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.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
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