《電子信息工程專業(yè)英語(yǔ)導(dǎo)論》

UNIT16

IntroductiontoComputerNetworks

^NewwordsandTechnicalTerms

peripheralcongestiontopologydismallymainframebusdartmuscleEthernetterminalcircularlybidirectionaltoken^TextDuringthe1950s,mostcomputersweresimilarinonerespect.Theyhadamainmemory,acentralprocessingunit(CPU),andperipherals.ThememoryandCPUwerecentraltothesystem.Sincethenanewgenerationofcomputinghasemergedinwhichcomputationanddatastorageneednotbecentralized.Ausermayretrieveaprogramfromoneplace,runitonanyofavarietyofprocessors,andsendtheresulttoathirdlocation.AsystemconnectingdifferentdevicessuchasPCs,printers,anddiskdrivesisanetwork.Typically,eachdeviceinanetworkservesaspecificpurposeforoneormoreindividuals.For^

example,aPCmaysitonyourdeskprovidingaccesstoinformationorsoftwareyouneed.APCmayalsobedevotedtomanagingadiskdrivecontainingsharedfiles.Wecallitafileserver.Oftenanetworkcoversasmallgeographicareaandconnectsdevicesinasinglebuildingorgroupofbuildings.Suchanetworkisalocalareanetwork(LAN).Anetworkthatcoversalargerareasuchasamunicipality,state,country,ortheworldiscalledawideareanetwork(WAN).Generallyspeaking,mostnetworksmayinvolvemanypeopleusingmanyPCs,eachofwhichcanaccessanyofmanyprintersorservers.Withallthesepeopleaccessinginformation,theirrequestsinevitablywillconflict[1].Consequently,thedevicesmustbeconnectedin^awaythatpermitsanorderlytransferofinformationforallconcerned.Agoodanalogyisastreetlayoutinalargecity.Withonlyonepersondrivingitmatterslittlewherethestreetsare,whichonesareone-way,wherethetrafficsignalsare,orhowtheyaresynchronized.Butwiththousandsofthecarsonthestreetsduringthemorningrushhour,abadlayoutwillcreatecongestionthatcausesmajordelays.Thesameistrueofcomputernetworks.Theymustbeconnectedinawaythatallowsdatatotravelamongmanyuserswithlittleornodelay.Wecalltheconnectionstrategythenetworktopology.Thebesttopologydependsonthetypesofdevicesanduserneeds.Whatworkswellforonegroupmayperformdismallyforanother.^Fig.4-16.Acommonbustopology.

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Somecommonnetworktopologiesaredescribedasfollowing.Fig.4-16showsacommonbustopology(orsimplybustopology)connectingdevicessuchasworkstations,mainframes,andfileservers.Theycommunicatethroughasinglebus(acollectionofparallellines).Acommonapproachgiveseachdeviceaninterfacethatlistenstothebusandexaminesitsdatatraffic.Ifaninterfacedeterminesthatdataaredestinedforthedeviceitserves,itreadsthedatafromthebusandtransfersittothedevice.Similarly,ifadevicewantstotransmitdata,theinterfacecircuitsensewhenthebusisemptyandthentransmitdata.Thisisnotunlikewaitingonafreewayentrancerampduringrushhour.Yousenseanopeningandeitherquicklydarttoitormuscle^

yourwaythrough,dependingonwhetheryou'redrivingasubcompactoralargetruck.Sometimes,twodevicestrytotransmitsimultaneously.Eachonedetectsanabsenceoftrafficandbeginstransmittingbeforebecomingawareoftheotherdevice’stransmission.Theresultisacollisionofsignals.Asthedevicestransmittheycontinuetolistentothebusanddetectthenoiseresultingfromthecollisions.Whenadevicedetectsacollisionitstopstransmitting,waitsarandomperiodoftime,andtriesagain.Thisprocess,calledCarrierSense,MultipleAccesswithCollisionDetection(CSMA/CD)willbediscussedlater.^

