Unit 8 Computer Network

Unit8ComputerNetworkSectionANetworkFundamentals•Theneedtoshareinformationandresourcesamongdifferentcomputershasledtolinkedcomputersystems,callednetworks,inwhichcomputersareconnectedsothatdatacanbetransferredfrommachinetomachine.Inthesenetworks,computeruserscanexchangemessagesandshareresources—suchasprintingcapabilities,softwarepackages,anddatastoragefacilities—thatarescatteredthroughoutthesystem.Theunderlyingsoftwarerequiredtosupportsuchapplicationshasgrownfromsimpleutilitypackagesintoanexpandingsystemofnetworksoftwarethatprovidesasophisticatednetworkwideinfrastructure.Inasense,networksoftwareisevolvingintoanetworkwideoperatingsystem.I.NetworkClassifications•Acomputernetworkisoftenclassifiedasbeingeitheralocalareanetwork(LAN),ametropolitanareanetwork(MAN),orawideareanetwork(WAN).ALANnormallyconsistsofacollectionofcomputersinasinglebuildingorbuildingcomplex.Forexample,thecomputersonauniversitycampusorthoseinamanufacturingplantmightbeconnectedbyaLAN.AMANisanetworkofintermediatesize,suchasonespanningalocalcommunity.AWANlinksmachinesoveragreaterdistance—perhapsinneighboringcitiesoronoppositesidesoftheworld.•Anothermeansofclassifyingnetworksisbasedonwhetherthenetwork’sinternaloperationisbasedondesignsthatareinthepublicdomainoroninnovationsownedandcontrolledbyaparticularentitysuchasanindividualoracorporation.（发明、商标等）不受专利权限制网络分类的另一种方式是根据网络的内部运行是基于无专利权保护的设计还是基于特定实体（如个人或公司）所拥有和控制的革新。前一种类型的网络称为开放式网络，后一种类型的网络称为封闭式网络，有时也称为专有网络。•Theinternetisanopensystem.Inparticular,communicationthroughouttheInternetisgovernedbyanopencollectionofstandardsknownastheTCP/IPprotocolsuite.Anyoneisfreetousethesestandardswithoutpayingfeesorsigninglicenseagreements.Anetworkoftheformertypeiscalledanopennetwork;anetworkofthelattertypeiscalledaclosed,orsometimesaproprietary,network.Incontrast,acompanysuchasNovellInc.mightdevelopsystemsforwhichitchoosestomaintainownershiprights,allowingthecompanytodrawincomefromsellingorleasingtheseproducts.Networksbasedonsuchsystemsareexamplesofclosednetworks.•Stillanotherwayofclassifyingnetworksisbasedonthetopologyofthenetwork,whichreferstothepatterninwhichthemachinesareconnected.Thebustopology,ringtopology,andstartopologyarethreepopulartopologies.Ofthese,thestarnetworkisperhapstheoldest,havingevolvedfromtheparadigmofalargecentralcomputerservingmanyusers.Asthesimpleterminalsemployedbytheseusersgrewintosmallcomputersthemselves,astarnetworkemerged.Apointtoemphasizeisthattheconnectionsbetweenmachinesinanetworkdonotneedtobephysical.Wirelessnetworks,usingradiobroadcasttechnology,arebecomingquitecommon.II.Protocols•Foranetworktofunctionreliably,itisimportanttoestablishrulesbywhichnetworkactivitiesareconducted.Suchrulesarecalledprotocols.Bydevelopingandadoptingprotocolstandards,vendersareabletobuildproductsfornetworkapplicationsthatarecompatiblewithproductsfromothervenders.Thus,thedevelopmentofprotocolstandardsisanindispensableprocessinthedevelopmentofnetworkingtechnologies.•Asanintroductiontotheprotocolconcept,letusconsidertheproblemofcoordinatingthetransmissionofmessagesamongcomputersinanetwork.Withoutrulesgoverningthiscommunication,allthecomputersmightinsistontransmittingmessagesatthesametimeormightfailtorelaymessageswhenthatassistanceisrequired.•Oneapproachtosolvingthisproblemisthetokenringprotocol,whichwasdevelopedbyIBMinthe1970sandcontinuestobeapopularprotocolinnetworksbasedontheringtopology.Inthisprotocol,allthemachinesinthenetworktransmitmessagesinonlyonecommondirection(Figure8A-1),meaningthatallmessagessentoverthenetworkmovearoundtheringinthesamedirectionbybeingforwardedfromcomputertocomputer.Whenamessagereachesitsdestination,thedestinationmachinekeepsacopyofitandforwardsacopyonaroundthering.Whentheforwardedcopyreachestheoriginatingcomputer,thatmachineknowthatthemessagemusthavereacheditsdestinationandremovesthemessagefromthering.Ofcourse,thissystemdependsoninter-machinecooperation.Ifamachineinsistsonconstantlytransmittingmessagesofitsownratherthanforwardingthoseoftheothermachines,nothingwillbeaccomplished.•Tosolvethisproblem,auniquebitpattern,calledatoken,ispassedaroundthering.Possessionofthistokengivesamachinetheauthoritytotransmititsownmessages;withoutatoken,amachineisonlyallowedtoforwardmessages.Normally,eachmachinemerelyrelaysthetokeninthesamemannerinwhichitrelaymessages.If,however,themachinereceivingthetokenhasmessagesofitsowntointroducetothenetwork,ittransmitsonemessagewhileholdingthetoken.Whenthismessagehascompleteditscyclearoundthering,themachineforwardsthetokentothenextmachineinthering.Likewise,whenthenextmachinereceivesthetoken,itcaneitherforwardthetokenimmediatelyortransmititsownnewmessagebeforesendingthetokenontothenextmachine.Inthismanner,eachmachineinthenetworkhasequalopportunitytointroducemessagesofitsownasthetokencirclesaroundthering.InanEthernetsystem,therighttotransmitmessagesiscontrolledbytheprotocolknownasCarrierSenseMultipleAccesswithCollisionDetection(CSMA/CD).Thisprotocoldictatesthateachmessagebebroadcasttoallthemachinesonthebus(Figure8A-2).•Anotherprotocolforcoordinatingmessagetransmiss