电子信息类专业英语第三章.ppt

UnitThreeDataCommunications,PassageADataCommunicationsPassageBCharacteristicsofCommunicationChannelsPassageCCommunicationEquipmentandSoftware,PassageADataCommunicationsTherapidgrowthofdatacommunicationshasbeeninfluencedprimarilybytheincreasingneedtomoveinformationtoandfromcomputers.Moderntime-sharedcomputerscancommunicatewithmanydatastationssimultaneously.Otherformsofdatatransmissionwhichthetelephoneandtelegraphlinesarecalledupontohandlearebatchdatatransmission,real-timetransmission(asinthecaseofairlinereservationsystems),bankingandcreditdata,man-computerconversationwiththeaidofgraphics,datacollectionsystemsandautomaticmeterreading.Thecombinationofcomputersanddatacommunicationplacessuchnewrequirementsonthesystems,whichmusthandlethem,thatonecanrefertothembetterasteleprocessingratherthantelecommunications.1,Teleprocessingstartedwiththeairlinereservationsystems,wherethedistributionofcomputerdataisnotasimportantasthemaintenanceofaninventoryofseatsforthewholesystemandtheabilitytoaccesssuchaninventoryrapidlyatanumberofdistantpoints.Anotherexampleisthatofthebanks,whodesiredtocentralizetheiraccountingandprovideaccesstoacentralfileforalltheirbranches.Basically,therearethreereasonsforthedevelopmentofteleprocessingsystems:therequirementforcentralizedfiles,theneedtodistributecomputerservicesandtheadvantagegainedbyhavingflexibilityinlocationoftheoperatingstaffwhichutilizesthesystem.,Initially,boththeairlineandbankingoperationscanbeestablishedonaprivatebasis,whichwouldminimizetheneedforswitchingoperations.Butinbothcases,theneedtoexpandbeyondprivateoperationssoonarises.Inthecaseoftheairlines,theneedarisestoaccessthefacilitiesofotherairlines;andinthecaseofthebanks,certaincommondatafacilitiesmaybesharedwithotherbanks.Eventhoughprivatenetworksmaysufficeformanyservices,theneedtoprovidedatacommunicationsbetweenurbancentersleadstoregionalandnationalnetworks.IntheU.S.,theBellSystem,theindependenttelephonecompaniesandspecializeddatacommunicationnetworksareinvolvedascarriersofsuchdata.,Themostapplicablemodelfordatacommunicationsrequirementsisthecentralcomputerconversingwithmanysimpleterminalssimultaneously.Differingfromtelephonecommunications,adatacommunicationsnetworkmustbeabletointerconnectawidevarietyofsubscribersequipment.Severaldifferenttypesofcomputerperipheralsmayappearasnetworkterminals,andsoalsocancomputersfunctioninginseveraldifferentways.2Terminalspeedsrangingfrom100bitspersecto10,000bitspersecarelikelytobeimportant.Manyterminalsincurrentuseareconstrainedbytheavailablecommunicationsservices,butmanyofthemhavevariablespeeds.Oneapproachtothissituationistoprovideabufferstoreattheterminal,butthisapproachisexpensive.Atpresent,multi-accesscomputersusuallyhandleterminalsofonlyonespeedorwithasmallnumberoffixedspeeds.Ithasbeenproposedthatfeedbackfromtheoutputdevicebeusedtopermitonesimpleoutputprogramtodriveterminalswithdifferentcharacteristics.Ofcourse,suchfeedbacksignalswouldhavetobehandledbythecommunicationsnetwork.,Atdatatechnologydevelops,eachmulti-accesscomputerisconnectedtoanincreasingnumberofterminals,makingitexpensiveanddifficulttoprovideseparatelinesfromthecomputertothenetworkforeachoftheterminals.Thus,aneedformultiplexedconnectionsbetweenthecomputerandtheterminalsarises.Butthisneedcannotnecessarilybemetinastraightforwardmanner.Onecharacteristicofadatacommunicationsnetworkdifferingfromconventionaltelephonenetworksisthatthetwoendsofthecommunicationlinkareverydifferent.Oneendisacomputercapableofflexiblebehavior,andtheotherendisarelativelyslowterminalwithnointelligence.Sincethetwoendsaresodifferent,itwouldappearthatthenetworkwouldcommunicatedifferentlywiththem.Theflowofinformationinadatacommunicationsystemisintermittentjustasitisinthecaseoftelephoneconversations.,However,insteadofminutesofconversation,onlyafewsecondswillelapsefortheinformationflowinonedirection,followedbyapauseandsomeflowofinformationintheoppositedirection.Theaccesstimesfortypicalcomputersinuseatthistimeareabout100ms,butthiswillbecomefasterasbettermemoriesareutilized.3Thetimescalefortheinterchangeofdatamessagesneedstobeofthisorder.,KEYWORDSreservation保留，预定graphics图示，图解inventory清单，报表teleprocessing远程(信息)处理，遥控处理constrain约束，强迫multi-access多路存取，多路进入feedback反馈，回授intelligence智能，智力，信息intermediate中间的elapse(时间)经过，消失,becalleduponto用来，(被)要求withtheaidof借助于，用，通过appearas作为出现belikelyto可能，大约，像要,NOTES1Thecombinationofcomputersanddatacommunicationplacessuchnewrequirementsonthesystems,whichmusthandlethem,thatonecanrefertothembetterasteleprocessingratherthantelecommunications.计算机和数据通信的结合向系统提出必须对这些信息进行处理的新要求，以至人们最好称它们为远程信息处理，而不是远程通信。“thatonetelecommunications”为结果状态从句。“whichmusthandlethem”定语从句修饰“system”。句中“place”有“放置”的含义，这里引申为“提出”。在科技英语中，有时用“one”泛指“人们”。,2Severaldifferenttypesofcomputerperipheralsmayappearasnetworkterminals,andsoalsocancomputersfunctioninginseveraldifferentways.几种不同的计算机外围设备可能作为网络终端出现，几种不同方式运行的计算机也可能作为网络终端出现。“soalsodifferentways”为并列句，该并列句主谓倒装，用于强调。句中“so”表示“appearasnetworkterminals”，翻译时应重复叙述，使译文表达更明确。,3Theaccesstimesfortypicalcomputersinuseatthistimeareabout100ms,butthiswillbecomefasterasbettermemoriesareutilized.目前所用的典型计算机存取时间大约为100毫秒，但当采用较好的存储器时，存取时间将会缩短。“butthiswillareutilized”为并列句。其中“asbettermemoriesareutilized”为时间状语从句。在科技英语中“as”运用很灵活，此句“as”作连词。,EXERCISES1.Fillintheblankswiththebestchoice.(1)Thetypesofdatapacketaremodedatafromdigitalterminalsand_modedatafromothersources.a.digital,nondigitalb.store-and-forward,nonstore-and-forwardc.packet,nonpacketputer,noncomputer(2)Ascomparedwithcircuitswitching,cessingpower,(3)ThemainelementsinARPAerfacemessageprocessors,hostsb.localterminals,IMPsc.multi-accesscomputers,peripheralequipmentd.hosts,sharingcomputers,2.True/False.(1)TheARPAnetworkisalong-distanceprivatedatanetwork,whichcanincludeseveralmulti-access,time-sharingcomputersystemforswitchingpackets.()(2)Thehostcandumpacompletemessageintotheinterfacemessageprocessors,whichcanbeconnectedby50,000bitspersecondcircuits.()(3)Thelargestpublicpacket-switchingnetwork,throughwhich200databasesareaccessible,intheU.S.serves250computers,andlinkstogethercomputersinothercountries.()(4)Thepublicpacket-switchednetworkwillbeenhancedbyprovidingforconnectionbetweenthepublictelephonenetworkandthepacketnetwork.