalsamplingfrequencyoforder6.8kilohertz(kHz)isrequiredtoconveyavoicechanneloccupyingtherange300Hzto3.4kHz.provev.证明minimumn.最小值frequencyn.频率ordern.数量级，命令occupyv.占据，占用rangen.范围Practicalequipments,however,normallyuseasamplingrateof8kHz,andif8-digitspersamplevalueareused,thevoicechannelbecomesrepresentedbyastreamofpulseswitharepetitionrateof64kHz.equipmentn.设备设备normallyadv.通常地通常地ratesn.速率速率streamn.流流repetitionn.重复重复Reexaminationofoursimpleexampleshowsusthatthespeechsignalofmaximumfrequency3.4kHzhasbeenrepresentedbyasignaloffrequency64kHz.reexaminationn.再审查，再看再审查，再看showv.展示，表演展示，表演maximumn.最大值最大值However,ifonly4-digitspersamplevaluehadbeenused,thequalityoftransmissionwoulddrop,andtherepetitionrateofthepulseswouldbereducedto32kHz.qualityn.质量质量transmissionn.传输传输dropv.下降下降reducev.减少减少Thusthequalityoftransmissionisdependentonthepulserepetitionrate,andfordigitalcommunicationsystemsthesetwovariablesmaybeinterchangedmostefficiently.variablen.变量变量interchangev.相互转换相互转换efficientlyadv.有效的有效的Digitaltransmissionprovidesapowerfulmethodforovercomingnoisyenvironments.powerfuladj.有力的，有力的，methodn.方法方法overcomev.克服克服environmentn.环境环境Noisecanbeintroducedintotransmissionpathinmanydifferentways:perhapsviaanearbylightningstrike,thesparkingofacarignitionsystem,orthethermallow-levelnoisewithinthecommunicationequipmentitself.Introducev.引入，介绍引入，介绍Lightningn.闪电闪电striken.打，打击sparkv.闪烁，打火球ignitionn.点火thermaladj.热的leveln.电平Itistherelationshipofthetruesignaltothenoisesignal,knownasthesignal-to-noiseratio,whichisofthemostinteresttothecommunicationengineer.relationshipn.关系关系Basically,ifthesignalisverylargecomparedtothenoiselevel,thenaperfectmessagecantakeplace;however,thisisnotalwaysthecase.basicallyadv.基本上的基本上的compareto比作比作perfectadj.完美的完美的messagen.消息，信息消息，信息takeplace发生发生Forexample,thesignalreceivedfromasatellite,locatedinfarouterspace,isveryweakandisatalevelonlyslightlyabovethatofthenoise.satelliten.卫星卫星locatev.位于位于slightlyadv.稍微稍微noisen.噪声噪声Alternativeexamplesmaybefoundwithinterrestrialsystemswhere,althoughthemessagesignalisstrong,soisthenoisepower.alternativeadj.替代的，迂回的替代的，迂回的terrestrialadj.地面的地面的Ifweconsiderbinarytransmission,thecompleteinformationaboutaparticularmessagewillalwaysbeobtainedbysimplydetectingthepresenceorabsenceofthepulse.considerv.考虑考虑completeadj.完整的完整的obtainv.获得获得detectv.检测检测presencen.出席，到场出席，到场absencen.缺席，不在缺席，不在Bycomparison,mostotherformsoftransmissionsystemsconveythemessageinformationusingtheshape,orlevelofthetransmittedsignal;parametersthataremosteasilyaffectedbythenoiseandattenuationintroducedbythetransmissionpath.bycomparison相比之下相比之下shapen.波形，形状波形，形状parametern.参数参数affectv.影响影响attenuationv.衰减衰减pathn.路径路径Consequentlythereisaninherentadvantageforovercomingnoisyenvironmentsbychoosingdigitaltransmission.consequentlyadv.因而因而inherentadj.固有的，本质的固有的，本质的advantagen.优势优势choosev.选择选择Sofarinthisdiscussionwehaveassumedthateachvoicechannelhasaseparatecoder,theunitthatconvertssampledamplitudevaluestoasetofpulses;anddecoder,theunitthatperformsthereverseoperation.sofar至此，迄今为止至此，迄今为止assumev.假定假定Thisneednotbeso,andsystemsareinoperationwhereasinglecodec(i.e.,coderanditsassociateddecoder)issharedbetween24,30,oreven120separatechannels.inoperation运转着，生效运转着，生效codec编解码器编解码器associatev.使发生联系，使联合使发生联系，使联合sharev.共用，分享共用，分享Ahigh-speedelectronicswitchisusedtopresenttheanaloginformationsignalofeachchannel,takeninturn,tothecodec.switchn.开关开关presentv.引见引见,给给,上演上演,提出提出,呈现呈现informationn.信息信息takeinturn轮流轮流Thecodecisthenarrangedtosequentiallysampletheamplitudevalue,andcodethisvalueintothe8-digitsequence.arrangev.安排安排sequentiallyadv.顺序的顺序的samplev.取样取样Thustheoutputtothecodecmaybeseenasasequenceof8pulsesrelatingtochannel1,thenchannel2,andsoon.output输出输出relateto涉及，对应涉及，对应Thisunitiscalledatimedivisionmultiplexer(TDM),andisillustratedinFig.1-2.divisionn.分开分开multiplexern.复用器复用器illustratev.描述，说明描述，说明Themultiplexingprinciplethatisusedisknownaswordinterleaving,sincethewords,or8-digitsequences,areinterleavedintime.beknownas被称作，被认为被称作，被认为interleavev.复用，间插复用，间插Thereadermayask,howdoesthedemultiplexerknowwhichgroupof8-digitsrelatestochannel1,2,andsoon?Clearlythisisimportant!demultiplexern.分路器分路器Atthereceiveterminalademultiplexerisarrangedtoseparatethe8-digitsequencesintotheappropriatechannels.terminaln.终端终端separatev.分开分开appropriateadj.适当的适当的Theproblemiseasilyovercomebyspecifyingaframeformat,whereatthestartofeachframeauniquesequenceofpulsescalledtheframecode,orsynchronizationword,isplacedsoastoidentifythestartoftheframe.specifyv.指定，详细说明指定，详细说明framen.帧帧uniqueadj.唯一的，独特的唯一的，独特的synchronizationn.同步字同步字identifyv.标识标识Acircuitofthedemultiplexerisarrangedtodetectthesynchronizationword,andtherebyitknowsthatthenextgroupof8-digitscorrespondstochannel1.Thesynchronizationwordreoccursonceagainafterthelastchannelhasbeenreceived.reoccurv.再发生，再出现再发生，再出现翻译要点：1、被动句翻译2、词类转换3、定语从句翻译4、宾语从句翻译5、状语从句翻译UNIT2AsynchronousSerialDatatransmission背景介绍ByfarthemostpopularserialinterfacebetweenacomputeranditsCRTterminalistheasynchronousserialinterface.byfar用来修饰比较级和最高级interfacen.接口CRT阴极射线管asynchronous异步的Thisinterfaceissocalledbecausethetransmitteddataandthereceiveddataarenotsynchronizedoveranyextendedperiodandthereforenospecialmeansofsynchronizingtheclocksatthetransmitterandreceiverisnecessary.synchronizev.使同步extendv.扩展，延伸specialadj.特殊的meansn.手段，方法clockn.时钟Infact,theasynchronousserialdatalinkisaveryoldformofdatatransmissionsystemandhasitsoriginintheeraofteleprinter.infact事实上事实上linkn.链路，线路，中继线链路，线路，中继线originn.起源，开始起源，开始teleprintern.电传打字机电传打字机Serialdatatransmissionsystemshavebeenaroundforalongtimeandarefoundinthetelephone(humanspeech),Morsecode,semaphore,andeventhesmokesignalsonceusedbynativeAmericans.semaphoren.旗语smokesignal烽火Thefundamentalproblemencounteredbyallserialdatatransmissionsystemsishowtosplittheincomingdatastreamintoindividualunits(i.e.,bits)andhowtogrouptheseunitsintocharacters.fundamentaladj.基本的，根本的encounterv.遇到splitv.分开incomingadj.输入的，进来的individualn.个别的，单个的unitn.单元，部件charactern.字符Forexample,inMorsecodethedotsanddashesofacharacterareseparatedbyaninter-symbolspace,whiletheindividualcharactersareseparatedbyaninter-characterspace,whichisthreetimesthedurationofaninter-symbolspace.dotn.点dashn.短划线separatev.分开inter-symbol符号间inter-character字符间spacen.空间，间隔durationn.持续时间Firstweexaminehowthedatastreamisdividedintoindividualbitsandthebitsgroupedintocharactersinanasynchronousserialdatalink.Thekeytotheoperationofthistypeoflinkisbothsimpleandingenious.Fig.2-1givestheformatofdatatransmittedoversuchalink.examinev.检查，查看streamn.流dividev.分开keyn.要点，关键ingeniousadj.精巧的Anasynchronousserialdatalinkissaidtobecharacteroriented,asinformationistransmittedintheformofgroupsofbitscalledcharacters.orientedv.面向，朝向Thesecharactersareinvariableunitscomprising7or8bitsof“information”plus2to4controlbitsandfrequentlycorrespondtoASCII-encodedcharacters.invariableadj.固定不变的comprising包括frequently通常的correspondto对应ASCIIInitially,whennoinformationisbeingtransmitted,thelineisinanidlestate.Traditionally,theidlestateisreferredtoasthemarklevel.Byconventionthiscorrespondstoalogical1level.initiallyadv.开始的开始的idleadj.空闲的，懒散的空闲的，懒散的traditionallyadv.传统的传统的referto称作称作markn.传号传号convention宗教会议宗教会议by习惯习惯logicaladj.逻辑的逻辑的Whenthetransmitterwishestosenddata,itfirstplacesthelineinaspacelevel(i.e.,thecomplementofamark)foroneelementperiod.ThiselementiscalledthestartbitandhasadurationofTseconds.wishv.希望placev.设置spacen.空号elementn.元素，单元Thetransmitterthensendsthecharacter,1bitatatime,byplacingeachsuccessivebitonthelineforadurationofTseconds,untilallbitshavebeentransmitted.successiveadj.连续的Thenasingleparitybitiscalculatedbythetransmitterandsentafterthedatabits.Finally,thetransmittersendsastopbitatamarklevel(i.e.,thesamelevelastheidlestate)foroneortwobitperiods.Nowthetransmittermaysendanothercharacterwheneveritwishes.parityn.奇偶校验，奇偶校验，calculaten.计算计算Atthereceivingendofanasynchronousserialdatalink,thereceivercontinuallymonitorsthelinelookingforastartbit.Oncethestartbithasbeendetected,thereceiverwaitsuntiltheendofthestartbitandthensamplesthenextNbitsattheircenters,usingaclockgeneratedlocallybythereceiver.continuallyadv.连续的monitorv.监测lookfor寻找detectv.检测endn.结束generatev.产生Aseachincomingbitissampled,itisusedtoconstructanewcharacter.Whenthereceivedcharacterhasbeenassembled,itsparityiscalculatedandcomparedwiththereceivedparitybitfollowingthecharacter.Iftheyarenotequal,aparityerrorflagissettoindicateatransmissionerror.constructv.建设，组成assemblev.装配，汇齐comparev.比较equaladj.相等的errorn.差错flagn.标志indicatev.指示Themostcriticalaspectofthesystemisthereceivertiming.Thefallingedgeofthestartbittriggersthereceiverslocalclock,whichsampleseachincomingbitatitsnominalcenter.criticaladj.紧要的，关键的aspectn.方面timingn.定时edgen.边，沿triggerv.触发，启动nominaladj.标称的，名义的SupposethereceiverclockwaitsT/2secondsfromthefallingedgeofthestartbitandsamplestheincomingdataeveryTsecondsthereafteruntilthestopbithasbeensampled.Fig.2-2showsthissituation.Asthereceiversclockisnotsynchronizedwiththetransmitterclock,thesamplingisnotexact.supposev.假设situationn.情况exactadj.精确的Themostobviousdisadvantageofasynchronousdatatransmissionistheneedforastart,parity,andstopbitforeachtransmittedcharacter.If7bitcharactersareused,theoverallefficiencyisonly70%.obviousadj.明显的disadvantagen.缺点overalladj.整个的efficiencyn.效率Alessobviousdisadvantageisduetothecharacter-orientednatureofthedatalink.WheneverthedatalinkconnectsaCRTterminaltoacomputer,fewproblemsarise,astheterminalisitselfcharacteroriented.However,ifthedatalinkisbeingusedto,say,dumpbinarydatatoamagnetictape,problemsarise.dueto由于naturen.特性dumpv.倾倒，转储magneticadj.磁的UNIT4Internet背景介绍TheInternetisagiantnetworkofcomputerslocatedallovertheworldthatcommunicatewitheachother.gianta.巨大的locatev.位于TheInternetisaninternationalcollectionofcomputernetworksthatallunderstandastandardsystemofaddressesandcommands,connectedtogetherthroughbackbonesystems.collectionn.收集，聚集standardn.标准addressn.地址commandn.命令backbonen.骨干Itwasstartedin1969,whentheU.S.DepartmentofDefenseestablishedanationwidenetworktoconnectahandfulofuniversitiesandcontractors.DepartmentofDefense国防部establishv.建立nationwidea.全国性的handfuln.少数，少量universityn.大学contractorn.订阅人，承包人Theoriginalideawastoincreasecomputingcapacitythatcouldbesharedbyusersinmanylocationsandtofindoutwhatitwouldtakeforcomputernetworkstosurviveanuclearwarorotherdisasterbyprovidingmultiplepathbetweenusers.Originala.最初的，开始的Increasev.增加Computev.计算Capacityn.容量Sharev.分享，共用Findout找出，发现，查明survivev.幸免于，幸存，生还nucleara.原子能的，核的disastern.灾难，天灾multiplea.许多的pathn.路径PeopleontheARPNETquicklydiscoveredthattheycouldexchangemessagesandconductelectronic“conferences”withdistantcolleaguesforpurposesthathadnothingtodowiththemilitaryindustrialcomplex.exchangev.交换conductv.进行，从事conferencen.会议militarya.军事的complexa.复杂的;n.综合企业Ifsomebodyelsehadsomethinginterestingstoredonhisorhercomputer,itwasasimplemattertoobtainacopy.obtainv.获得copyn.拷贝Overtheyears,additionalnetworksjoinedwhichaddedaccesstomoreandmorecomputers.Thefirstinternationalconnection,toNorwayandEngland,wereaddedin1973.TodaythousandsofnetworksandmillionsofcomputersareconnectedtotheInternet.ItisgrowingsoquicklythatnobodycansayexactlyhowmanyusersOntheNet5.TheInternetisthelargestrepositoryofinformationwhichcanprovideveryverylargenetworkresources.Thenetworkresourcescanbedividedintonetworkfacilitiesresourcesandnetworkinformationresources.Thenetworkfacilitiesresourcesprovideustheabilityofremotecomputationandcommunication.Thenetworkinformationresourcesprovideusallkindsofinformationservices,suchasscience,education,business,history,haw,art,andentertainment,etc.repositoryn.仓库，资料丰富的地方resourcen.资源facilityn.设备，工具entertainmentn.