lecturer kartik krishnan lectures 1-3 communications and computer networksnew

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1、SFWR4C03:ComputerNetworksandComputerSecurityJanuary5-82004Lecturer:KartikKrishnanLectures1-3CommunicationsandComputerNetworksThefundamentalpurposeofacommunicationsystemistheexchangeofdatabetweentwoparties.Anexampleistheexchangeofvoicesignalsbetweentwotelephonesonthesamenetwork.Itisoftenimpracticalf

2、ortwocommunicatingdevicestobedirectly,point-to-pointconnected.Thisisforone(orboth)ofthefollowingcontingencies:1.Thedevicesareveryfarapart,andhencecannotshareadedicatedlink.2.Thereisasetofdevices,eachofwhichrequiresalinktomanyoftheothersatvarioustimes.Thesolutiontothisproblemistoattacheachdevicetoac

3、ommunicationnet-work.Thetwocategoriesintowhichcommunicationnetworksareclassi¯ed:localareanetworks(LANs)andwideareanetworks(WANs).WideAreaNetworksWideareanetworksgenerallycoveralargegeographicalarea,typicallymultiplecities,countries,orcontinents.Typically,aWANconsistsofanumberofin-terconnectedswitch

4、ingnodescalledrouters.Atransmissionfromanydeviceisroutedthroughtheseinternalnodestothespeci¯eddestinationdevice.Theseswitchingnodesarenotconcernedwiththecontentofthedata;rather,theirpurposeistoprovideaswitchingfacilitythatwillmovethedatafromnodetonodeuntiltheyreachtheirdestination.Theswitchingemplo

5、yedisusually:1.CircuitSwitching:Inacircuitswitchingnetwork,adedicatedcommu-nicationspathisestablishedbetweentwostationsthroughthenodesofthenetwork.Datageneratedbythesourcestationaretransmittedalongthededicatedpathasrapidlyaspossible.Sincethepathisalreadyes-tablishedapriorithereisnodelayinvolved.The

6、mostcommonexampleofcircuitswitchingisthetelephonenetwork.Disadvantagesarethecostinvolved;forexampleonepaysa¯xedrateforaphonecallevenwhenthetwopartiesdonottalk.2.PacketSwitching:Dataissentoutinasequenceofsmallchunkscalledpackets.Eachpacketispassedthroughthenetworkfromnodetonodealongsomepathleadingfr

7、omsourcetodestination.Ateachswitching1-3-1node,theentirepacketisreceived,storedbrie°yinaqueue,andthentransmittedtothenextnode.Thisiscommonlyusedincomputertocomputercommunications.Anexampleofpacketswitchingi