(),3.Answerthequestions.(1)Whatarethedifferencesbetweenpacket-switchingnetworkandconventionalstore-and-forwardnetwork?(2)WhatisthefunctionoftheARPAnetwork?,PassageBCharacteristicsofCommunicationChannelsDatacommunicationsistheelectronictransmissionofinformationincludingdata,televisionpictures,soundandfacsimiles.Itusuallyinvolvesacomputer,amodem,software,andaprinter.Withthisequipment,youcancommunicatewithafriendinSt.Louis,Missouri,orParis,France,sendingandreceivinganythingfromamanuscripttoasimplemessageoverthetelephonelines.Usingthesamemethod,ahome-boundchildcaninteractwithateacherintheclassroom,anofficeworkercanworkathome,andadoctorcanaccessaremotecomputerforresearchdata.,Thereasonsforusingcommunicationssystemareconvincing.Itisexpedientandefficient,lessensjobstress,decreasescarpollution,savestimeandmoney,allowsthehometoserveasanoffice,andpromotesdistantlearninginwhichstudentscanshareinformationandcomputerresearchfindings.,Whenyouconnectonecomputertoanother,youusehardwareandsoftware.Inthemajorityofcases,thehardwareconsistsofequipmentthatsendsthedataoversometypeofcommunicationsline,suchasatelephoneline.Thesoftwarecontrolstheflowofthisdata.Thenecessaryhardwareconsistofamodemandtelephonelines.Themodemmodulatesthecomputeroutputtoanacceptablesignalfortransmissionandthendemodulatesthesignalbackforcomputerinput.Themodemonthetransmittingcomputerconvertsthedigitalsignalstomodulatedanalogsignaltonesandtransmitsthemoverthetelephonelines.1Thereceivingcomputersmodemtransformstheincominganalogsignalsbacktotheirdigitalequivalentsinordertounderstandthem.,TransmissionRateThetransmissionrateofacommunicationschannelisdeterminedbyitsbandwidthanditsspeed.Thebandwidthistherangeoffrequenciesthatachannelcancarry.Sincetransmitteddatacanbeassignedtodifferentfrequencies,thewiderthebandwidth,themorefrequencies,andthemoredatacanbetransmittedatthesametime.,Thespeedatwhichdataistransmittedisusuallyexpressedasbitspersecondorasabaudrate.Bitspersecond(bps)isthenumberofbitsthatcanbetransmittedinonesecond.Usinga10-bitbytetorepresentacharacter(7databits,1start,1stop,and1paritybit),a2,400bpstransmissionwouldtransmit240characterspersecond.Atthisrate,a20-page,single-spacedreportwouldbetransmittedinapproximatelyfiveminutes.Thebaudrateisthenumberoftimespersecondthatsignalbeingtransmittedchanges.Witheachchange,oneormorebitscanbetransmitted.Atspeedsupto2,400bps,usuallyonlyonebitistransmittedpersignalchangeand,thus,thebitspersecondandthebaudratearethesame.Toachievespeedsinexcessof2,400bps,morethanonebitistransmittedwitheachsignalchangeand,thus,thebpswillexceedthebaudrate.,DirectionofTransmissionThedirectionofdatatransmissionisclassifiedaseithersimplex,half-duplex,orfull-duplex.Insimplextransmission,dataflowsinonedirectiononly.Simplexisusedonlywhenthesendingdevice,suchasatemperaturesensor,neverrequiresaresponsefromthecomputer.Forexample,ifacomputerisusedtocontrolthetemperatureofabuilding,numeroussensorsareplacedthroughoutit.Eachsensorisconnectedtothecomputerwithasimplextransmissionlinebecausethecomputeronlyneedstoreceivedatafromthetemperaturesensorsanddoesnotneedtosenddatabacktothesensors.2,Inhalf-duplextransmission,datacanflowinbothdirectionsbutinonlyonedirectionatatime.Anexampleisacitizensbandradio.Theusercantalkorlistenbutnotdobothatthesametime.Half-duplexisoftenusedbetweenterminalsandacentralcomputer.Infull-duplextransmission,datacanbesentinbothdirectionsatthesametime.Anormaltelephonelineisanexampleoffull-duplextransmission.3Bothpartiescantalkatthesametime.Full-duplextransmissionisusedformostinteractivecomputerapplicationsandforcomputer-to-computerdatatransmission.,TransmissionModes:AsynchronousandSynchronousInasynchronoustransmissionmode(Figure3.1),individualcharacters(madeupofbits)aretransmittedatirregularintervals,forexample,whenauserentersdata.Todistinguishwhereonecharacterstopsandanotherstarts,theasynchronouscommunicationmodeusesastartandastopbit.Anadditionalbitcalledaparitybitissometimesincludedattheendofeachcharacter,andtheydetectifoneofthedatabitshasbeenchangedduringtransmission.Theasynchronoustransmissionmodeisusedforlowerspeeddatatransmissionandisusedwithmostcommunicationsequipmentdesignedforpersonalcomputers.,Figure3.1,Inthesynchronoustransmissionmode(Figure3.1),largeblocksofdataaretransmittedatregularintervals.Timingsignalssynchronizethecommunicationsequipmentatboththesendingandreceivingendsandeliminatetheneedforstartandstopbitsforeachcharacter.Error-checkingbitsandstartandendindicatorcalledsyncbytesarealsotransmitted.Synchronoustransmissionrequiresmoresophisticatedandexpensiveequipment,butitdoesgivemuchhigherspeedsandaccuracythanasynchronoustransmission.,KEYWORDSbps(bitspersecond)每秒钟传输的比特baudrate波特率simplextransmission单工传输half-duplextransmission半双工传输full-duplextransmission全双工传输asynchronoustransmission异步传输synchronoustransmission同步传输,NOTES1Themodemonthetransmittingcomputerconvertsthedigitalsignalstomodulatedanalogsignaltonesandtransmitsthemoverthetelephonelines.发送端计算机的调制解调器把数字信号调制为模拟信号，并通过电话线发送出去。主句结构为“Themodemconvertsandtransmits”。2Eachsensorisconnectedtothecomputerwithasimplextransissionlinebecausethecomputeronlyneedstoreceivedatafromthetemperaturesensorsanddoesnotneedtosenddatabacktothesensors.每一个传感器都通过一根单工传输线连接到计算机上，这是因为计算机只需要接收来自温度传感器的数据，而不需要向传感器发送数据。,3Anadditionalbitcalledaparitybitissometimesincludedattheendofeachcharcter,andtheydetectifoneofthedatabitshasbeenchangedduringtransmissiom.在每个字符的后面有时还包括一个称为奇偶校验位的附加位，它们测试数据位在传输过程中是否被意外改变。they指的是aparitybit。,EXERCISES1.Fillintheblankswiththebestchoice.(1)Thetermmodemisderivedfromthewords.a.modulateemphasisb.modularemulationc.multiplexordemultiplexerd.modulatedemodulate(2)Datatransmissionspeedismeasuredin.a.bpsbytespersecondb.bpsbitspersecondc.bpsblockspersecondd.bpsbufferspersecond(3)Thetransmissionmethodwhichusesstartstopbitsaftereachcharacteris.a.synchronousb.asynchronousc.fullduplexd.noneofabove,(4)A(n)erface(5)Devicesormediathatoperateatdifferentspeedscanbesynchronizedbyusinga.a.bufferb.datasetc.lined.modem(6)Thetransmissionmethodwhichsendsmultiplecharactersinablockis.a.synchronousb.asynchronousc.simplexd.half-duplex(7)Intransmission,datacanbesentinbothdirectionsatthesametime