娱乐，乐趣ThegoalofyouruseoftheInternetisexchangingmessagesorobtaininginformation.WhatyouneedtoknowisthatyoucanexchangemessagewithothercomputersontheInternetanduseyourcomputerasaremoteterminalondistantcomputers6.Buttheinternaldetailsofthelinkarelessimportant,aslongasitworks.Ifyouconnectcomputerstogetheronanetwork,eachcomputermusthaveauniqueaddress,whichcouldbeeitherawordoranumber.Forexample,theaddressofSamscomputercouldbeSam,oranumber.TheInternetisahugeinterconnectedsystem,butitusesjustahandfulofmethodtomovedataaround.UntiltherecentexplosionofpublicinterestintheInternet,thevastmajorityofthecomputersontheNetusetheUnixoperatingsystem.Asaresult,thestandardUnixcommandsforcertainInternetserviceshaveenteredtheonlinecommunityslanguagesasbothnounsandverbstodescribetheservicesthemselves.SomeoftheservicesthattheInternetcanprovideare:Mail,Remoteuseofanothercomputer(Telnet),FileTransfer(FTP),News,andLiveconversation.explosionn.爆炸，扩张，激增majorityn.多数，大多数Themostcommonlyusednetworkserviceiselectronicmail(E-mail),orsimplyasmail.Mailpermitsnetworkuserstosendtextualmessagestoeachother.Computersandnetworkshandledeliveringthemail,sothat7communicatingmailusersdonothavetohandledetailsofdelivery,anddonothavetobepresentatthesametimeorplace.Thesimplestwaytoaccessafileonanotherhostistocopyitacrossthenetworktoyourlocalhost.FTPcandothis.Presently,auserwithanaccountonanyInternetmachinecanestablishaliveconnectiontoanyothermachineontheNetfromtheterminalinhisownofficeorlaboratory.It8isonlynecessarytousetheUnixcommandthatsetsuparemoteterminalconnection(Telnet),followedbytheaddressofthedistantmachine.BeforeyoucanusetheInternet,youmustchooseawaytomovedatabetweentheInternetandyourPC.Thislinkmaybeahigh-speeddatacommunicationcircuits,alocalareanetwork(LAN),atelephonelineoraradiochannel.Mostlikely,youwilluseaModemattachedtoyourtelephonelinetotalktotheInternet.Naturally,thequalityofyourInternetconnectionandservice,likemanyotherthingsinlife9,isdictatedbytheamountofmoneyyouarewillingtospend.choosev.选择，挑选attachv.缚，系，附加modemn.调制解调器dictatev.命令，支配Althoughalltheseservicescanwellsatisfytheneedsoftheusersforinformationexchange,adefiniterequirementisneededfortheusers.Notonlyshouldtheusersknowwheretheresourceslocate,butalsoheshouldknowsomeoperatingcommandsconcerned10.Toeasethesearchingburdenoftheusers,recentlysomeconvenientsearchingtoolsappear,suchasGopher,WWWandNetscape.satisfyvt.满足，使满意definitea.明确的，确切的，肯定的requirementn.需要，要求WorldWideWeb(WWW)isanetworkedhypertextprotocolanduserinterface.ItprovidesaccesstomultipleservicesanddocumentslikeGopherdoesbutismoreambitiousinitsmethod.AjumptootherInternetservicecanbetriggeredbyamouseclickona“hot-linked”word,image,oriconontheWebpage.hypertextn.超级文本interfacen.接口，界面ambitiousn.有抱负的，雄心勃勃的triggervt.发射，引起iconn.画像，肖像，图标As11moreandmoresystemsjointheInternet,andasmoreandmoreformsofinformationcanbeconvertedtodigitalform,theamountofstuffavailabletoInternetuserscontinuestogrow.Atsomepointsverysoonafterthenationwide(andlaterworldwide)Internetstartedtogrow,peoplebegantotreattheNetasacommunity,withitsowntraditionandcustoms.stuffn.原料，资料，东西communityn.团体，社会traditionn.传统，惯例.Forexample,somebodywouldaskaquestioninaconference,andacompletestrangerwouldsendbackananswer:afterthesamequestionswererepeatedseveraltimesbypeoplewhohadntseentheoriginalanswers,somebodyelsegatheredalistof“frequentlyaskedquestions”andplaceditwherenewcomerscouldfindit.UNIT3TheISONetworkingStandards1TheearlydevelopmentofLANs,MANs,andWANswaschaoticinmanyways.Theearly1980ssawtremendousincreasesinthenumbersandsizesofnetworks.Ascompaniesrealizedtheycouldsavemoneyandgainproductivitybyusingnetworkingtechnology,theyaddednetworksandexpandedexistingnetworksalmostasrapidlyas2newnetworktechnologiesandproductscouldbeintroduced.chaotica.混乱的，无序的tremendousa.极大的，非常的，惊人的companyn.公司productivityn.生产率，生产能力networkv.联网addv.增加，添加Bythemid-1980s,thesecompaniesbegantoexperiencegrowingpainsfromalltheexpansionstheymade.Itbecameharderfornetworksthatuseddifferentspecificationsandimplementationstocommunicatewitheachother3.Theyrealizedthattheyneededtomoveawayfromproprietarynetworkingsystems.introducev.引入，导入painn.痛苦，疼痛specificationn.规格，规范，说明书implementationn.工具，器具，方法，手段Proprietarysystemsareprivatelydeveloped,owned,andcontrolled.Inthecomputerindustry,proprietaryistheoppositeofopen.Proprietarymeansthatoneorasmallgroupofcompaniescontrolsallusageandevolutionofthetechnology.Openmeansthatfreeusageofthetechnologyisavailabletothepublic.proprietarya.专有的，专用的privatelyad.私人地，隐私地evolutionn.发展，进化，演进usagen.利用，利用率Toaddresstheproblemofdifferentnetworksystemsbeingincompatibleandincapableofcommunicatingwitheachother4,theInternationalOrganizationforStandardization(ISO)researchednetworkschemes,suchasDECnet,SNA,andTCP/IP,tofindasetofrules.Asaresultofthisresearch,theISOcreatedanetworkmodelthatcouldhelpvendorscreatenetworksthatwouldbecompatiblewith,andinteroperatewith,othernetworks.interoperatea.互控的，互动的，交互运行的addressv.向讲话，提出；寻址compatiblea.适合的，一致的，兼容的standardizationn.标准化schemen.方案，设计，方法modeln.模型，模特vendorn.卖主，小贩，供应商TheOSIreferencemodel,releasedin1984,wasthedescriptiveschemetheycreated.Itprovidedvendorswithasetofstandardsthatcouldenablegreatercompatibilityandinteroperabilitybetweenthevarioustypesofnetworktechnologiesthatwereproducedbymanycompaniesaroundtheworld.releasev.发布，发表descriptivea.描述的，描写的，说明的objectiven.目的，目标acceleratev.加速，加快，促进TheOSIreferencemodelistheprimarymodelfornetworkcommunications.AprimaryobjectiveoftheOSIreferencemodelistoacceleratethedevelopmentoffuturenetworkingproducts.Althoughthereareothermodelsinexistence,mostnetworkvendorstodayrelatetheirproductstotheOSIreferencemodel,especiallywhentheywanttoeducateusersontheuseoftheirproducts.Theyconsideritthebesttoolavailabletoteachpeopleaboutsendingandreceivingdataonanetwork5.objectiven.目的，目标acceleratev.加速，加快，促进frameworkn.框架，结构，组织，机构visualizev.设想，想象，使可见packetn.包，分组TheOSIreferencemodelallowsyoutoviewthenetworkfunctionsthatoccurateachlayer.Moreimportantly,theOSIreferencemodelisaframeworkyoucanusetounderstandhowinformationtravelsthroughoutanetwork.Inaddition,theOSIreferencemodelcanbeusedtovisualizehowinformation,ordatapackets,travelsfromapplicationprograms,throughanetworkmedium,tootherapplicationprogramsthatarelocatedinanothercomputeronanetwork,evenifthesenderandthereceiverhavedifferenttypesofnetworkmedia.frameworkn.框架，结构，组织，机构visualizev.设想，想象，使可见packetn.包，分组IntheOSIreferencemodel,therearesevennumberedlayers.Eachlayerillustratesaparticularnetworkfunction.Thisseparationofnetworkingfunctionsiscalledlayering.Dividingthenetworkintothesesevenlayersprovidesthefollowingadvantages:TheapplicationlayeristheOSIlayerthatisclosesttotheuser.Itprovidesnetworkservices,suchasfileaccessandprinting,totheusersapplications.ItdiffersfromtheotherlayersinthatitdoesnotprovideservicestoanyotherOSIlayer,butrather,onlytoapplicationsoutsidetheOSImodel.Theapplicationlayerestablishestheavailabilityofintendedcommunicationpartners.Italsosynchronizesandestablishesanagreementonproceduresforerrorrecoveryandcontrolofdataintegrity.IfyouwanttorememberLayer7inasfewwordsaspossible,thinkofthebrowsers.integrityn.完整，完全，完善，综合browsern.浏览器ensurev.保证，担保，确保Thepresentationlayerensuresthattheinformationthattheapplicationlayerofonesystemsendsoutisreadablebytheapplicationlayerofanothersystem.Ifnecessary,thepresentationlayertranslatesbetweenmultipledataformatsbyusingacommonformat.Thislayeralsoisresponsibleforcompressionandencryption.IfyouwanttorememberofLayer6inasfewwordsaspossible,thinkofacommondataformat.multiplea.多重的，多路的，多倍的formatn.格式compressionn.压缩encryptionn.加密Asitsnameimplies,thesessionlayerestablishes,manages,andterminatessessionsbetweentwocommunicatinghosts.Thesessionlayerprovidesitsservicestothepresentationlayer.Italsosynchronizesdialoguebetweenthetwohostspresentationlayersandmanagestheirdataexchange.Inadditiontosessionregulation,thesessionlayeroffersprovisionsforefficientdatatransfer,classofservice,andexceptionreportingofsessionlayer,presentationlayer,andapplicationlayerproblems.IfyouwanttorememberLayer5inasfewwordsaspossible,thinkofdialoguesandconversations.terminatev.终结transfern.传输，转移summarizev.总结，概括Thetransportlayersegmentsdatafromthesendinghostssystemandreassemblesthedataintoadatastreamonthereceivinghostssystem.Theboundarybetweenthetransportlayerandthesessionlayercanbethoughtofastheboundarybetweenapplicationprotocolsanddata-flowprotocols.Whereastheapplication,presentation,andsessionlayersareconcernedwithapplicationissues,thelowerfourlayersareconcernedwithdatatransportissues.segmentn.部分，段，环节；v.分段，分节assemblev.组装，装配boundaryn.分界线，边界protocoln.协议issuen.事件，事情，（杂志的）一期，问题Thetransportlayerattemptstoprovideadatatransportservicethatshieldstheupperlayersfromtransportimplementationdetails.Specifically,suchissueashowreliabletransportbetweentwohostsisaccomplishedintheconcernofthetransportlayer.Inprovidingcommunicationservice,thetransportlayerestablishes,maintains,andproperlyterminatesconnection-orientedcircuits.Inprovidingreliableservice,transporterrordetection-and-recoveryandinformationflowcontrolareused.IfyouwanttorememberLayer4inasfewwordsaspossible,thinkoftheflowcontrolandreliability.shieldv.保护，掩盖，避开reliablea.可靠的orientv.使面向，面对Thenetworklayerisacomplexlayerthatprovidesconnectivityandpathselectionbetweentwohostsystemsthatmightbelocatedongeographicallyseparatednetworks.IfyouwanttorememberLayer3inasfewwordsaspossible,thinkofpathselection,routing,andlogicaladdressing.connectivityn.连接，连通separatev.分离，分开transitn.经过，通过，运行，过渡Thedatalinklayerprovidesthetransitofdataacrossaphysicallink.Insodoing,thedatalinklayerisconcernedwithphysical(asopposedtological)addressing,network(sometimescalledlogical)topology,networkmediaaccess,anderrordetection.IfyouwanttorememberLayer2inasfewwordsaspossible,thinkofframesandmediaaccesscontrol.topologhn.拓扑framen.帧，框架accessn.v.接入，连通Thephysicallayerdefinestheelectrical,mechanical,procedural,andfunctionalspecificationsforactivating,maintaining,anddeactivatingthephysicallinkbetweenendsystems.Suchcharacteristicsasvoltagelevels,timingofvoltagechanges,physicaldatarates,maximumtransmissiondistances,physicalconnectors,andother,similar,attributesaredefinedbyphysicallayerspecifications.IfyouwanttorememberLayer1inasfewwordsaspossible,thinkofsignalsandmedia.procedurala.程序上的，程序性的activatev.激活，建立maintainv.维持，维护，保持raten.速率transmissionn.传输attributev.把归因于，归咎于；n.属性，本质，特征UNIT5CloudComputing1Thecloudcomputingisonekindofemergingbusinesscomputingmodel.Itdistributesthedutytocalculatetotheresourcepoolwhichthemassivecomputersconstitute,enableseachkindofapplicationsystemaccordingtoneedtogainthecomputationstrength,thestoragespaceandallkindsofsoftwareservice2.Thiskindofresourcepooliscalled“thecloud”.“Thecloud”isvirtualcomputationresourcesthatcanmaintainandmanageitself,usuallyforsomelarge-scaleservercluster,includingcalculatingserver,storageserver,thebroadbandresourcesandsoon3.emerginga.新兴的，出现的distributev.分配，散布massivea.大量的，巨大的pooln.联营，水池virtuala.虚拟的computationn.计算，估计Thecloudcomputingwillconcentrateallcomputationresources,andcanbemanagedautomaticallythroughthesoftwarewithoutintervention.Theuserswhousecloudcomputingwillnotbeworriedbythetediousdetailsandconcentratehisownbusiness.CloudcomputingisarecenttrendinITthatmovescomputinganddataawayfromdesktopandportablePCsintolargedatacenters4.concentratev.集中，浓缩automaticallyad.自动地，不经思索地intervenev.干涉，干预tediousa.沉闷的，单调乏味的ThemajorcloudproviderssuchasGoogle,MicrosoftandAmazonhavebuiltandareworkingonbuildingtheworldslargestdatacentersacrosstheUnitedStatesandelsewhere.Eachdatacenterincludeshundredsofthousandsofcomputerservers,coolingequipmentandsubstationpowertransformers.substationn.