《电子信息工程专业英语导论》.ppt

UNIT16IntroductiontoComputerNetworks NewwordsandTechnicalTermsperipheralcongestiontopologydismallymainframebusdartmuscleEthernetterminalcircularlybidirectionaltoken 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 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 表4 1 电工类国内主要学术期刊 表4 2 电子 通信类国内主要学术期刊 表4 3 自动化 控制类国内主要学术期刊 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 表4 6自动化 控制类国外主要学术期刊 表4 7计算机类国外主要学术期刊 表4 8电工 电子类国外主要学术期刊 Notes 1 Withallthesepeopleaccessinginformation theirrequestsinevitablywillconflict 在这么多人存取信息的情况下 不可避免地会产生冲突 本句中的With表示 在 的情况下 而accessinginformation是现在分词短语 修饰前面的thesepeople back 2 Eachonecancommunicatedirectlywitheitherorbothofitsneighborsbutnobodyelse 每一个设备只能与它相邻的一边或两边之间进行的通信 而不能与此外其他设备直接通信 back 3 ApplicationsfromonePCthuscanaccessdatastoredonotherswithoutrequiringamainframetocoordinatecommunications 一个PC上的应用程序可以访问其他机器上的数据 而用不着大型机对通信进行协调 storedonothers是过去分词短语 作定语修饰它前面的名词 数据 而withoutrequiringamainframetocoordinatecommunications表示条件 back 4 Moretimeisspentrelayingmessagesmeantforothersthanin forexample abustopology 这样 比起总线拓扑来 要花更多的时间来为其他的站点转发数据 本句中 relayingmessages是一个现在分词短语作状语 表示原因 而meantforothers是一个过去分词短语 修饰messages back