《电子信息工程专业英语导论》

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.^信息类国内、外重要学术期刊一般认为，信息学科主要包括电工、电机、电子、通信、计算机、自动化等学科，下面列出了国内、外信息类主要的学术期刊。国内信息类学术期刊

国内重要学术期刊一般由中国科学院、科技部、教育部以及各专业学会主办，如《自动化学报》由中国自动化学会主办、《计算机学报》由中国计算机学会主办等。

^1中国电机工程学报2电工技术学报3电力系统自动化4电网技术5电气自动化6电气传动7中小型电机8电工技术杂志9微电机10微特电机11高电压技术12电测与仪表表4-1.电工类国内主要学术期刊^1电子学报2通信学报3中国激光4电子与信息学报5半导体学报6红外与微米波学报7电子技术应用8电波科学学报9电路与系统学报10微电子学11微波学报12固体电子学研究与进展13真空科学与技术学报14量子电子学报15激光与光电子学进展16红外与激光工程表4-2.电子、通信类国内主要学术期刊^1自动化学报2控制理论与应用3控制与决策4信息与控制5系统工程与电子技术6机器人7系统仿真学报8系统工程学报9模式识别与人工智能10仪器仪表学报11电机与控制学报12电力系统及其自动化学报13数据采集与处理14传感器学报表4-3.自动化、控制类国内主要学术期刊^1计算机学报2软件学报3计算机研究与发展4小型微型计算机系统5计算机辅助设计与图形学学报6中国图像图形学报7计算机集成制造系统-CIMS8计算机科学9遥感学报10计算机工程11计算机应用表12计算机应用研究13计算机工程与设计14计算机应用与软件15计算机工程与应用16微型计算机表4-4.计算机类国内主要学术期刊

^国外信息类学术期刊

国外信息类学术期刊众多，除鼎鼎大名的《Science》与《Nature》外，主要以美国IEEE会刊、ACM会刊、英国IEE会刊为主。表4-5.电信类国外主要学术期刊1IEEETransactionsonCommunications2IEEETransactionsonWirelessCommunications3IEEETransactionsonCircuitsandSystems4IEEETransactionsonSignalProcessing^5EuropeanTransactionsonTelecommunications6EuropeanTransactionsonTelecommunicationsandRelatedTechnologies7IEEETransactionsonMobileComputing8IEEEWirelessCommunications9IEEECommunicationsLetters10IEEECommunicationsMagazine11IEEESignalProcessingMagazine12IEEProceedings-Communications^1IEEETransactionsonAutomaticcontrol2Automatic3InternationalJournalofControl4System&ControlLetters5IEEETransactionsonControlSystemsTechnology6IEEEControlSystemsMagazine7IEEETransactionsonSystemsManandCyberneticsPartB-Cybernetics8IEEProceedings-ControlTheoryandApplications9IEEETransactionsonRoboticsandAutomation10IEEETransactionsonIndustrialElectronics11IEEETransactionsonFuzzySystems12IEEEIntelligentSystems表4-6自动化、控制类国外主要学术期刊^1Computers&Structures

2IEEESoftware3Computers4IEEETransactionsonComputers5IEEETransactionsonSoftwareEngineering6ACMComputingSurveys7ACMTransactionsonComputersSystems8ACMTransactionsonDatabaseSystems9ACMTransactionsonInformationSystems10ACMTransactionsonMathematicalSoftware11ACMTransactionsonProgrammingLanguagesandSystems12ACMTransactionsonSoftwareEngineeringandMethodology表4-7计算机类国外主要学术期刊^1IEEEElectronDeviceLetters2IEEETransactionsonInstrumentationandMeasurement3IEEETransactionsonMicrowaveTheoryandTechniques4IEEETransactionsonElectronDevices5IEEETransactionsonPowerDelivery6IEEETransactionsonPowerSystems7IEEEMicrowaveandWirelessComponentsLetters表4-8电工、电子类国外主要学术期刊^8IEEETransactionsonAntennasandPropagation9IEEProceedings-RadarandNavigation10IEEProceedings–VisionImageandSignalProcessing11IEEProceedings–MicrowavesAntennasandPropagation12IEEProceedings–ElectricPowerApplications13IEEProceedings–CircuitsDevicesandSystems14IEE-ElectronicsLetters^Notes[1]Withallthesepeopleaccessinginformation,theirrequestsinevitab