变电站，变电所WiththelargescaleproliferationoftheInternetaroundtheworld,applicationscannowbedeliveredasservicesovertheInternet5.Themaingoalofcloudcomputingistomakeabetteruseofdistributedresources,combinethemtoachievehigherthroughputandbeabletosolvelargescalecomputationproblems.Cloudcomputinghassupercomputingpower.proliferationn.增殖，扩散capacityn.容量，能力Thousandsofcomputersformasuperserverincloudserviceswhichprovideuserswithpowerfulcomputinganddataprocessingcapacitythatishardtorealizeforapersonalcomputer6.Asaresultthisreducestheoverallcost.Incloudcomputing,usersaccessthedata,applicationsoranyotherserviceswiththehelpofabrowserregardlessofthedeviceusedandtheuserslocation.Cloudcomputinghasbeenenvisionedasthenext-generationcomputingmodelforitsmajoradvantagesinon-demandself-service,ubiquitousnetworkaccess,locationindependentresourcepoolingandtransferenceofrisk7.CloudComputingisthelatestdevelopmentsofcomputingmodelsafterdistributedcomputing,parallelprocessingandgridcomputing.ThedefinitionofcloudcomputingprovidedbyNationalInstituteofStandardsandTechnology(NIST)saysthat:“Cloudcomputingisamodelforenablingconvenient,on-demandnetworkaccesstoasharedpoolofconfigurablecomputingresources(e.g.,networks,servers,storageapplicationsandservices)thatcanberapidlyprovisionedandreleasedwithminimalmanagementeffortorserviceproviderinteraction”8.envisionv.想象，预想ubiquitousa.普遍存在的，无所不在的transferencen.转移，转让parallela.平行的，类似的gridn.格子，网格definitionn.定义，清晰度configurablea.可配置的，结构的“Cloudcomputing”simplymeans“Internetcomputing”,generallytheInternetisseenascollectionofclouds;thusthewordcloudcomputingcanbedefinedasutilizingtheInternettoprovidetechnologyenabledservicestothepeopleandorganizations9.CloudcomputingenablesconsumerstoaccessresourcesonlinethroughtheInternet,fromanywhereatanytimewithoutworryingabouttechnical/physicalmanagementandmaintenanceissuesoftheoriginalresources10.GoogleAppsistheparamountexampleofcloudcomputing,itenablesuserstoaccessservicesviathebrowseranddeployedonmillionsofmachinesovertheInternet.maintenancen.维护，维修originala.原始的，最初的paramounta.最重要的，主要的browsern.浏览器，阅读器.Cloudcomputingischeaperthanothercomputingmodels;zeromaintenancecostisinvolvedsincetheserviceproviderisresponsiblefortheavailabilityofservicesandclientsarefreefrommaintenanceandmanagementproblemsoftheresourcemachines11.Theprincipalservicemodelsbeingdeployedare:SoftwareasaService(SaaS)、PlatformasaService(PaaS)、InfrastructureasaService(IaaS).Acloudclientconsistsofcomputerhardwareand/orcomputersoftwarewhichreliesoncloudcomputingforapplicationdelivery12.Acloudapplicationdelivers“SoftwareasaService(SaaS)”overtheInternet,thuseliminatingtheneedtoinstallandruntheapplicationontheuserssystem.Softwaresareprovidedasaservicetotheconsumersaccordingtotheirrequirement,whichenableconsumerstousetheservicesthatarehostedonthecloudserver13.GoogleAppsisthemostwidelyusedSaaS.eliminatev.消除，排除hostv.主持，托管Platformservices“PlatformasaService(PaaS)”provideacomputingplatformusingthecloudinfrastructure14.Clientsareprovidedplatformsaccess,whichenablesthemtoputtheirowncustomizedsoftwaresandotherapplicationsontheclouds.Throughthisservicedeveloperscangetaholdofallthesystemsandenvironmentsrequiredforthelifecycleofsoftware,suchasitsdeveloping,testing,deployingandhostingofwebapplications.platformn.平台，月台customizev.定制，自定义environmentn.环境，外界Infrastructureservices“InfrastructureasaService(IaaS)”providetherequiredinfrastructureasaservice.Rentprocessing,storage,networkcapacity,andotherbasiccomputingresourcesaregranted,enableconsumerstomanagetheoperatingsystems,applications,storage,andnetworkconnectivity.Theclientneednotpurchasetherequiredservers,datacenterorthenetworkresources.Alsothekeyadvantagehereisthatcustomersneedtopayonlyforthetimedurationtheyusetheservice15.Asaresultcustomerscanachieveamuchfasterservicedeliverywithlesscost.grantv.同意，允许connectivityn.连接，联通，连接性UNIT6IntroductiontoOpticalFiberCommunication1Oneofthemostimportanttechnologicaldevelopmentsduringthe1980shasbeentheemergenceofopticalfibercommunicationasamajorinternationalindustry.Oneindicationoftheextentofthisdevelopmentisthetotallengthofinstalledfiber,whichwasestimatedtobe3.2millionkilometersintheU.S.alonebytheendof19872.Over90%ofthisfiberwasplacedinserviceduringthetimeperiodof19821987.Long-haultrunkinstallationshavebeendominated,accountingforabout95%ofthefiberintheU.S.emergencen.浮现，出现haulvt.用力拖；n.拖，拉，拖运的距离long-haul长运距的，长途的trunkn.干线，中继线，中继线路，局内线dominatevt.支配，统治；处于支配地位accountn.帐目，报道；说明accountfor解释，(数量等)占Althoughtelecommunicationistherationaleformostofthecurrentinterestinfiberoptics,thiswasnotthecaseduringtheearlydaysofthetechnology.Theresearcherswhoproducedthefirstcladglassopticalfibersintheearly1950swerenotthinkingofusingthemforcommunications;theywantedtomakeimagingbundlesforendoscopy.Fiberopticswasalreadyawell-establishedcommercialtechnologywhenthefamouspaperbyKaoandHockham,suggestingtheuseoflow-lossopticalfibersforcommunication,appearedin19663.rationalen.(某事物的)基本理由，理论基础clada.穿衣的，(金属)包层n.覆盖层，(用壳)包盖bundlen.捆，束endoscopyn.内窥镜Thefirstlow-loss(20dB/km)silicafiberwasdescribedinapublicationwhichappearedinOctoberof1970.Thedateofthispublicationissometimescitedasthebeginningoftheeraoffibercommunication.Althoughthisdevelopmentdidreceive4considerableattentionintheresearchcommunityatthetime,itwasfarfrominevitablethatamajorindustrywouldevolve.The20dB/kmlossfigurewasstilltoohighforlong-halltelecommunicationsystems.Thefiberswerefragile,andawaytoprotectthemwouldhavetobefound.silican.石英，二氧化硅publicationn.发表，出版物citevt.引用，举(例)communityn.团体，社会inevitablea.不可避免的，必然的，合情合理的evolvevt.使发展，推论；进展，发展，进化figuren.外形，图形，人物，数字fragilea.脆的，易碎的Therewerenosuitablelightsources.Researchersdidnotknowwhetherfieldterminationandsplicingofopticalcableswouldeverbepractical.Finally,therewereseriousdoubtsastowhetherthesecomponentscouldeverbeproduced5economicallyenoughforthetechnologytoplayamajorroleinthemarketplace.Althoughthetechnologicalbarriersappearedformidable,theeconomicpotentialwasverysignificant.Asaconsequence,researchanddevelopmentactivityexpandedrapidly,andanumberofimportantissueswereresolvedduringtheearly1970s.marketplacen.市场barriern.隔板，障碍，界限formidablea.可怕的，难以应付的，庞大的consequencen.结果，结论Duringthemiddleandlate1970s,therateofprogresstowardmarketableproductsacceleratedastheemphasisshiftedfromresearchtoengineering6.FiberswithlossesapproachingtheRayleighlimit7of2dB/kmatawavelengthof0.8m,0.3dB/kmat1.3m,and0.15dB/kmat1.55m,wereproducedinthelaboratory.Microbendlossproblemswereovercomethroughtheuseofimprovedfibercoatingsandcablingtechniques.Ruggedcablesandmultifibreconnectorswereproducedforfieldinstallation.acceleratevt.vi.加速Rayleighn.瑞利(人名)ruggeda.强壮的，艰苦的Roomtemperaturethresholdcurrentsforcommercialgalliumaluminumarsenidelasersoperatinginthe0.8to0.85mspectralregionwerereducedtothe20to30mArange,andprojectedlifetimesinthe100,000to1,000,000hourrangewereclaimedforbothlasersandLEDs.Lightsourcesandimprovedphotodetectorswhichoperatednear1.3mweredevelopedtotakeadvantageofthelowfibrelossanddispersioninthislongerwavelengthregion.Severalmajorfieldtrialswereundertakenduringthisperiod,includingAT&TsAtlantaexperimentandChicagoinstallation,andJapanssubscriberaccessproject.thresholdn.门槛，阀，门限galliumaluminumarsenide(GaAlAs)lasers镓铝砷激光器photodetectorn.光电探测器，光电检测器projectvt.设计，抛出n.项目，方案reliabilityn.可靠性trialn.(好坏、性能等的)试验Improvementsincomponentperformance,cost,andreliabilityby19808ledtomajorcommitmentsonthepartoftelephonecompanies.Fibersoonbecamethepreferredtransmissionmediumforlong-haultrunks.Someearlyinstallationsused0.8mlightsourcesandgraded-indexmultimodefiber,butby1983,designersofintercitylinkswerethinkingintermsof1.3m,single-modesystems.Thesingle-modefibre,usedinconjunctionwitha1.3mlaser,providesabandwidthadvantagewhichtranslatesintoincreasedrepeaterspacingsforhighdataratesystems.Dataratesforinstalledfiberopticsystemshaverecentlymovedintogigabitpersecondrange.Suchsystemsusethespectrallypuredistributed-feedbacklaserstominimizefibredispersioneffects.Fibresdesignedforlowdispersionat1.55mwavelength,whichcorrespondstominimumfiberloss,arenowcommonlyusedinlongdistancetransmission.Theuseofwavelengthmultiplexingtofurtherincreasethefibreinformationcapacityisbecomingmorewidespread.gigabitn.吉比特，千兆比特spectrally-purea.光谱纯的distributed-feedback(DFB)laser分布反馈激光器Thepotentialoffibreopticsinotherareasisonlybeginningtoberealized.Fibreopticnetworksforcomputersystemsandofficesarebecomingmoreprominent.Inthetelephonesystem,theuseoffibreopticsforinterconnectingcentralofficeswithinametropolitanareaandforlowerlevelsintheswitchinghierarchyisstillincreasingrapidly.Fibrelinkstothehomehavebeenusedindemonstrationprojects.potentialn.潜力，可能prominenta.突出的，杰出的，重要的metropolitana.首都的，主要城市的hierarchyn.分类等级upgradevt.使升级videoa.使视频的，电视的Manyobserversbelievethatnationaltelephonesystemswilleventuallybeupgradedtohandlevideobandwidthsbyusingfibreoptics.ThesewidebandsubscriberloopsystemswouldprovideaccesstoservicessuchasPicturephone,videoentertainment.Widespreadinstallationofthesebroadbandserviceswillbecomeeconomicallyfeasible.widebandn.宽频带，宽波段entertainmentn.招待，娱乐broadbandn.宽频带，宽波段feasiblea.可实行的，行得通的，可能的UNIT7SynchronousDigitalHierarchy1SDHisaninternationalstandardforhigh-speedsynchronousopticaltelecommunicationnetworks-aSynchronousDigitalHierarchy.WorkstartedonSDHstandardsinCCITTsStudyGroupXVIIIinJune1986.Theobjectivewastoproduceaworldwidestandardforsynchronoustransmissionsystemswhichprovidesnetworkoperatorswithaflexibleandeconomicnetwork.standardn.标准hierarchyn.体系，分层结构synchronousa.同步的approvev.批准，审订，通过tributarya.支流的，从属的，辅助的mapv.绘制，设计，映射，变换noden.节点，结InNovember1988thefirstSDHstandardswereapproved-G.707,G.708andG.709.Thesestandardsdefinetransmissionrates,signalformat,multiplexingstructuresandtributarymappingsfortheNetworkNodeInterface(NNI)-theinternationalstandardinterfaceforSynchronousDigitalHierarchy.InadditiontodefiningstandardscoveringtheNII,CCITTalsoembarkedonseriesofstandardsgoverningtheoperationofsynchronousmultiplexers(G.781,G.782andG.783)andSDHNetworkManagement2(G.784).ItisthestandardizationoftheseaspectsofSDHequipmentthatwilldelivertheflexibilityrequiredbynetworkoperatorstocost-effectivelymanagethegrowthinbandwidthandprovisioningofnewcustomerservicesexpectedinthenextdecade3.embarkv.从事，开始搞provisionn.准备，预备essencen.本质，精华，核心inessence本质上，实质上advanceda.先进的，前进的，高深的maintenancen.维护，保养approximatelyad.近似的，大约地TheSDHstandardsarebasedontheprinciplesofdirectsynchronousmultiplexingwhichisthekeytocost-effectiveandflexibletelecommunicationnetworking.Inessence,itmeansthatindividualtributarysignalsmaybemultiplexeddirectlyintoahigherrateSDHsignalwithoutintermediatestagesofmultiplexing.SDHNetworkElementscanthenbeinterconnecteddirectlywithobviouscostandequipmentsavingsovertheexistingnetwork.AdvancednetworkmanagementandmaintenancecapabilitiesarerequiredtoeffectivelymanagetheflexibilityprovidedbySDH.Approximately5%oftheSDHsignalstructureisallocatedtosupportingadvancednetworkmanagementproceduresandpractices.TheSDHsignaliscapableoftransporting4allthecommontributarysignalsfoundintodaystelecommunicationnetworks.ThismeansthatSDHcanbedeployedasoverlaytotheexistingsignaltypes.Inaddition,SDHhastheflexibilitytoreadilyaccommodatenewtypesofcustomerservicesignalsthatnetworkoperatorswillwishtosupportinthefuture.overlayn.覆盖层，涂善层accommodatev.供应，提供，容纳unifyv.使一致，使一体SDHcanbeusedinalltraditionaltelecommunicationsapplicationareas.SDHthereforemakesitpossibleforaunifiedtelecommunicationnetworkinfrastructuretoevolve.ThefactthatSDHprovidesasinglecommonstandardforthistelecommunicationsnetworkmeansthatequipmentsuppliedbydifferentmanufacturersmaybeinterconnecteddirectly.Now,letstakealookatthenetwork“buildingblocks”andhowtheyareconfigured.ThesenetworkelementsarenowalldefinedinCCITTstandardsandprovidemultiplexingorswitchingfunctions.configurev.