计算机网络与互联网英语chap7

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1、Chapter7Packets,Frames,AndErrorDetection(Parity,Checksums,andCRCs)PartIIIPacketTransmissionSmallestpieceofinformationWhenyousendamessagebetweentwohosts,thesmallestpieceofinformationmaybe:BitCharacter(e.g.RS232)SwitchingWhenshouldthispieceofinformationbesent?Assoonasitisava

2、ilable->circuitswitchingWhentheentiremessageisready->messageswitchingAfterXnumberofbits/characters->packetswitchingSwitchingcontentCircuitsw.Messagesw.Packetsw.P1P2P3P4P1P2P3P4P3P4报文报文报文ABCDABCDABCDt连接建立数据传送报文P2P1连接释放ConnectingtotheNetworkTheProblemCannotaffordindividualn

3、etworkconnectionperpairofcomputersReasonsInstallingwiresconsumestimeandmoneyMaintainingwiresconsumesmoney(esp.long-distanceconnections)SolutionNetworkhasSharedcentralcoreManyattachedstationsTheConceptofPacketsMostcomputersdonottransferdataasanarbitrarystringofcontinuousbit

4、s.InsteadthenetworksystemdividesdataintosmallbockscalledpacketssendindividuallyAlsoreferredcalledpacketnetworksorpacketswitchingnetworksTheProblemWithSharingDemandhighSomeapplicationshavelargetransfersSomeapplicationscannotwaitNeedmechanismforfairnessPacketSwitchingPrincip

5、leSolutionforfairnessDividedataintosmallunitscalledpacketsAlloweachstationopportunitytosendapacketbeforeanystationsendsanotherFormoftime-divisionmultiplexingIllustrationofPacketSwitchingAcquiresharedmediumSendonepacketIllustrationofPacketSwitchingDividingdataintosmallpacke

6、tsallowstime-divisionmultiplexingEachpacketleavesthesourceandisswitchedontothesharedcommunicationchannelthroughamultiplexorAtthedestination,thepacketisswitchedthroughademultiplexortothedestinationPacketDetailsDependonunderlyingnetworkMinimum/maximumsizeFormatHardwarepacket

7、calledaframeEachhardwaretechnologyusesdifferentpacketformatFrameorhardwareframedenotesapacketofaspecificformatonaspecifichardwaretechnologyExampleFrameFormatUsedwithRS-232RS-232ischaracter-orientedNeedtodefineastandardformatfordatatoindicatethebeginningandendoftheframeSpe

8、cialcharactersStartofheader(soh)Endoftext(eot)DefiningtheFramingStandardCanchoosetwounuse