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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 信息類國內 外重要學術期刊一般認為 信息學科主要包括電工 電機 電子 通信 計算機 自動化等學科 下面列出了國內 外信息類主要的學術期刊 國內信息類學術期刊國內重要學術期刊一般由中國科學院 科技部 教育部以及各專業(yè)學會主辦 如 自動化學報 由中國自動化學會主辦 計算機學報 由中國計算機學會主辦等 表4 1 電工類國內主要學術期刊 表4 2 電子 通信類國內主要學術期刊 表4 3 自動化 控制類國內主要學術期刊 表4 4 計算機類國內主要學術期刊 國外信息類學術期刊國外信息類學術期刊眾多 除鼎鼎大名的 Science 與 Nature 外 主要以美國IEEE會刊 ACM會刊 英國IEE會刊為主 表4 5 電信類國外主要學術期刊 表4 6自動化 控制類國外主要學術期刊 表4 7計算機類國外主要學術期刊 表4 8電工 電子類國外主要學術期刊 Notes 1 Withallthesepeopleaccessinginformation theirrequestsinevitablywillconflict 在這么多人存取信息的情況下 不可避免地會產生沖突 本句中的With表示 在 的情況下 而accessinginformation是現在分詞短語 修飾前面的thesepeople back 2 Eachonecancommunicatedirectlywitheitherorbothofitsneighborsbutnobodyelse 每一個設備只能與它相鄰的一邊或兩邊之間進行的通信 而不能與此外其他設備直接通信 back 3 ApplicationsfromonePCthuscanaccessdatastoredonotherswithoutrequiringamainframetocoordinatecommunications 一個PC上的應用程序可以訪問其他機器上的數據 而用不著大型機對通信進行協調 storedonothers是過去分詞短語 作定語修飾它前面的名詞 數據 而withoutrequiringamainframetocoordinatecommunications表示條件 back 4 Moretimeisspentrelayingmessagesmeantforothersthanin forexample abustopology 這樣 比起總線拓撲來 要花更多的時間來為其他的站點轉發(fā)數據 本句中 relayingmessages是一個現在分詞短語作狀語 表示原因 而meantforothers是一個過去分詞短語 修飾messages back