使成形，使具形体，构成LineTerminalMultiplexer(LTM):LTMcanacceptanumberoftributarysignalsandmultiplexthemtotheappropriateopticalSDHratecarrier,i.e.STM-1,STM-4orSTM-16.TheinputtributariescaneitherbetheexistingPDHsignalssuchas2,34and140Mbit/sorlowerrateSDHsignals.LTMformsthemaingatewayfromthePDHtotheSDH.gatewayn.门口，入口，通道Add-DropMultiplexer(ADM)5:aparticulartypeofterminalmultiplexerdesignedtooperateinathroughmodefashion.WithintheADMitispossibletoaddchannelsto,ordropchannelsfromthe“through”signal.ADMisgenerallyavailableattheSTM-1andSTM-4interfaceratesandmayadd/dropavarietyoftributarysignals,i.e.2,34or140Mbit/s.TheADMfunctionisoneofthemajoradvantagesresultingfromtheSDHsincethesimilarfunctionwithinaPDHnetwork,requiredbanksofhardwiredback-backterminals.moden.方式，样式，模式varietyn.变化，多样化，各种各样SynchronousDXC:thesedeviceswillformthecornerstoneofthenewsynchronousdigitalhierarchy.Thycanfunctionassemi-permanentswitchesfortransmissionchannelsandcanswitchatanylevelfrom64kbit/suptoSTM-1.GenerallysuchdeviceshaveinterfacesatSTM-1orSTM-4.TheDXCcanberapidlyreconfigured,undersoftwarecontrol,toprovidedigitalleasedlinesandotherservicesofvaryingbandwidth.cornerstonen.基石，柱石，基础permanenta.永久的，持久的leasev.出租virtuala.虚的，假想的，虚拟的containern.容器Forclarity,asingleframeintheSTM-1canberepresentedbya2-dimensionalmap(seeFig.7-1).The2-dimensionalmapcomprises9rowsand270columnsofboxes.Eachboxrepresentsasingle8-bitsbytewithinthesynchronoussignal.Sixframingbytesappearinthetopleftcornerofthe2-dimensionalmap.Theseframingbytesactasamarker,allowing6anybyteintheframetobeeasilylocated.sectionn.节，段，部分dimensionala.维的，度的markern.记号，符号，标志intactad.;a.未经触动的（地），完整的（地），未受损的（地）capacityn.容量，能力，容纳量Thesignalbitsaretransmittedinasequencestartingwiththoseinthefirstrow.Theorderoftransmissionisfromlefttoright.Aftertransmissionofthelastbyteintheframe(thebytelocatedinrow9,column270),thewholesequencerepeatsstartingwiththe6framingbytesofthefollowingframe.Asynchronoustransportframecomprisestwodistinctandreadilyaccessiblepartswithintheframestructure-avirtualcontainerpartandasectionoverheadpart.VirtualContainer(VC)isarrangedmainlyforusersinformationtobetransmittedthroughthenetwork,buttheSectionOverhead(SOH)providesthefacilitiesrequiredtosupportandmaintainthetransportationofaVCbetweennodesinasynchronousnetwork,suchasalarmingmonitoring,bit-errormonitoringanddatacommunicationchannels.distincta.不同的，有区别的，清晰的，明显的accessiblead.很快地，容易地，简便地，直接地overheada.可接入的assemblev.集合，装配，聚集UNIT8WDM1EvenvisionariessuchasAlbertEinsteinandlsaacNewton,whocontributedsignificantlytoourunderstandingofthepropertiesoflightanditsfundamentalimportance,wouldnotlikelyimaginethecommunicationsnetworksoftoday.Highwaysoflightspantheglobe,transmittingmassiveamountsofinformationinthetwinklingofaneye2.Theequivalentsofmillionsoftelephonecallsaretransmittedonasinglefiber,thinnerthanahumanhair.Astoundingastheseadvancesmayseem,weareonlyatthebeginningofwhatispossible3.visionaryn.幻想家，梦想家contributev.捐献，贡献significantadv.重大的，效果显著的，具有特殊意义的propertyn.所有物，房产，所有权fundamentaladj.基础的，重要的，本性的spanv.跨过，延伸twinklev.闪烁，闪耀，眨眼equivalentadj.相同的，同等trafficn.交通，通信量，交易Thecurrentexplosionoftrafficintheworldwidenetworksisampleevidenceofthespeedwithwhichweareadoptingnewcommunicationstechnologies.Thegrowthofwirelesssystemsandtheinternetarewell-documentedphenomena.Nomatterwhatapplicationisgeneratingtraffic,mostofthistrafficwillbecarriedbytheunifyingopticallayer4.ampleadj.充足的，充分的，宽敞的evidencen.证据，证词phenomenan.单数为phenomenonn.现象unifyv.使一致，使化一，统一Forthisreason,thegrowthofvariousapplicationssuchastelephony(whethercellularorfixed),internet,videotransmission,computercommunicationanddatabaseaccessleadsdirectlytoanincreaseinthedemandplacedontheopticalnetwork.Itisverylikelythattheopticalnetworkwillbeusedtoconveylargeamountsofvideoinformationinthefuture.videoadj.电视的，视频的，录像的ThemoststrikingrecentadvancesinopticalnetworkinghavetakenplaceinthefieldofWavelengthDivisionMultiplexing(WDM).Theseadvanceshavebenefitedbothterrestrialandsubmarinesystems,increasedavailablecapacitiesbyseveralordersofmagnitudeand,correspondingly5,reducedcosts.strikingadj.引人注意的，显著的advancev.前进，提升，进展magnituden.广大，巨大，重要Untilquiterecently,itwaspossibletosendonlyonewavelength,orcolor,oflightalongeachfiber6.Alotofefforthasthereforebeenconcentratedinmaximizingtheamountofinformationthatcanbetransmittedusingasinglewavelength.Commercialsystemswillsoonbeabletocarry40Gbit/sonasinglewavelength,whileinthelabs320Gbit/ssystemshavealreadybeendemonstrated.correspondingadj.符合的，对应的，一致的lyadv.commercialadj.商业的，商务的demonstratev.示范，展示，演示WDM,ontheotherhand,makesitpossibletotransmitalargenumberofwavelengthsusingthesamefiber7,effectivelysendinga“rainbow”ofcolors,wheretherewasonlyonecolorbefore8.Alreadytoday,commerciallyavailablesystemscantransmit400Gbit/sofinformationonasinglefiber.Thatisequivalenttotransmittingapproximately200feature-lengthfilmspersecond.Recently,ateamofresearchersfromBellLabsdemonstratedlong-distance,error-freetransmissionof3.28Tbit/soverasingleopticalfiber.rainbown.虹，彩虹approximateadj.近似的，大概的lyadv.ThemajoradvancethathasledtotheWDMrevolutionhasbeentheinventionoftheOpticalAmplifier(OA).BeforetheinventionoftheOA,wavelengthhadtobeconvertedintoelectronicform,thenbackintoopticalformandthenretransmittedintothenextspanoffiber.Thiswasrelativelyexpensive,sincetheopticalcomponentsinvolvedarehighlyspecializeddevices9.TheOA,however,canboostthesignalpowerofallwavelengthsinthefiber,thuseliminatingtheeliminatingtheneedforseparateregenerators,andallowingmanywavelengthstosharethesamefiber.Advancesinopticalamplifierdesignhavebeenconsiderable.approximateadj.近似的，大概的lyadv.boostv.上推，增加，提高eliminatev.除去，淘汰considerableadj.相当大的，相当多的simultaneousadj.同时发生的，同时存在的，同时的lyadv.First,theoperatingwindowhasexpandedfrom12nm,inthefirsgeneration,toabout80nmtoday.ThisallowstheOAtoamplifymoresignalssimultaneously.Second,thedevelopmentofgainequalizationtechniqueshasenabledamuchflatterresponseandallowsanumberoftheseamplifierstobeconnectedinseries.Therehavealsobeenadvancesinthefibersthemselves.Intheearlydaysofopticalsystems,opticalfiberswerenotbuiltformulti-wavelengthtransmission.Todaysfibers,ontheotherhand,aredesignedtohavewidetransmissionwindowsandareoptimizedforhigh-capacity,multiple-wave-lengthtransmission.equalizationn.相等，均等，平等optimizev.乐观的考虑，使尽可能完善Thegrowingdemandonopticalnetworkisacomplexissue.Ontheonehand,thegrowthincapacitydemandisextraordinary,andthisinitself10wouldbeabigenoughchallengetomeet.However,thisisaccompaniedbyanincreasingvarietyofservicesandapplications,aswellasmuchmoreexactingrequirementsforqualitydifferentiation,Forexample,thereisquiteadifferenceinthequalityrequirementforasignalbeingusedtotransmitanemergencytelephonecallorlivevideocoverageofamedicaloperation,ascomparedwithanE-mailthatisnoturgentandcanarriveafterseveralhours.extraordinarya.非常的，特别的，非凡的exactingadj.苛求的，严格的differentiationa.分化，变异，演变medicaladj.医学的，医术的，医疗的However,thesameopticalinfrastructureisexpectedtosupportthiswidevarietyofservices.InternetProtocol(IP)traffic,inparticular,isgrowingexponentially.Insomepartsoftheworld,itisexpectedthatIPwillconstitutethemajorityoftrafficinthenearfuture.Therefore,existingnetworkswillhavetobeprogressivelyoptimizedtohandlevarioustypesoftraffic.WDMhasamajoradvantageinthisregard,whichisthatthedifferenttypesoftrafficcanbeassignedtodifferentwavelengths,asrequired.infrastructuren.基础，基础结构exponentialadj.指数的，幂的constitutev.构成，组成，设立majorityn.多数，半数以上inthisregard在这一点上，关于此事Fortunately,wewillsoonbeinapositiontorouteindividualwavelengthsflexiblythroughanopticalnetwork.Featuressuchasadd/dropandcross-connectionintheopticaldomainarebeingmadepossiblebyadvanceinphotonics11.Iwouldliketodrawattentiontoafewrecentadvancesinthisarea.Firstly,theso-calleddigitalwrapperisintheprocessofbeingstandardizedintheinternationalbodies.Asecondsignificantdevelopmentistheall-opticalcross-connect.BellLabshasrecentlyunveileditsall-opticalcrossconnectcalledtheLambdaRouter.BasedonMicroElectroMechanicalSwitching(MEMS)technology,itconsistsofmicroscopicmirrorsthattilt,andthusre-directopticalsignals.flexibleajd.易弯曲的，可变通的，灵活的domainn.领域，领地，范围wrappern.包装者，包装物，覆盖物standardizev.使与标准比较，使合标准，使标准化twofoldadj.两倍的，两重的identifyv.使等同于，识别，鉴定unveilv.使公诸于众，揭露，展出microscopicadj.显微镜的，微小的，细微的implementv.贯彻，完成，履行Itissuchatechnologythatwillenableustobuildnetworksthatarepurelyoptical12.Asmoreroutingfunctionsareimplementedintheopticalplane,moresophisticatedintelligenceisneededtocontrolandmanagethenetwork.Controlsystemsarebeingdevelopedfortheseopticalrouterswithwhichitwillbeabletobuildopticalnetworksthatcanbeeasilyconfiguredinresponsetodemand13,andwhichalsohaveself-healingpropertiesandfastrestorationtimesintheorderoffiftytoahundredmilliseconds,muchthesameastodaysSDHandSONETnetworks.sophisticatev.使复杂，使精致intelligencen.智力，只能，理解力routern.路由器restorationn.恢复，复位，复原aspectn.样子，外表，方面Afurtheraspecttoconsider14isaccesstotheopticalnetwork.Mostuserswouldliketohavedirectaccesstotheopticalnetworkandtheenormouscapacityitprovides.Thiswilltakeplaceinstages.Multi-wavelengthopticalsystemsarerapidlyspreadingoutfromthecoretowardstheenduser.Inregionalandmetropolitanareas,therequirementsaresomewhatdifferentfromthelong-distancearea.ThedreamofFiberToTheHome(FTTH)ordesktopisyettomaterialize,mainlybecauseofthecost-sensitivenatureofthispartofthenetwork.Inthenearfuture,residentialaccessmayremaincopper-based,usingtechnologiessuchasADSLtoboostthecapacityoftraditionalcopperlines.However,forbusinessoffices,opticaltechnologywillbeusedtobringbandwidthtotheend-used.Currently,alotofFiberToTheBuilding(FTTB)networksarebeingdeployedinvolvingATMandSDHaccessequipmentatcustomerpremises.ThenextstepistouseWDMtechnologyfortheseapplications.WDMwillfirstbeusedinindustrialandcampusLocalAreaNetwork(LAN)environments.Weareatthebeginningofarevolutionincommunicationsnetworks,whereincreasingcapacity,varietyofapplications,andqualityofserviceareplacingenormousdemandsontheopticalnetwork.Therevolutionofopticalnetworkisjustbeginning,andisadvancingveryswiftlytowardsafutureonlineworldinwhichbandwidthisessentiallyunlimited,reliableandlow-cost.enormousadj.巨大的，庞大的metropolitanadj.大城市的，大都会的materializev.使物质化，使具体化residentialadj.居住的，长住的，居留的deployv.