OnepopularcommonbusnetworkisanEthernet.ItscommonbustypicallyisEthernetcable,whichconsistsofcopper,opticalfiber,orcombinationsofboth.Itsdesignallowsterminals,PCs,diskstoragesystems,andofficemachinestocommunicate.AmajoradvantageofanEthernetistheabilitytoaddnewdevicestothenetworkeasily.Anothercommonconnectingarrangementisthestartopology,showninFig.4-17.Itusesacentralcomputerthatcommunicateswithotherdevicesinthenetwork.Controliscentralized;ifdevicewantstocommunicate,itdoessoonlythroughthecentralcomputer.Thecomputer,inturn,routesthedatatoitsdestination.Centralizationprovidesafocalpointfor^responsibility,anadvantageofthestartopology.Thebustopology,however,hassomeadvantagesoverastartopology.Thelackofcentralcontrolmakesaddingnewdeviceseasybecausenodeviceneedstobeawareofothers.Inaddition,thefailureorremovalofadeviceinabusnetworkdoesnotcausethenetworktofail.Inastartopology,thefailureofthecentralcomputerbringsdowntheentirenetwork.^Fig.4-17.Thestartopology.^Startopologiesofteninvolveasinglemainframecomputerthatservicesmanyterminalsandsecondarydevices.Withappropriateterminalemulationsoftware,PCscancommunicatewiththemainframe.Datatransfersbetweenterminalsorbetweenterminalsandstoragedevicesoccuronlythroughthemaincomputer.InaringtopologyshowninFig.4-18,devicesareconnectedcircularly.Eachonecancommunicatedirectlywitheitherorbothofitsneighborsbutnobodyelse[2].Ifitwantstocommunicatewithadevicefartheraway,itsendsamessagethatpassesthrougheachdeviceinbetween.^Fig.4-18.Aringtopology.^

Aringnetworkmaybeeitherunidirectionalorbidirectional.Unidirectionalmeansthatalltransmissionstravelinthesamedirection.Thus,eachdevicecancommunicatewithonlyoneneighbor.Bidirectionalmeansthatdatatransmissionstravelineitherdirection,thatis,adevicecancommunicatewithbothneighbors.RingtopologiessuchasIBM'stokenringnetworkoftenconnectPCsinasingleofficeordepartment.ApplicationsfromonePCthuscanaccessdatastoredonotherswithoutrequiringamainframetocoordinatecommunications[3].Instead,communicationsarecoordinatedby^passingatokenamongallthestationsinthering.Astationcansendsomethingonlywhenitreceivesthetoken.Adisadvantageoftheringtopologyisthatwhenonestationsendstoanother,allstationsinbetweenareinvolved.Moretimeisspentrelayingmessagesmeantforothersthanin,forexample,abustopology[4].Moreover,thefailureofonestationcausesabreakintheringthataffectscommunicationsamongallthestations.Manycomputernetworksarecombinationsofvarioustopologies.Fig.4-19showsapossiblecombination.^Fig.4-19.Combinationsofvarioustopologies.^信息類國(guó)內(nèi)、外重要學(xué)術(shù)期刊一般認(rèn)為，信息學(xué)科主要包括電工、電機(jī)、電子、通信、計(jì)算機(jī)、自動(dòng)化等學(xué)科，下面列出了國(guó)內(nèi)、外信息類主要的學(xué)術(shù)期刊。國(guó)內(nèi)信息類學(xué)術(shù)期刊

國(guó)內(nèi)重要學(xué)術(shù)期刊一般由中國(guó)科學(xué)院、科技部、教育部以及各專業(yè)學(xué)會(huì)主辦，如《自動(dòng)化學(xué)報(bào)》由中國(guó)自動(dòng)化學(xué)會(huì)主辦、《計(jì)算機(jī)學(xué)報(bào)》由中國(guó)計(jì)算機(jī)學(xué)會(huì)主辦等。