.a.simplexb.half-duplexc.full-duplexd.noneofabove,(8)Whenthesendingdevice,suchasatemperaturesensor,neverrequiresaresponsefromthecomputer,weshouldusetransmission.a.full-duplexb.half-duplexc.simplexd.noneofabove(9)Usinga10-bitbytetorepresentacharacter(7databits,1start,1stop,and1paritybit),a2,400bpstransmissionwouldtransmitcharactersintwoseconds.a.120b.240c.480d.720(10)scansapieceofpaperandconvertsitsimageintocodedformfortransmissionoverthetelephonesystem.a.Scannerb.Modemc.Printerd.Faxmachine,2.Answerthequestions.(1)Describethebasiccomponentsofacommunicationssystem.(2)Listanddescribethethreetypesofdatatransmission(direction)thatareused.(3)Describethetransmissionmodes:asynchronousandsynchronous.,PassageCCommunicationEquipmentandSoftwareIfaterminalorapersonalcomputeriswithinapproximately100feetofanothercomputer,thetwodevicescanusuallybedirectlyconnectedbyascale.Over1,000feet,however,theelectricalsignalweakenstothepointthatsometypeofspecialcommunicationsequipmentisrequiredtoincreaseorchangethesignaltotransmititfarther.Avarietyofcommunicationsequipmentexiststoperformthistask,buttheequipmentthatauserismostlikelytoencounterismodems,fax/modems,faxmachine,etc.,ModemsAmodemconvertsthedigitalsignalsofaterminalorcomputertoanalogsignalsthataretransmittedoveracommunicationschannel.Italsoconvertsanalogsignalsitreceivesintodigitalsignalsthatareusedbyaterminalorcomputer.Thewordmodemcomesfromacombinationofthewordsmodulate,whichmeanstochangeintoasoundoranalogsignal,anddemodulate,whichmeanstoconvertananalogsignalintoadigitalsignal.Amodemisneededatboththesendingandreceivingendsofacommunicationschannel.,Anexternalmodemisaseparate,orstand-alone,devicethatisattachedtothecomputerorterminalbyacableandtothetelephoneoutletbyastandardtelephonecord.Anadvantageofanexternalmodemisthatitcanbeeasilymovedfromoneterminalorcomputertoanother.Aninternalmodemisacircuitboardthatisinstalledinsideacomputerorterminal.Internalmodemsaregenerallylessexpensivethancomparableexternalmodemsbutonceinstalled,theyarenotaseasyasmove.,Fax/ModemsBesidessendingandreceivingdata,manymodemstodaycananswerthetelephonethewayanansweringmachinedoesandstorethemessageontheharddisk.Someevenhavecomprehensivevoice-mailcapabilitiessimilartobusinessvoice-mailsystems.Furthermore,themajorityofmodemshavefacsimile(fax)capabilities;thatis,theycansendgraphicimagesandtextbetweendistantlocations.Suchmodemsemulatefaxmachines,allowingyoutoreceiveandsendfaxtransmissionsdirectlyviayourcomputer.Afax/modemisdifferentfromastandardfaxmachineinthewayithandlesdocuments.Thefax/modemcanonlysenddocumentsthatarealreadyinthecomputer;theycandisplaythedocumentsonthecomputerscreenorprintthemwhentheyreceivethem.Howdoesthefax/modemuserfaxdocumentsnotinputtedintothecomputer?Onewayistouseascannertoscandocumentsintothecomputer.Nevertheless,thetraditionalfaxmachineismoreversatilethanthefax/modem.Itisalsomoreexpensive.,FaxMachinesThededicatedfaxmachineisacombinationscanner,faxmodem,andprinter.Thismachinescansapieceofpaperandconvertsitsimageintocodedformfortransmissionoverthetelephonesystem.Ontheotherend,afaxmachinerec