展开，调度，部署通信英语UNIT9GSM(GlobalSystemforMobileCommunication)Thesuccessofmobilesystemsacrosstheworldisasignthatcommunicationismovingtowardsamorepersonalized,convenientsystem.success成功，成就sign标记，符号personalize个性化convenient便利的，方便的Peoplewhohavetouseamobilephoneonbusinesssoonbegintorealizethattheabilitytophoneanytime,anyplaceinonespersonalliferapidlybecomesanecessity,notaconvenience.business业务，生意realize实现，认识到ability能，能力necessity必要性，需要Thespeedandrapiditywithwhichthepersonalcommunicationsrevolutiontakesplaceis,unlikefixedtransmissionsystems,highlydependentontechnologyandcommunicationstandards.rapidity快，迅速revolution革命，彻底改变fixed固定的standard标准Formobilethethreekeyelementstoachievingservicetake-uparethecost,thesizeandtheweightofthephone,andthecostandqualityofthelink.elementachievetake-upcostqualitylinkIfanyofthesearewrong,especiallythefirsttwo,thenmarketgrowthisliabletobeseverelyrestricted.especiallymarketgrowthliableseverelyrestrictThefixedtelephoneserviceisglobalandtheinterconnectionvariesfromcoaxialcabletoopticalfibreandsatellite.global全球的interconnection互联vary变化，改变coaxial同轴的，共轴的cable电缆，有线satellite卫星Thenationalstandardsaredifferent,butwithcommoninterfacesandinterfaceconversion,interconnectioncantakeplace.standard标准common公共的，共同的interface接口conversion转化，变换Formobiletheproblemisfarmorecomplex,withtheneedtoroamcreatinganeedforcomplexnetworksandsystems.problem问题complex复杂的roam漫游create产生Thusinmobilethequestionofstandardsisfarmorecrucialtosuccessthanfixedsystems.Inaddition,thereisalsothevexedquestionofspectrumallocationinthemobilearea.crucial决定性的，紧要关头的vexed争论不休的，棘手的spectrum频谱allocation配给，分配Mobilesystemsoriginallyoperatedinanalogmodeinthe450MHzband,movinglaterto900MHzwithdigitalGSMandthento1,800MHzwithpersonalcommunicationsystems.Thehistoryofmobilitycansplitintogenerations.analog模拟的mode方式，样式mobility流动性，移动性split裂开，破裂generation一代Thefirstgenerationsystemsweretheadvancedmobilephonesystems(AMPS)intheUS,totalaccesscommunicationsystem(TACS)inmostofEuropeandNordicmobiletelephonesystem(NMT);whichwereallanaloguesystems.advancedtotalaccessNordicThesecondgenerationisvarymuchdominatedbythestandardfirstsetoutinEuropebythegroupspecialmobile(GSM)committee,whichwasdesignedasaglobalmobilecommunicationsystem.dominatespecialcommitteeTheGSMsystemisbasedonacellularcommunicationsprinciplewhichwasfirstproposedasaconceptinthe1940sbyBellSystemengineersintheUS.base以-为基础cellular蜂窝状的principle原则，原理propose提议，建议concept概念Theideacameoutoftheneedtoincreasenetworkcapacityandgotroundthefactthatbroadcastmobilenetworks,operatingindenselypopulatedareas,couldbejammedbyaverysmallnumberofsimultaneouscalls.IdeacapacitybroadcastdenselypopulatejamsimultaneousThepowerofthecellularsystemwasthatitallowedfrequencyreuse.powercellularThecellularconceptisdefinedbytwofeatures,frequencyreuseandcellsplitting.featurereusecellsplittingFrequencyreusecomesintoplaybyusingradiochannelsonthesamefrequencyincoverageareasthatarefarenoughapartnottocauseco-channelinterference.comeintoplaychannelcoverageapartcauseco-channelinterferenceThisallowshandlingofsimultaneouscallsthatexceedthetheoreticalspectralcapacity.HandlingsimultaneousexceedtheoreticalspectralcapacityCellsplittingisnecessarywhenthetrafficdemandonacellhasreachedthemaximumandthecellisthendividedintoamicro-cellularsystem.cellsplittingtrafficdemandmaximumdividemicro-cellularTheshapeofcellinacellularsystemisalwaysdepictedasahexagonandtheclustersizecanbeseven,nineortwelve.shapecellulardepicthexagonclusterUNIT104G1Intelecommunications,4Gisthefourthgenerationofmobilecommunicationtechnologystandards.A4Gsystemprovidesmobileultra-broadbandInternetaccess,forexampletolaptopswithUSBwirelessmodems,tosmartphones,andtoothermobiledevices.Conceivableapplicationsincludeamendedmobilewebaccess,IPtelephony,gamingservices,high-definitionmobileTV,videoconferencing,3Dtelevision,andcloudcomputing.laptopn.笔记本电脑modemn.调制解调器smartphonen.智能手机conceivablea.想得到的，可想象的amendv.修改，改善，改进Two4Gcandidatesystemsarecommerciallydeployed:theMobileWiMAXstandard(firstusedinSouthKoreain2006),andthefirst-releaseLongTermEvolution(LTE)standard(inOslo,NorwayandStockholm,Swedensince2009).Ithashoweverbeendebatedifthesefirst-releaseversionsshouldbeconsideredtobe4Gornot2.InMarch2008,theInternationalTelecommunicationsUnion-Radiocommunicationssector(ITU-R)specifiedasetofrequirementsfor4Gstandards,namedtheInternationalMobileTelecommunicationsAdvanced(IMT-Advanced)specification,settingpeakspeedrequirementsfor4Gserviceat100Mb/sforhighmobilitycommunication(suchasfromtrainsandcars)and1Gb/sforlowmobilitycommunication(suchaspedestriansandstationaryusers)3.definitionn.定义，清晰度conferencen.会议，讨论，协商candidaten.候选人，候选者commerciallyad.商业上，通商上deployv.部署，展开Asopposedtoearliergenerations,a4Gsystemdoesnotsupporttraditionalcircuit-switchedtelephonyservice,butall-InternetProtocol(IP)basedcommunicationsuchasIPtelephony.Thespreadspectrumradiotechnologyusedin3Gsystemsisabandonedinall4GcandidatesystemsandreplacedbyOFDMA，multi-carriertransmissionandotherfrequency-domainequalization(FDE)schemes,makingitpossibletotransferveryhighbitratesdespiteextensivemulti-pathradiopropagation(echoes)4.Thepeakbitrateisfurtherimprovedbysmartantennaarraysformultiple-inputmultiple-output(MIMO)communications.releasev.释放，发射，发布debatev.辩论，争论，讨论specifyv.指定，详细说明LTEAdvanced(LongTermEvolutionAdvanced)isacandidateforIMT-Advancedstandard,formallysubmittedbythe3GPPorganizationtoITU-Tinthefall2009,andexpectedtobereleasedin2013.Thetargetof3GPPLTEAdvancedistoreachandsurpasstheITUrequirements.LTEAdvancedisessentiallyanenhancementtoLTE.Itisnotanewtechnology,butratheranimprovementontheexistingLTEnetwork.ThisupgradepathmakesitmorecosteffectiveforvendorstoofferLTEandthenupgradetoLTEAdvancedwhichissimilartotheupgradefromWCDMAtoHSPA5.LTEandLTEAdvancedwillalsomakeuseofadditionalspectrumsandmultiplexingtoallowittoachievehigherdataspeeds.CandidateSubmitSurpassExistingUpgradeSpectrumJustwhenLTEandWiMaxarevigorouslypromotingintheglobaltelecommunicationsindustry,theformer(LTE)isalsothemostpowerful4Gmobilecommunicationsleadingtechnology,andquicklyoccupiedtheChinesemarket.TD-LTE,oneofthetwovariantsoftheLTEairinterfacetechnologies,isnotyetmature,butmanydomesticandinternationalwirelesscarriersoneafteranotherturntoTD-LTE6.IBMsdatashowthat67%oftheoperatorsareconsideringLTE,becausethisisthemainsourceoftheirfuturemarket7.Whileonly8%oftheoperatorsareconsideringtheuseofWiMAX.vigorouslyOccupyVariantCarrierOperatorTD-LTEisnotthefirst4Gwirelessmobilebroadbandnetworkdatastandard,butitisChinas4GstandardthatwasamendedandpublishedbyChinaslargesttelecomoperator-ChinaMobile8.Afteraseriesoffieldtrials,itisexpectedtobereleasedintothecommercialphaseinthenexttwoyears.AmendPublishTrialReleaseThemainreason4Gisfasterthan3GisbecauseofOrthogonalFrequency-DivisionMultiplexing(OFDM).Itsoundscomplicated,butitsthesametechnologyusedinWi-Fi,ADSLbroadbandanddigitalTV9.OFDMisatechniqueforsqueezingmoredataontothesameamountofradiofrequency10.Italsoreduceslatencyandinterference.Dataissplitupandsentviasmallchunksoffrequencyinparallel,thereforeincreasingthecapacityofthenetwork11.squeezev.挤压，挤interferencen.干扰，干涉latencyn.潜伏，潜在因素chunkn.大块，数据块configurationn.配置，结构Multiple-inputandmultiple-output,orMIMO,isanotherreason4Gisabletoprovidefasterspeeds.Itissimplytheuseofmultipleantennaarraysatboththetransmitterandreceivertoimprovecommunicationperformance12.Theperformanceofradiocommunicationsdependsonanantennasystem,termedsmartorintelligentantenna.Intheearly1990s,tocaterforthegrowingdatarateneedsofdatacommunication,manytransmissionschemeswereproposed.AntennaArrayPerformanceSmartIntelligentSchemeOnetechnology,spatialmultiplexing,gainedimportanceforitsbandwidthconservationandpowerefficiency13.Spatialmultiplexinginvolvesdeployingmultipleantennasatthetransmitterandatthereceiver.Thisallowsmoredatatobetransferredwithoutrequiringadditionalbandwidthordrawingmorepower14.Themostcommonconfigurationcurrentlyisa2x2MIMO,foundinmanysmartphonesandsometablets.tabletn.写字板，平板电脑Asdatarateshaveincreasedovertheyears,higherperformanceapplicationshavebeenenabledonthemobilehandset.Contextawarenesswillallowphonestosenseandreacttoauserssurroundings.Forexample,imagineawomanisoutshoppinginalocalmall.ShepassesbyaposterofherfavoriteTVstar.Herphoneandthepostersenseeachotherandshereceivesinformationdirectlyonherphoneaboutshowtimesorevenanoffertodownloadthestarslatesthitsingle.ContextSurroundingPosterFavoriteUNIT11CircuitSwitchingandPacketSwitching1Therearetwobasictypesofswitchingtechniques:circuitswitchingandmessageswitching.Incircuitswitching,atotalpathofconnectedlinesissetupfromtheorigintothedestinationatthetimethecallismade,andthepathremainsallocatedtothesource-destinationpair(whetherusedornot)untilitisreleasedbythecommunicatingparties2.Theswitches,calledcircuitswitches(orofficeexchangeintelephonejargon),havenocapabilityofstoringormanipulatingusersdataontheirwaytothedestination3.destinationn.目的地，终点allocatevt.分配，分派，配给releasevt.释放，放松，发布jargonn.行话，土语manipulatevt.处理，操纵Thecircuitissetupbyaspecialsignalingmessagethatfindsitswaythroughthenetwork,seizingchannelsinthepathasitproceeds4.Oncethepathisestablished,areturnsignalinformsthesourcetobegintransmission.Directtransmissionofdatafromsourcetodestinationcanthentakeplacewithoutanyinterventiononthepartofthesubnet.seizevt.抓住，占领，俘获proceedvi.进行，继续informvt.通知，告诉interventionn.干预，干涉，妨碍subnetn.子网，分网Inmessageswitching,thetransmissionunitisawell-definedblockofdatacalledamessage.Inadditiontothetexttobetransmitted5,amessagecomprisesaheaderandachecksum.Theheadercontainsinformationregardingthesourceanddestinationaddressesaswellasothercontrolinformation6;thechecksumisusedforerrorcontrolpurposes.Theswitchingelementisacomputerreferredtoasamessageprocessor7,withprocessingandstoragecapabilities.Messagestravelindependentlyandasynchronously,findingtheirownwayfromsourcetodestination8.Firstthemessageistransmittedfromthehosttothemessageprocessortowhichitisattached9.subnetn.子网，分网blockn.一批，一组，一块headern.头，头部checksumn.检验和，检验项regardv.与有关，涉及Oncethemessageisentirelyreceived,themessageprocessorexaminesitsheader,andaccordinglydecidesonthenextoutgoingchannelonwhichtotransmitit10.Ifthisselectedchannelisbusy,themessagewaitsinaqueueuntilthechannelbecomesfree,atwhichtimetransmissionbegins.Atthenextmessageprocessor,themessageisagainreceived,stored,examined,andtransmittedonsomeoutgoingchannel,andthesameprocesscontinuesuntilthemessageisdeliveredtoitsdestination.Thistransmissiontechniqueisalsoreferredtoasthestore-and-forwardtransmissiontechnique11.headern.头，头部checksumn.检验和，检验项regardv.与有关，涉及attachvt.缚，系，捆entirelyad.完全地，彻底地accordinglyad.相应地queuen.行列，长队variationn.变动，变化，变更Avariationofmessageswitchingispacketswitching.Herethemessageisbrokenupintoseveralpiecesofagivenmaximumlength,calledpackets.Aswithmessageswitching12,eachpacketcontainsaheaderandachecksum.Packetsaretransmittedindependentlyinastore-and-forwardmanner.Withcircuitswitching,thereisalwaysaninitialconnectioncostincurredinsettingupthecircuit13.Itiscost-effectiveonlyinthosesituationswhereoncethecircuitissetupthereisaguaranteedsteadyflowofinformationtransfertoamortizetheinitialcost14.Thisiscertainlythecasewithvoicecommunicationinthetraditionalway,andindeedcircuitswitchingisthetechniqueusedinthetelephonesystem.Communicationamongcomputers,however,ischaracterizedasbursty.Burstinessisaresultofthehighdegreeofrandomnessencounteredinthemessage-generationprocessandthemessagesize,andofthelowdelayconstraintrequiredbytheuser15.incurvt.招致，惹起guaranteevt.保证，担保steadya.平稳的，不变的amortizevt.缓冲，分摊Theusersanddevicesrequirethecommunicationresourcesrelativelyinfrequently;butwhentheydo,theyrequirearelativelyrapidresponse.Ifafixeddedicatedend-to-endcircuitweretobesetupconnectingtheendusers16,thenonemustassignenoughtransmissionbandwidthtothecircuitinordertomeetthedelayconstraintwiththeconsequencethattheresultingchannelutilizationislow17.Ifthecircuitofhighbandwidthweresetupandreleasedateachmessagetransmissionrequest,thentheset-uptimewouldbelargecomparedtothetransmissiontimeofthemessage,resultingagaininlowchannelutilization18.DedicatedAssignDelayConstraintConsequenceUtilizationTherefore,forburstyusers(whichcanalsobecharacterizedbyhighpeak-to-averagedataaterequirements),store-and-forwardtransmissiontechniquesofferamorecost-effectivesolution,sinceamessageoccupiesaparticularcommunicationslinkonlyforthedurationofitstransmissiononthatlink;therestofthetimeitisstoredatsomeintermediatemessageswitchandthelinkisavailableforothertransmissions.Thusthemainadvantageofstore-and-forwardtransmissionovercircuitswitchingisthatthecommunicationbandwidthisdynamicallyallocated,andtheallocationisdoneonthefinebasisofaparticularlinkinthenetworkandaparticularmessage(foraparticularsource-destinationpair).burstyn.突发性，突发randomnessn.随机，机遇constraintn.强迫，强制，制约relativelyad.相对地，比较地responsen.回答，响应dedicateda.专用的assignvt.分配，指派，委派utilizationn.利用dynamicallyad.动态地，有生气地Packetswitchingachievesthebenefitsdiscussedsofarandoffersaddedfeatures.Itprovidesthefulladvantageofthedynamicallocationofthebandwidth,evenwhenmessagesarelong.Indeed,withpacketswitching,manypacketsofthesamemessagemaybeintransmissionsimultaneouslyoverconsecutivelinksofapathfromsourcetodestination,thusachievingapipeliningeffectandreducingconsiderablytheoveralltransmissiondelayofthemessageascomparedtomessageswitching.benefitn.