^1中國(guó)電機(jī)工程學(xué)報(bào)2電工技術(shù)學(xué)報(bào)3電力系統(tǒng)自動(dòng)化4電網(wǎng)技術(shù)5電氣自動(dòng)化6電氣傳動(dòng)7中小型電機(jī)8電工技術(shù)雜志9微電機(jī)10微特電機(jī)11高電壓技術(shù)12電測(cè)與儀表表4-1.電工類國(guó)內(nèi)主要學(xué)術(shù)期刊^1電子學(xué)報(bào)2通信學(xué)報(bào)3中國(guó)激光4電子與信息學(xué)報(bào)5半導(dǎo)體學(xué)報(bào)6紅外與微米波學(xué)報(bào)7電子技術(shù)應(yīng)用8電波科學(xué)學(xué)報(bào)9電路與系統(tǒng)學(xué)報(bào)10微電子學(xué)11微波學(xué)報(bào)12固體電子學(xué)研究與進(jìn)展13真空科學(xué)與技術(shù)學(xué)報(bào)14量子電子學(xué)報(bào)15激光與光電子學(xué)進(jìn)展16紅外與激光工程表4-2.電子、通信類國(guó)內(nèi)主要學(xué)術(shù)期刊^1自動(dòng)化學(xué)報(bào)2控制理論與應(yīng)用3控制與決策4信息與控制5系統(tǒng)工程與電子技術(shù)6機(jī)器人7系統(tǒng)仿真學(xué)報(bào)8系統(tǒng)工程學(xué)報(bào)9模式識(shí)別與人工智能10儀器儀表學(xué)報(bào)11電機(jī)與控制學(xué)報(bào)12電力系統(tǒng)及其自動(dòng)化學(xué)報(bào)13數(shù)據(jù)采集與處理14傳感器學(xué)報(bào)表4-3.自動(dòng)化、控制類國(guó)內(nèi)主要學(xué)術(shù)期刊^1計(jì)算機(jī)學(xué)報(bào)2軟件學(xué)報(bào)3計(jì)算機(jī)研究與發(fā)展4小型微型計(jì)算機(jī)系統(tǒng)5計(jì)算機(jī)輔助設(shè)計(jì)與圖形學(xué)學(xué)報(bào)6中國(guó)圖像圖形學(xué)報(bào)7計(jì)算機(jī)集成制造系統(tǒng)-CIMS8計(jì)算機(jī)科學(xué)9遙感學(xué)報(bào)10計(jì)算機(jī)工程11計(jì)算機(jī)應(yīng)用表12計(jì)算機(jī)應(yīng)用研究13計(jì)算機(jī)工程與設(shè)計(jì)14計(jì)算機(jī)應(yīng)用與軟件15計(jì)算機(jī)工程與應(yīng)用16微型計(jì)算機(jī)表4-4.計(jì)算機(jī)類國(guó)內(nèi)主要學(xué)術(shù)期刊

^國(guó)外信息類學(xué)術(shù)期刊

國(guó)外信息類學(xué)術(shù)期刊眾多，除鼎鼎大名的《Science》與《Nature》外，主要以美國(guó)IEEE會(huì)刊、ACM會(huì)刊、英國(guó)IEE會(huì)刊為主。表4-5.電信類國(guó)外主要學(xué)術(shù)期刊1IEEETransactionsonCommunications2IEEETransactionsonWirelessCommunications3IEEETransactionsonCircuitsandSystems4IEEETransactionsonSignalProcessing^5EuropeanTransactionsonTelecommunications6EuropeanTransactionsonTelecommunicationsandRelatedTechnologies7IEEETransactionsonMobileComputing8IEEEWirelessCommunications9IEEECommunicationsLetters10IEEECommunicationsMagazine11IEEESignalProcessingMagazine12IEEProceedings-Communications^1IEEETransactionsonAutomaticcontrol2Automatic3InternationalJournalofControl4System&ControlLetters5IEEETransactionsonControlSystemsTechnology6IEEEControlSystemsMagazine7IEEETransactionsonSystemsManandCyberneticsPartB-Cybernetics8IEEProceedings-ControlTheoryandApplications9IEEETransactionsonRoboticsandAutomation10IEEETransactionsonIndustrialElectronics11IEEETransactionsonFuzzySystems12IEEEIntelligentSystems表4-6自動(dòng)化、控制類國(guó)外主要學(xué)術(shù)期刊^1Computers&Structures

2IEEESoftware3Computers4IEEETransactionsonComputers5IEEETransactionsonSoftwareEngineering6ACMComputingSurveys7ACMTransactionsonComputersSystems8ACMTransactionsonDatabaseSystems9ACMTransactionsonInformationSystems10ACMTransactionsonMathematicalSoftware11ACMTransactionsonProgrammingLanguagesandSystems12ACMTransactionsonSoftwareEngineeringandMethodology表4-7計(jì)算機(jī)類國(guó)外主要學(xué)術(shù)期刊^1IEEEElectronDeviceLetters2IEEETransactionsonInstrumentationandMeasurement3IEEETransactionsonMicrowaveTheoryandTechniques4IEEETransactionsonElectronDevices5IEEETransactionsonPowerDelivery6IEEETransactionsonPowerSystems7IEEEMicrowaveandWirelessComponentsLetters表4-8電工、電子類國(guó)外主要學(xué)術(shù)期刊^8IEEETransactionsonAntennasandPropagation9IEEProceedings-RadarandNavigation10IEEProceedings–VisionImageandSignalProcessing11IEEProceedings–MicrowavesAntennasandPropagation12IEEProceedings–ElectricPowerApplications13IEEProceedings–CircuitsDevicesandSystems14IEE-ElectronicsLetters^Notes[1]Withallthesepeopleaccessinginformation,theirrequestsinevitab