专业外语第15单元资料讲解.docx

专业外语第15单元资料讲解.docx

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专业外语第15单元资料讲解

专业外语第15单元

PassageADataCommunications

Therapidgrowthofdatacommunicationshasbeeninfluencedprimarilybytheincreasingneedtomoveinformationtoandfromcomputers.

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.

KEYWORDS

reservation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

PassageBCharacteristicsofCommunicationChannels

Datacommunicationsistheelectronictransmissionofinformationincludingdata,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

　　·promotesdistantlearninginwhichstudentscanshareinformationandcomputerresearch findings.

Whenyouconnectonecomputertoanother,youusehardwareandsoftware.

Inthemajorityofcases,thehardwareconsistsofequipmentthatsendsthedataoversometypeofcommunicationsline,suchasatelephoneline.

Thesoftwarecontrolstheflowofthisdata.Thenecessaryhardwareconsistofamodemandtelephonelines.

Themodemmodulatesthecomputeroutputtoanacceptablesignalfortransmissionandthendemodulatesthesignalbackforcomputerinput.

Themodemonthetransmittingcomputerconvertsthedigitalsignalstomodulatedanalogsignaltonesandtransmitsthemoverthetelephonelines.[1]

Thereceivingcomputer’smodemtransformstheincominganalogsignalsbacktotheirdigitalequivalentsinordertounderstandthem.

TransmissionRate

Thetransmissionrateofacommunicationschannelisdeterminedbyitsbandwidthanditsspeed.

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.

DirectionofTransmission

　Thedirectionofdatatransmissionisclassifiedaseithersimplex,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:

AsynchronousandSynchronous

Inasynchronoustransmissionmode（Figure3.1）,individualcharacters（madeupofbits）aretransmittedatirregularintervals,forexample,whenauserentersdata.Todistinguishwhereonecharacterstopsandanotherstarts,theasynchronouscommunicationmodeusesastartandastopbit.

Anadditionalbitcalledaparitybitissometimesincludedattheendofeachcharacter,andtheydetectifoneofthedatabitshasbeenchangedduringtransmission.

Theasynchronoustransmissionmodeisusedforlowerspeeddatatransmissionandisusedwithmostcommunicationsequipmentdesignedforpersonalcomputers.

Figure3.1

Inthesynchr