利益，好处，恩惠simultaneouslyad.同时地，同时发生consecutivead.连续地，连贯地pipelinev.用管道输送proceduren.过程，步骤entityn.存在，实体Ittendstorequiresmallerstorageallocationattheintermediateswitches.Italsohasbettererrorcharacteristicsandleadstomoreefficienterrorrecoveryprocedures,asitdealswithsmallerentities.Needlesstosay19,packetswitchingpresentsdesignproblemsofitsown,suchastheneedtoreorderpacketsofagivenmessagethatmayarriveatthedestinationnodeoutofsequence20.AllocationIntermediateCharacteristicsUNIT12Multimedia1Theword“multimedia”isbeingusedtodescribeamixtureofhardware,softwareandapplications,withaconsequentconfusioninpeoplesmindsastowhatitis2.Itisaconfusionsituationthatcouldleadtoamissedopportunityforindustrytodevelopanewmarketplace.Thetermmultimedialiterallymeansmanyformsofcommunicationandinasensewehavealwaysbeenimmersedinamultimediaenvironmentbecausewearebeingsthatrespondtosound,light,touch,andideas.Thisisessentiallyaninteractiveenvironment.multimedian.多媒体describev.描述，形容mixturen.混合，混合物consequenta.作为结果的，随之发生的confusionn.混，混乱状态opportunityn.机会，良机literallyad.逐字地，字面地inasense从某种意义上说immersev.沉浸，使陷入environmentn.环境，周围Inthemodernpresentationofmultimediawenowhavethetechnologythatcanbegintomovetowardsanability,alreadyheldbythehumanbeing,inthatbyusingcomputertechnology,suchasCD-ROM,notonlycanwebringtogetheraudioandvisualimagesbutalsobeinteractivewithothercomputerusers3.presentationn.呈现，展示visuala.视觉的audioa.听觉的，声音的Perhapsthebiggestreasonfortheexplosion4ofmultimediaofferingsistheenormousadvancesinpersonaldesktopcomputers.Thiscomputeradvancewouldappeartobringmultimediafacilitieswithinthereach5ofthemajorityofhomesforaboutUS$1000;thissystemmightconsistofahigh-speedprocessorwithhugestoragesofRAMandharddisc,acolourmonitor,CD-ROM,soundsystem,languageandlearningsystemsandWindowsandLotusApplications.Thissortofspecificationisaimedatthepersonal,homecomputermarketandisprobablymoreforgamesandeducation.enormousa.巨大的，庞大的facilityn.设备，工具majorityn.多数ThefirstelementassociatedwithmultimediaisperhapstheCD-ROM,nowconsideredamaturetechnology6.TheCD-ROMisusedtostoredatathatrepresentsvisualimageryandcurrentunitscanstoreupto650Mbytesofdata.ThestandardsofmultimediacanbefoundinwhatisknownastheITUWhiteBoot,whichbringstogethervariousstandardsforCD-COMandvideocompressionanditisessentialthatthesetwoareasofstandardsaretakentogether7.monitora.监视器maturea.成熟的Acrucialelementintheprovisionofmultimediaistheuseofcompressedvideo.WithoutcompressiontheamountofstoragethatwouldhavetobeprovidedbytheCD-ROMwouldbeimmense,oryouwouldonlybeabletostore15secondsofafullmotionpicture.Ifcompressionisappliedtothemotionpicturesexpertsgroup(MPEG-1)standard,thena650MbytesCD-ROMcanholdeither72minutesofvideo,or19hoursofaudio,or7000stillphotographs.representv.描述，讲述compressionn.压缩，浓缩immensea.广大的，巨大的ThestandardsformovingpicturesarebyMPEGasinternationalstandards.ThetwolevelsareMPEG-1,whichsetstheprovisionofconsumerqualityofvideoandaudioandMPEG-2,whichsetsthehigherstandardsforprofessional,broadcastimagesandalsolookstodefinehighdefinitiontelevision8(HDTV).MPEG-2looksatmorethanpictures;itlooksattheassociatedsound,storagemethods,bitratesneeded,codingalgorithms,etc.ModernencodersarenotonlyusedintheCD-ROMarea,butalsoasameansoftransmittingsignals,theyuseanumberofdigitalsignalprocessortoformavisionencoder.photographn.照片professionala.职业的，专业的definitionn.定义，清晰度Ifweconsiderthebusinessareaofmultimediaapplications,theprovisionofvideoiscrucialandthatvideoinformationmustcometouserinrealtimeandthereforebeprocessedinrealtime.Thismeansthatanysystemmustconformto9theCCITTMPEG-2standard.Thetechnologytodothishasmovedontremendouslyinthelastfewyearsduetotheneedtoreducethecostoftransmittingdataoverthenetwork.Theencodersanddecodersforvideoarenowmanufacturedwithaminimumofintegratedcircuits.Whatthentheproblemsforthedesigner?Theindividualelementsofmultimediaarealreadyhandledbyastandardpersonalcomputer,suchthingsasaudio,graphicsandtext,butthemajorchallengeisinrealtimevideo,for10thisisthemajorbusinessrequirementandanecessityifmultimediasystemsaretobecomestandarditemsforthedesktop.Thisrealtimeprocessingrequiresavastamountofdatastorageandthisisafunctionoftheamountofinformationcontainedinacolourpicture11,forinstanceanormal640480pixelcolourpicturerequireswellover1Mbytesofstoragetogivetherequiredquality,thoughforfullmotionvideothisisincreasedupto100Mbytesofdata.Thespreadofmultimediausersisnotjustafunctionoftechnology,anditiscertainlynotastoppableprocess.Thekeytoitstake-upisinthesuccessfulsellingofvideoandtelevisionservices,whichareoftengatheredunderthegeneraltitleofvideo-on-demand(VOD)interactiveservices.AtypicalVODarchitecturetakestrunkandaccessnetworksintoaccount12.Thevideoserveristhekeytoon-demandservices.Thevideoserverwillstorealargenumberofprogramsthatcanbeinstantlyaccessedbyalargenumberofindividualsubscribers;itisstoredinacompressedformtosavespace.architecturen.结构，组织trunkn.干线instantlyad.立刻，即刻Theuseofmultimediaisincreasingallthetimeanditisestimatedthatintheperiodfrom1991to1998thevolumeofmultimedia,notonlyforpersonalbutalsobusinessuse,willincreasefromzeroto$1.9billionforthesoftwareelementalone13.Multimediaisnolongeratoyoragametechnology.Itismovingintoourlivesinmanyways.Itisaffectingourbanking,shopping,education,andtravelandworkpatterns.Yet,itisanincreasingelementinourlives.estimatev.估计toyn.玩具UNIT13IPTV1Overthepastfewmonths,therehasbeenincreasedinterestintheuseofIPnetworkstodeliverbroadcast-qualityTV.IPTVisarelativelyrecentbuzzwordhavingemergedonlythreeorfouryearsago2.Inanutshell,itmeansthedeliveryoftelevisionoverInternetProtocol(IP)networks.Butforthetelecommunicationindustry,whichdrivesittothemarket,IPTVmeansmuchmore.buzzwordn.时髦术语，口号emergev.出现，浮现nutshelln.极小的东西inanutshell简括地说，一句话marketn.市场Forthem,IPTVisanentirelynewmultimediaexperienceextendingthebordersofconventionalbroadcasttelevision;itisanintegrated,“all-embracing”mediaplatformofferingabundleofdiversecontentandcommunicationservicesfromasingleprovideroverasinglenetworktoasingleuserdevice-allwithasinglepayment3.entirelyad.完全地，全部地experiencen.经历extendv.伸长，延伸bordern.边界integratev.综合，集成embracev.拥抱，怀抱；利用，抓住，采用；包围，包括，包含platformn.平台，站台，讲台bundlen.一捆，一束，一大堆diversea.多种多样的，形形色色的contentn.内容，含量，目录paymentn.支付，支付的款项unreliablea.不可靠的Whyuse30-year-oldtechnologydesignedforslow,unreliable,unicastcommunicationstotransportvideo?TheansweristhatIPisamagicalplatformtobreedsuccessandithasbecometotallypervasive4.ThesecondmotivationisthatIPcanoffermorefunctionalitythantraditionalTV.Theopen,extensiblenatureofIPholdsthepromiseofmanymoreservicesinthefuture.IPinfrastructurealsocanbeleveragedtoprovidevideotelephonyandremotemonitoring,aswellasnewusesofvideowithinapplicationsthathaventbeenthoughtofyet5.Thefinalfactorthatisemergingiscostsavings.unreliablea.不可靠的unicasta.单播的magicala.迷人的，神秘的，有魔力的breedv.养育，培养，教养pervasivea.遍布的，充满的motivationn.动力，动机，诱因functionalityn.功能extensiblea.可延伸的，可扩展的promisen.允诺，诺言；出息，前途infrastructuren.基础结构leveragev.举债经营TherearedivergingviewsaboutwhatIPTVreallymeans6.Tobroadcasters,IPTVissimplyanewemergingplatformfordistributingdigitaltelevisionchannelstohomeconsumersusingaTVscreen.Tothetelecomindustry,IPTVissynonymouswithanewbroadbanddigitaltechnology,offeringvoice,dataandvideo.IPTViscomplementarytoexistingsatellite,cableandterrestrialsystems,althoughinsomecasesitmaybecomeavigorouscompetitortothem.divergev.分叉，分歧；发散distributev.分配，分发，分送；散布synonymousa.同义的complementarya.补充的，补足的terrestriala.地面的vigorousa.朝气蓬勃的Sofar,thetelecomindustryhasmerelybeenprovidingtelecommunicationservicessuchasvoiceconnectionsbetweentwopoints7.Thetelecomcompanieswerenotatallconcernedwiththecontentoftheinformationcarried.TheyarenowgettinginvolvedinIPTVastheyfacingdecreasingsubscriberrevenuesfromtheirtraditionalvoiceandbroadbandcommunication8.Theyareintheprocessofmovingintothecontent-providingdomainwithnorealexpertiseandexperienceinprovidingtelevisionservicestothegeneralpublic,simplyinordertoimprovetheirbalancesheetandbringthecustomersback.Indeed,IPTVisbeginningtolooklikeapromisingnewbusinessopportunityfortelecomoperators.merelyad.仅仅，只不过revenuen.（年）收入，收益domainn.领域，领地，范围expertisen.专长，专门的技能banlancen.平衡，均衡sheetn.表格However,telecomswillhavetoresolvesomemajortechnicalchallenges.Oneproblemistheend-to-endtransmissionqualityduetolimitednetworkbandwidthwhichdecreaseswiththedistancefromtheexchangecabinet.NewmodulationstrategiessuchasADSL2+andVDSLarebeingintroduced,butthecostconcernedisrelativelyhigh.TheintroductionofHigh-DefinitionTelevision(HDTV)maybequiteachallenge,evenifadvancedcodingtechnologiessuchasITU/MPEG4orMicrosoftVC1areused9.Furthermore,Telecomnetworksdonotgenerallyservethehundredsofthousandsandevenmillionsofsimultaneoususers.Alackofinternationalstandardization,whichiscurrentlythemainobstacletoestablishinghorizontalmarketsandequipmentinter-operability,isnowbeingaddressedbytheInternationalTelecommunicationsUnion10.challengen.挑战cabinetn.机柜strategen.战略，策略obstaclen.障碍horizontala.同行业的，同一层次的IPTVcanpotentiallyofferamyriadofnewinnovativeservicesandapplicationstotheuser,manyofwhicharealreadypossiblewithdigitaltelevision.ThemaindifferencescomefromthefactthatIPTVusesatwo-waycommunicationchannel,sotheusercaninteractdirectlywiththecontentandserviceprovider11.Theinteractivelinkbetweentheproviderandtheuserenablessendingindividualvideostreamstoindividualdevicesinthehomeattheusersrequest,incontrasttothebroadcastmodelwhereallchannelsaresenttoallusersallthetime.Suchpersonalizedservicesarebecomingincreasinglypopular,particularlywithentertainmentandeducation.potentiallyad.潜在地，有可能地myriadn.无数innovativea.创新的interacta.交互的personalizev.个人化entertainmentn.娱乐Whilethetechnicalissuesarenoteasy,theyarenotinsurmountable.Thelegalandregulatoryissues,however,arearealnightmare.OneoftheoutstandingissuesistodeterminewhichnationalregulatorybodyisresponsiblefortheregulationofIPTVservices.ThisdependsontheverydefinitionofIPTV.SomecountriesconsiderIPTVabroadcastserviceandsomeatelecomservice.Notonlydothelegalregimesvaryfromcountrytocountry,theyarealsodifferentfortelecomsandbroadcasters12.insurmountablea.难以逾越的legala.法律的，合法的，法定的regulatoryn.管理的，控制的nightmaren.噩梦，梦魇regimen.政治制度，社会制度，体制；政权IPTVisstillinitsinfancy.Itsmarketsizeisstillsmallbutitisgrowingrapidly.Theprospectsforthecommercialsuccessaregreat.TheprincipledriversforIPTVaretheincumbenttelecomandInternetcompanies.Sofar,broadcastershavebeenmarginallyinvolvedintheIPTVprocess.Itisnowtimetoraisetheawarenessofbroadcasters,sothattheycanplayamoreactiveroleintheIPTVdevelopments13.infancyn.婴幼年期，初期prospectn.展望，前景，前程incumbenta.成为责任的，义不容辞的marginallyad.边沿，勉强够格地awarenessn.意识，认识UNIT14ADSL1ConnectiontotheInternetcanbemadeinvariousways.OnemightconnectacomputertotheInternetoveraregularmodem,onanIntegratedServicesDigitalNetworkline,throughalocalareanetworkintheoffice,throughacablemodem,andthroughadigitalsubscriberline(DSL)connection.ADSL(AsymmetricDigitalSubscriberLine)isahigh-speedInternetaccessservicethatutilizesexistingcoppertelephoneslinestosendandreceivedataatspeedsthatfarexceedconventionaldial-upmodems2.Thefastestdial-upmodemsareratedat57kilobitspersecond(Kbps),andusuallyoperateatabout53Kbpsundergoodconditions.Bycomparison,ADSLallowsdatastreamspeedsfrom1.5to8megabitspersecond(Mbps),dependingonthegradeofADSLservicepurchased.variousa.各种各样的；多种多样的regulara.规则的；经常的，习惯性的，普通的integrateda.综合的；集成的modema.调制解调器subscribern.用户；订户asymmetrica.不对称的accessn.接通，接入，存取utilizev.利用，使用exceedv.超过，胜过；越出dialv.