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 adatacommunicationsnetworkmustbeabletointerconnectawidevarietyofsubscriber sequipment Severaldifferenttypesofcomputerperipheralsmayappearasnetworkterminals andsoalsocancomputersfunctioninginseveraldifferentways 2 Terminalspeedsrangingfrom100bitspersecto10 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 3 Thetimescalefortheinterchangeofdatamessagesneedstobeofthisorder 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 NOTES 1 Thecombinationofcomputersanddatacommunicationplacessuchnewrequirementsonthesystems 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 thatone telecommunications 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 2 Severaldifferenttypesofcomputerperipheralsmayappearasnetworkterminals 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 soalso differentways 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 3 Theaccesstimesfortypicalcomputersinuseatthistimeareabout100ms 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 butthiswill areutilized 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 thepacketswitchinghastheadvantagewhenischeap a transmissionmediumputingpowerc bufferstoraged processingpower 3 ThemainelementsinARPAnetworkareand a interfacemessageprocessors 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 It isexpedientandefficient lessensjobstress decreasescarpollution savestimeandmoney allowsthehometoserveasanoffice and promotesdistantlearninginwhichstudentscanshareinformationandcomputerresearchfindings Whenyouconnectonecomputertoanother youusehardwareandsoftware Inthemajorityofcases thehardwareconsistsofequipmentthatsendsthedataoversometypeofcommunicationsline suchasatelephoneline Thesoftwarecontrolstheflowofthisdata Thenecessaryhardwareconsistofamodemandtelephonelines Themodemmodulatesthecomputeroutputtoanacceptablesignalfortransmissionandthendemodulatesthesignalbackforcomputerinput Themodemonthetransmittingcomputerconvertsthedigitalsignalstomodulatedanalogsignaltonesandtransmitsthemoverthetelephonelines 1 Thereceivingcomputer smodemtransformstheincominganalogsignalsbacktotheirdigitalequivalentsinordertounderstandthem 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 3 Bothpartiescantalkatthesametime 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 NOTES 1 Themodemonthetransmittingcomputerconvertsthedigitalsignalstomodulatedanalogsignaltonesandtransmitsthemoverthetelephonelines 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 Themodem converts andtransmits 2 Eachsensorisconnectedtothecomputerwithasimplextransissionlinebecausethecomputeronlyneedstoreceivedatafromthetemperaturesensorsanddoesnotneedtosenddatabacktothesensors 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 3 Anadditionalbitcalledaparitybitissometimesincludedattheendofeachcharcter 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 bps bytespersecondb bps bitspersecondc bps blockspersecondd bps bufferspersecond 3 Thetransmissionmethodwhichusesstartstopbitsaftereachcharacteris a synchronousb asynchronousc fullduplexd noneofabove 4 A n convertsdatafrompulseformtowaveformandbackagain a modemb controlunitc channeld interface 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 afaxmachinereconvertsthetransmittedcodeandprintsoutafacsimileoftheoriginalsheetofpaper Sometimescommunicationsequipmentispreprogrammedtoaccomplishitsdesignedcommunicationstasks Othertimes theusermustloadaprogrambeforetransmittingdata Theseprograms referredtoascommunicationssoftware canperformanumberoftasksincludingdialing ifaswitchedtelephonelineisused filetransferanddataencryption etc Dialingsoftwareallowsyoutostore review selectanddialtelephonenumbersofcomputersthatcanbecalled Thesoftwareprovidesavarietyofmeaningfulmessagestoassistyouinestablishingaconnectionbeforetransmittingdata Forexample apersonwhousesapersonalcomputerathometocommunicatewithacomputerattheofficecouldusedialingsoftwaretoestablishthecommunicationsconnection Thesoftwarewoulddisplaytheofficecomputer stelephonenumberontheuser spersonalcomputerscreen Theuserwouldentertheappropriatecommandforthedialingsoftware workingwithamodem tobegindialingtheofficecomputerandtoestablishaconnection Duringthe10or15secondsthatthisprocesstakes thesoftwarewoulddisplaymessagestoindicatespecificallywhatwashappening suchas DIALING CARRIERDETECT whichmeansthattheofficecomputerhasanswered and CONNECTED toindicatethatthecommunicationsconnectionhasbeenestablishedanddatatransmissioncanbegin Filetransfersoftwareallowsyoutomoveoneormorefilesfromonesystemtoanother Generally youhavetoloadthefiletransfersoftwareonboththesendingandreceivingcomputers Dataencryptionprotectsconfidentialdataduringtransmission Dataencryptionistheconversionofdataatthesendingendintoanunrecognizablestringofcharactersorbitsandthere conversionofthedataatthereceivingend Withoutknowinghowthedatawasencrypted someonewhointerceptedthetransmitteddatawouldhaveadifficulttimedeterminingwhatthedatameant