拨号，打电话给ADSLusesstandardtelephonelinestotransmitupstreamanddownstreamdataonadigitalfrequency,whichsetsthesedatastreamsapartfromtheanalogsignalstelephonesandfaxmachinesuse3.BecausetheADSLsignalisoperatingonadifferentfrequency,thetelephonecanbeusednormally,evenwhensurfingtheWebwithADSLservice.TheonlyrequirementwillprobablybeinexpensiveDSLfiltersoneachphoneorfaxline,toremoveany“whitenoise”onthelinethatmightbegeneratedfromADSLservice4.upstreama.上行（流）downstreama.下行（流）faxn.传真机normallyad.正常，一般地surfv.冲浪；浏览inexpensivea.不贵的The“asymmetric”inADSLreferstothefactthatthedownstreamdatarate,orthedatacomingtoyourcomputerfromtheInternet,istravelingfasterthanupstreamdata,orthedatatravelingfromyourcomputertotheInternet5.UpstreamdataratesareslowerbecauseWebpagerequestsarefairlyminisculedatastringsthatdonotrequiremuchbandwidthtohandle6.ADSLwillplayacrucialroleastelephonecompanies,andotherserviceproviders,enternewmarketsfordeliveringinformationinvideoandmultimediaformats7.Newbroadbandcablingwilltakedecadestoreachallprospectivesubscribers,butsuccessofthesenewserviceswilldependuponreachingasmanysubscribersaspossibleduringthefirstfewyears.Bybringingmovies,television,videocatalogs,corporateLANs,andtheInternetintohomesandsmallbusinesses,ADSLwillmakethesemarketsviable,andprofitable,fortelephonecompaniesandapplicationsuppliersalike.cruciala.决定性的；关键的；至关重要的providern.提供者videon.视频；录像；图像formatn.格式cablingn.成缆；铺缆decaden.十年，十年期prospectivea.期望中的；未来的catalogn.目录；目录册；目录内容corporatea.公司的；团体的viablea.有望成功的；可实施的；切实可行的profitablea.有益的；有盈利的twistv.绞扭；对绞duplexa.双工的AnADSLcircuitconnectsanADSLmodemoneachendofatwisted-pairtelephoneline,creatingthreeinformationchannels-ahighspeeddownstreamchannel,amediumspeedupstreamchannel,andaduplextelephonechannel,dependingontheimplementationoftheADSLarchitecture.ADSLdependsuponadvanceddigitalsignalprocessingandcreativealgorithmstosqueezesomuchinformationthroughtwisted-pairtelephonelines.Inaddition,manyadvanceshavebeenrequiredintransformers,analogfilters,andA/Dconverters.Longtelephonelinesmayattenuatesignalsatonemegahertzbyasmuchas90dB,forcinganalogsectionsofADSLmodemstoworkveryhardtorealizelargedynamicranges,separatechannels,andmaintainlownoisefigures.architecturen.结构；体系；架构algorithmn.算法squeezev.挤压；榨；紧握transformern.变压器；变换器convertern.转换器attenuatev.衰减；衰耗掉megahertzn.兆赫兹sectionn.段；部分dynamica.动态的；不断变化的Tocreatemultiplechannels,ADSLmodemsdividetheavailablebandwidthofatelephonelineinoneoftwoways:FrequencyDivisionMultiplexing(FDM)orEchoCancellation.FDMassignsonebandforupstreamdataandanotherbandfordownstreamdata.ThedownstreampathisthendividedbyTimeDivisionMultiplexingintooneormorehighspeedchannelsandoneormorelowspeedchannels.EchoCancellationassignstheupstreambandtooverlapthedownstream,andseparatesthetwobymeansoflocalechocancellation,atechniquewellknowninV.32andV.34modems8.echon.回声；反响cancellationn.删除，删去assignv.分配；指定overlapv.重叠，互搭ADSLworksontheprinciplethatsincevoicedoesnotuseallofthebandwidthavailablefromastandardcoppertwisted-pairline;itispossibletomaintainahigh-speeddataconnectionatthesametime9.Todothis,ADSLsplitsthe1MHzmaximumbandwidthofacopperwireconnectioninto4KHzchannels,onlythebottom4KHzchannelusedforplainoldtelephonesystem(POTS)conversations,faxandanaloguemodemdata.Theother256availablechannelsareusedforparalleldigitalcommunication.Beingasymmetric,192channelsof4KHzareusedforthedownlinkandonly64fortheuplink.InthiswayADSLcanbeviewedassimplytakingaserialstringofdigitaldataandconvertingitintoaparallelstring,therebyincreasingdatathroughput10.splitv.劈开，切开；裂开plaina.简单的；不复杂的；单纯的；无装饰的；坦白的；普通的conversationn.对话；交谈parallela.并行的；平行的throughputn.生产量；吞吐量；通过量ThemodulationtechniqueisknownasDiscreteMulti-tone(DMT),theencodinganddecodingisperformedattheexchangeandattheuserendrespectively,inthesamewayitisbyaconventionaldial-upmodem.Anearliersystem,calledCarrier-lessAmplitudePhase(CAP),abletouseallthebandwidthabove4KHzasasingletransmissionchannel,hadtheadvantageofbeingcloselyrelatedtotheQuadratureAmplitudeModulation(QAM)techniqueusedbyhigh-speedmodemsatratesabove9.6Kbit/sandwasalsocheapertoimplement.However,DMTisamorereliable,sophisticatedandflexibletechnology,andhassucceededinbecomingtheuniversallyadoptedstandard.modulationn.调制discreta.离散的；不连续的multitonen.多音调carrierlessa.无载波的，载波消掉的phasen.相位；相角quadraturea.正交；90度相位差reliablea.可靠的，可信赖的sophisticateda.复杂的；高级的；尖端的flexiblea.灵活的succeedv.接替；继承；成功；办妥universallyad.普遍地；普通地；一般地；通用地；全体地adoptv.采用，采纳Attheuserend,theADSLmodemcollectshigh-frequencydigitaldataandassemblesitfortransmissiontothePCornetwork.Attheexchangeend,aDigitalSubscriberLineAccessMultiplexer(DSLAM)connectstheADSLusertothewiderInternet,aggregatingincomingADSLlinesintoasingledataconnectionfortransmissionontovoiceanddatanetworks11.ThephonesignalsaresenttotheswitchedtelephonenetworkandthedigitaldatarouteddatatotheInternetviaahigh-speedbackbone.assemblev.拼装；组装aggregatev.聚集；集合；总计routev.为选择路由backbonen.主干；骨干Ethernetn.以太网routern.路由器ADSLmodemshaveevolvedovertheyearsandarenowavailableinanumberofforms.SomeconnecttoaPCviaaUSBport,othersviaEthernet.MostdevicesallowanInternetconnectiontobesharedacrossseveralPCs.Integratedmodem/routerssupportanetworkofPCs,somecomingwithanintegratedfirewalltoprovidevaryinglevelsofprotectionagainstunauthorizedaccess.firewalln.防火墙protectionn.防护；保护unauthorizeda.非授权的；未授权的UNIT15EPONTechnology1Nowadays,sincemoreandmoreInternetbusinesseshavegraduallyenteredintonumeroushouseholdswiththerapiddevelopmentoftheInternet,bottlenecksofaccessnetworkareemerging2.EPONtechnology,whichcombinesamatureEthernettechnologyandhigh-bandwidthPONtechnology,isanidealaccessmethodtoachieveintegratedservices3.graduallyad.逐步地，渐渐地；按部就班地numerousa.很多的，许多的；数不清的emergevi.出现，浮现；暴露；摆脱maturea.成熟的；仔细考虑过的Inrecentyears,thetelecommunicationbackbonenetworkhasbeenupgradedfromtimetotimewhilemorefibersarelaidanddevicesoflargercapacitieshavecomeintouse.Butasfortheaccessnetwork,thecopperlineisstilltheleadingchoice.ThetremendousincreaseinInternetserviceshasprickeduptheshortageoftheaccessnetworkscapacity.Theaccessnetworkwhichiscalled“lastmile”stillremainsthebottleneckbetweenhigh-speedLANsandthehigh-capacitybackbonenetwork.accessn.接近，进入integratevt.使一体化；使整合；使完整upgradevt.提升；使升级；capacityn.容量；才能；性能tremendousa.极大的，巨大的；可怕的，惊人的；prickv.刺；竖起，加剧Themostwidelydeployed“broadband”solutionstodayareDSLandCMnetworks.Althoughtheyareimprovementscomparedto56kbpsdial-uplines,theyarestillunabletoprovideenoughbandwidthforemergingservicessuchasvideo-on-demand(VOD),interactivegaming,ortwo-wayvideoconferencing4.Anewkindofaccesstechnologyisrequiredforthetimewithfollowingfeatures:inexpensive,simple,upgradeable,andbeingabletoprovidebundledvoice,dataandvideoservices.EPON,whichmergesthelow-costEthernettechnologyandlow-costopticnetworkarchitecture,isthebestrepresentativeofthefuture-orientednextgenerationaccessnetworktechnologies5.solutionn.解答；解决（办法）；解释；dial-up拨号interactivea.互相作用的；交互式的；互动的conferencen.会议；讨论；（正式）讨论会bundledv.收集，归拢，把塞入architecturen.体系结构；（总体、层次）结构Ethernetpassiveopticalnetworks(EPON)arenext-generationsubscriberaccessnetworktechnologiesthatdeployopticalaccesslinesbetweenanopticallineterminator(OLT)andmultipleopticalnetworkunits(ONU)and/oropticalnetworkterminators(ONT)6.OLTislocatedinnetworksideandONU/ONTisincustomerside.EPONwillbeutilizedinprovidingsubscriberswithaccessbandwidthsof10Mbpsupto10Gbpsperusertobeneededforhigh-qualityInternetservices.TheOLT（OpticalLineTerminal）residesintheCentralOffice(CO)andconnectstheopticalnetworktothemetropolitan-areanetworkorwide-areanetwork,alsoknownasthebackboneorlong-haulnetwork.OLTisbothaswitchorrouterandamulti-serviceplatformwhichprovidesEPON-orientedopticalinterfaces.Besidesthenetworkassemblingandaccessfunctions,OLTcouldalsoperformbandwidthassignments,networksecurityandmanagementconfigurationsaccordingtothecustomersdifferentQoSrequirements.metropolitana.大都会的；大城市的routern.路由器platformn.台；站台；平台interfacen.界面；接口；交界面assemblevt.装配，组合assignmentn.分给，分配；任务，工作securityn.安全；保证，担保TheONU（OpticalNetworkUnit）islocatedeitherattheend-userlocationoratthecurbandprovidesopticalinterfaceswhichareconnectedtotheOLTandserviceinterfacesatuserssidesuchasvoice,dataandvideo7.TheODN（OpticalDistributedNetwork）isanopticaldistributionnetworkandismainlycomposedofoneormorepassiveopticalsplitterswhichconnecttheOLTandONU.Itsfunctionistosplitdownstreamsignalfromonefiberintoseveralfibersandcombineopticalupstreamsignalsfrommultiplefibersintoone.Opticalsplitterisasimpledevicewhichneedsnopowerandcouldworkinanall-weatherenvironment.Thetypicalsplittershaveasplittingratioof2,4,8,16or32andcanbeconnectedtoeachother.ThelongestdistancetheODNcouldcoveris20Km.configurationn.组合，布置；结构，构造distributionn.分配，分布splittern.分裂机；分流器upstreama.向上游的；逆流而上的Inthedownstreamdirection(fromOLTtoONUs),aPONisapoint-to-multipointnetwork.Intheupstreamdirection,aPONisamultipoint-to-pointnetwork:multipleONUstransmitalltowardsoneOLT.Directionalpropertiesofapassivesplitter/combineraresuchthatanONUstransmissioncannotbedetectedbyotherONUs.EPONsystemadoptsthebroadcastmethodindownstreamdirection.EthernetframesofdifferentONUsaretransmittedinonedownstreamtimeslot.ThepacketwhichcarriesalltheONUsdatareachesdifferentONUsandeachONUonlyextractsitsowndataframesandtransmitthemtotheuserswhileothersframesarediscarded.environmentn.环境，外界；周围propertyn.特性，属性；财产timeslotn.时隙packetn.小包；信息包，分组extractvt.提取；拔出；选取discardvt.丢弃，抛弃；解雇However,datastreamsfromdifferentONUstransmittedsimultaneouslystillmaycollide.Thus,intheupstreamdirection(fromusertonetwork),PONshouldemploysomechannelseparationmechanismtoavoid8datacollisionsandfairlysharethetrunkfiberchannelcapacityandresources.OnepossiblewayofseparatingtheONUsupstreamchannelsistouseawavelength-divisionmultiplexing(WDM),inwhicheachONUoperatesonadifferentwavelength.Whileitisasimplesolution(fromatheoreticalperspective),itremainscost-prohibitiveforanaccessnetwork.collidevi.相撞；碰撞；冲突separationn.分离，分开；间隔，距离mechanismn.机制，机能;结构，机械装置simultaneouslyad.同时地；一壁；齐InaTimeDivisionMultiplexing(TDM)PON,simultaneoustransmissionsfromseveralONUswillcollidewhenreachingthecombiner.Inorderto9preventdatacollisions,eachONUmusttransmitinitsowntransmissionwindow(timeslot).OneofthemajoradvantagesofaTDMPONisthatallONUscanoperateonthesamewavelength.TheOLTwillalsoneedasinglereceiver.AtransceiverinanONUmustoperateatthefulllinerate,eventhoughthebandwidthavailabletotheONUislower.However,thispropertyalsoallowstheTDMPONtoefficientlychangethebandwidthallocatedtoeachONUbychangingtheassignedtimeslotsize,orevenemploystatisticalmultiplexingtofullyutilizethebandwidthavailableinthePON10.cost-prohibitive成本高昂的preventvt.预防；阻碍；阻止wavelengthn.波长；波段allocatevt.分配，分派；把拨给statisticala.统计的；统计学的utilizevt.利用，使用ItispossiblethatEPONsandpoint-to-pointopticalEthernetofferthebestpossibilityforaturnaroundinthetelecomsector11.Asserviceprovidersinvestinopticalaccesstechnologies,thiswillenablenewapplications,stimulatingrevenuegrowthanddrivingmoretrafficontobackboneroutes.UNIT16CurrentSituationandtheFutureintheTelecommunicationworld1Todaystelecommunicationnetworksarecharacterizedbyspecialization.Thismeansthatforeveryindividualtelecommunicationserviceatleastonenetworkexiststhattransportsthisservice.Afewexamplesofexistingpublicnetworksaredescribedbelow:Thetelexnetworktransportstelexinformation,i.e.messagesofcharacters,transportedatverylowspeed.Thecharactersarecodedbasedonaspecific5-bitcode.POTS(plainoldtelephoneservice)istransportedviathepublicswitchedtelephonenetwork(PSTN).Thisubiquitousnetworkoffersthecustomersclassicaltwo-wayvoiceconversation.ComputerdataaretransportedinthepublicdomaineitherbyapacketswitcheddatanetworkbasedonX.25protocolsorinaverylimitednumberofcountriesbyacircuitswitcheddatanetworkbasedonX.21protocols2.Televisionsignalscanbetransportedinthreeways:broadcastviaradiowavesusinggroundantenna,bythecoaxialtreenetworkofcommunityantennaTVnetworkorrecentlyviaasatellite,usingtheso-calleddirectbroadcastsystem.Intheprivatedomain,computerdataaremainlytransportedbyLANs(LocalAreaNetwork).ThemostfamousonesareEthernet,tokenbusandtokenring(IEEE802series).Eachofthesenetworkswasspeciallydesignedforthatspecificserviceandisoftennotatallapplicabletotransportanotherservice.Forinstance,theoriginalCATVnetworksdidnotallowthetransportationofPOTS;orthePSTNdoesnottransportTVsignals;orthetransferofvoiceoveranX.25networkisveryproblematicbecauseoftoolargeend-to-enddelayandjitteronthisdelay.Onlyinlimitedandspecialcasescanservicetypesotherthantheonethenetworkwasoriginallydesignedforbetransportedoverit3.ThisisforinstancethecaseforthePSTNwhichiscapableoftransportingcomputerdataatalimitedspeed,ifmodemsareprovidedatbothendsofthenetwork.Animportantconsequenceofthisservicespecializationistheexistenceofalargenumberofoftenworld-wideindependentnetworks,eachrequiringitsowndesignphase,manufacturingandmaintenance4.Inaddition,thedimensioningofeachnetworkmustbedoneforeveryindividualservicetype.Evenifresourcesarefreelyavailableinonenetwork,theycannotbeusedbyanotherservicetype.Forexample,thepeakhoursinthetelephonenetworkarebetween9a.m.and5p.m.,whereasthepeakhoursintheCATVnetworkareduringevening.Sinceresourcepoolingisimpossibleeachnetworkmustbedimensionedforitsworstcasetrafficconditionswhichisthepeakhourtraffic.Afirststep,albeitalimitedonetowardsasingleuniversalnetwork,istheintroductionofNISDNinwhichvoiceanddataaretransportedoverasinglemedium.ThisnetworkcannottransportTVsignalsduetoitslimitedbandwidthcapabilities,soaspecialTVnetworkisstillrequired.EveninNISDNtheintegrationofnarrowbandservicessuchasdataandvoicecanbeconsideredasbeingratherlimited:theuseraccesstothenetworkisfullyintegrated,eitherbyabasicaccessorprimaryrateinterface.However,insidethenetworktherewillstillexistforsometimeapacketswitchedandacircuitswitchednetworkastwooverlaynetworksincapableoftransportingothertraffictypesandeachdimensionedeitherforvoiceorX.25data5.Anotherimportantconsequenceofthisservicespecializationistheinabilityofthenetworktobenefithighlyfromtheprogressmadeintechnologyandcodingalgorithms.Forinstance,currentdigitalNISDNswitchesaredesignedfor64kbit/svoicechannels.However,withthecurrentprogressinspeechcodingandchiptechnology,bitratesof32kbit/s,13kbit/sandevenlowerwillbeusedinthefuture.Theexistingswitchesandtransmissionsystemsarenotdirectlysuitedandthusneedanadaptation,orwillnotefficientlyusetheirinternalresourcesfortheselowerspeedbitrates.WhendesigningthefutureBISDNnetwork,onemusttakeintoaccountallpossibleexistingandfutureservices.Supposeanetworkiscapableoftransportingaspecificservice,e.g.acircuitswitchedservicewithachannelrateof70Mbit/s.Supposealsothatitisspecificallydesignedtotransportthisbitrate.Someyearslateranewteleserviceof,forexample,40Mbit/sappearsonthescene.Thiswouldmeanthatthenetworkdesignedforthatservice(i.e.70Mbit/s)willbecapableoftransportingthenewteleservice,butwithalargeinefficiency:only40outofthe70Mbit/savailablewillbeused.Thisexampleisnotunrealistic.Itisverylikelythatinthefuturenewserviceswillemergewhichhavenotyetbeenidentified,andofwhichtherequirementsareunknowntoday.Ascanbeconcludedfromtheaboveexamples,thenetworksoftodayareveryspecializedandsufferfromalargenumberofdisadvantages,themostimportantbeing:ServiceDependenceEachnetworkisonlycapableoftransportingonespecificserviceforwhichitwasintentionallydesigned.Onlyinalimitednumberofcasesandbyusingadditionalequipment(e.g.amodem)andwithaninefficientuseofitsresourcescanitbeadaptedtootherservices6.InflexibilityAdvancesinaudio,videoandspeechcodingandcompressionalgorithmsandprogressinVeryLargeSystemsIntegration(VLSI)technologyinfluencethebitrategeneratedbyacertainserviceandthuschangetheservicerequirementsforthenetwork.Inthefuture,newserviceswithunknownrequirementswillappear.Forthetimebeingitisyetunclear,e.g.whattherequirementsintermsof7bitrateforHDTVwillbe.Aspecializednetworkhasgreatdifficultiesinadaptingtochangingornewservicerequirements.InefficiencyTheinternalavailableresourcesareusedinefficiently.Resourceswhichareavailableinonenetworkcannotbemadeavailabletoothernetworks.Takingintoaccountalltheseconsiderationsonflexibility,servicedependenceandresourceusage,itisconsequentlyveryimportantinthefuturethatonlyasinglenetworkexistsandthatthisnetworkofthefuture(BISDN)isservice-independent.Thisimpliesasinglenetworkcapableoftransportingallservices,sharingallitsavailableresourcesbetweenthedifferentservices.Asingleservice-independentnetworkwillnotsufferfromthedisadvantagesdescribedabove,butitwillhavethefollowingmainadvantages:Flexibleandfuture-safeAdvancesinthestateoftheartofcodingalgorithmsandVLSItechnologymayreducethebandwidthofexistingteleservices.Anetworkcapableoftransportingalltypesofserviceswillbeabletoadaptitselftochangingornewneeds.Allavailableresourcescanbesharedbetweenallservices,suchthatanoptimalstatisticalsharingoftheresourcescanbeobtained.LessexpensiveSinceonlyonenetworkneedstobedesigned,manufacturedandmaintained,theoverallcostsofthedesign,manufacturing,operationsandmaintenancewillbesmaller.UNIT17NGN1Themarketforinformationandcommunicationstechnologyiscurrentlyundergoingastructuralchange.Theclassictelecommunicationnetworkswereplannedandimplementedforthetransferofspecificdatasuchastelephonecallsorpuredatapackages.Therecentgrowthincompetition,newrequirementsforthemarketandtechnologicaldevelopmentshavefundamentallychangedthetraditionalattitudesofthetelecommunicationsindustry.Thepresentindustryischaracterizedbytherapidgrowthofbroadbandconnections,theconvergenceprocessesofvariousnetworktechnologiesandtheemergenceofauniformIPstandardforindividualandmasscommunications2.undergov.历经；经历；忍受structurala.结构上的；构造上的transferv.转移；传送；传输purea.纯净的，纯粹的packagen.包；分组competitionn.竞争；比赛fundamentallyad.基本地；基础地；主要地attituden.姿态；态度；看法characterizev.以为特征；表征convergencen.收敛；会聚；会合；集中emergencen.浮现；出现uniforma.划一的；统一的；相同的；一致的Traditionaltelecommunicationsoperatorsfindthemselvesconfrontedwithahostofnewchallenges3.Inparticular,theirpreviouslysuccessfulfixed-networkbusinessiscomingincreasinglyunderpressure.Newcommunicationpossibilities,suchastelephoningviatheInternet,andalsogrowingmarketsharesinmobiletelephonyarecausingagreatdealofconcern4.Tocounteracttheselosses,thenetworkoperatorsareinvestingmorestronglyinbroadband，thegrowthdriver.Thetraditionallyfamiliarmarketboundariesbetweenfixednetworks,mobiletelephonyanddatanetworksaredisappearingmoreandmorequickly.confrontv.使面对；使面向；使遭遇hostn.一大群；许多challengen.挑战pressuren.压力；压迫；挤压possibilityn.可能性concernn.关心的事；关切counteractv.对抗；抵抗；抵制lossn.丢失；遗失；丧失；亏损investv.投资；投入boundaryn.边界；分界线；界限Thisgivesthecustomertheadvantagethathecancallonanextremelywiderangeofservices,regardlessofhisaccesstechnology5.Thisdevelopmentrequiresametainfrastructurebeyondtheexisting,subordinatednetworks-acorenetworkforalltheaccessnetworks.ThisnewnetworkiscalledtheNextGenerationNetwork.TheInternetProtocolisthemostsignificantintegrationfactorbecauseitisavailablegloballyand,atleastinprinciple,itcanusealmostalltheservicesandapplicationsinallthenetworks6.extremelyad.极度地；极大地regardlessa.毫不留心的；毫不在意的infrastructuren.基础设施；基础结构subordinatev.使成为下级；使成为次要coren.核心；芯integrationn.综合；集成Inface,NextGenerationNetwork（NGN）isabroadtermtodescribesomekeyarchitecturalevolutionsintelecommunicationcoreandaccessnetworksthatwillbedeployedovernext5-10years7.ThegeneralideabehindNGNinthatonenetworktransportsallinformationandservices(voice,dataandallsortsofmediasuchasvideo)byencapsulatingtheseintopackets.LikeitisontheInternet,NGNarecommonlybuiltaroundtheInternetprotocol,andthereforetheterm“all-IP”isalsosometimesusedtodescribethetransformationtowardsNGN.termn.术语；专门名词；学期；任期；期限describev.描述；描写architecturala.建筑学的evolutionn.进化；发展；演进encapsulatev.打包；封装definitionn.定义；限定AccordingtoITU-T,thedefinitionofNGNis:ANextGenerationNetwork(NGN)isapacket-basednetworkabletoprovideservicesincludingTelecommunicationServicesandabletomakeuseofmultiplebroadband,QoS-enabledtransporttechnologiesandinwhichservice-relatedfunctionsareindependentfromunderlyingtransport-relatedtechnologies.Itoffersunrestrictedaccessbyuserstodifferentserviceproviders.Itsupportsgeneralizedmobilitywhichwillallowconsistentandubiquitousprovisionofservicestousers.underlyinga.在下面的；基本的；基础的unrestrictv.不限制；不约束consistenta.协调的；一致的；连贯的；相容的；一致的ubiquitousa.普遍存在的；无处不在的ThebasicpremiseforNGNisthearchitectureonseveralindependentlevels.Theseincludetheaccessarea,thecorenetworkarea,thecontrollevelandtheservicemanagementlevel.TheconnectionofsubscribersandterminalstotheNGNcanbeachievedwithvariousaccesstechnologies.TheinformationandtransmissionformatsofthevariousnetworksmustbeconvertedintoinformationthatiscomprehensiblefortheNGN8.ThiscallsforGatewaysfortheconnectionofbusinessandprivatecustomers.ThecorenetworkoftheNGNisanIPnetwork.ThisisastandardizedtransportplatformconsistingofvariousIProutersandswitches9.Theconnectioncontroloftheindividualcomponentsiscarriedoutbythecontrollevel.Standardandvalue-addedservicescanthenbeprovidedviatheservicemanagementlevel.premisen.假设；前提comprehensiblea.可理解的；易懂的gatewayn.关口；网关standardizev.使标准化platformn.平台TheprerequisitefortheefficientuseoftheNGNisanetworkaccesswithhighbandwidthforthesubscriber.Itistobeexpectedthataccessnetworksinthefuturewillbeabletoprovidebandwidthsofupto100Mbit/sforprivatesubscribers10.Forbusinesscustomers,transmissionratesinthegigabyterangeareconceivable.ThepossibleoptionsforaccesstotheIPbackbonewouldincludecopper,cable,fiber-opticandwirelessconnections.prerequisiten.先决条件；前提expectv.期望；盼望gigabyten.千兆字节conceivablea.可想象的；可得到的ForvoiceapplicationoneofthemostimportantdevicesinNGNisasoftswitch-aprogrammabledevicethatcontrolsVoiceoverIPcalls.ItenablescorrectintegrationofdifferentprotocolswithinNGN.Themostimportantfunctionofthesoftswitchiscreatingtheinterfacetotheexistingtelephonenetwork,PSTN,throughsignalingGateways(SG)andMediaGateway(MG).However,thesoftswitchasatermmaybedefineddifferentlybythedifferentequipmentmanufacturersandhavesomewhatdifferentfunctions.ASignalingGatewayisanetworkcomponentsolelyresponsiblefortranslatingsignalingmessages(i.e.informationaboutcallestablishmentandteardown)betweenonemedium(usuallyIP)andanother(PSTN)11.softswitchn.软交换programmablea.可编程的solelyad.单独地；唯一地；仅仅responsiblea.负责的translationn.翻译；转换disparatea.全异的，根本不同的Forexample,asignalinggatewaymighttranslatebetweenISUPandSIP.AMediaGatewayactsasatranslationunitbetweendisparatetelecommunicationsnetworkssuchasPSTN,NextGeneration,2Gand3G,radioaccessnetworksorPBX.MediaGatewaysenablemultimediacommunicationsacrossNextGenerationNetworkovermultipletransportprotocolssuchasATMandIP.BecausetheMGconnectsdifferenttypesofnetworks,oneofitsmainfunctionsistoconvertbetweenthedifferenttransmissionandcodingtechniques.TranslateGatewayProtocolconvertThemarketfortelecommunicationsservicesintheworldhasdevelopedextremelydynamicallysincebeingliberalized.However,aweakeningoftheaverageannualgrowthonthevariousmarketsistobeexpectedbytheendofthedecade.Thebusinesswithbroadbandconnectionsisbeingtreatedasparticularlylucrativeinordertocompensateforthemarket-sharelossesofthefixednetworkinparticular.Thenetworkoperatorsareattemptingtoprovideamoreefficientandcost-effectiveprovisionofserviceswiththecurrentconversionoftheentirenetworkinfrastructuretoIPtechnology.Theaimistounitfixed,mobileanddatanetworkstogetherandsotoprovidevariousservicesviaatransparentnetwork-thesocalledNetworkGenerationNetwork.dynamicallyad.动态的liberalizev.使自由化weakenv.削弱；减弱；变弱averagea.平均的annuala.每年的；年度的lucrativea.生利的；赚钱的compensatev.补偿；赔偿conversionn.转换；变换aimn.目标；目的transparenta.透明的；清澈的；可穿透的