网络通信_ch7_数据链路控制ppt课件.ppt

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1、BilingualCourseNetworkCommunications网络通信(ForMasterStudentsintheDepartmentofElectronicEngineering)Chapter7DataLinkControlInstructor:Dr.TianshuangQiuFacultyoftheElectronicInformationandElectricalEngineeringFallof20101OutlineBecauseofthepossibilityoftransmissionerrors,andbec

2、ausethereceiverofdatamayneedtoregulatetherateatwhichdataarrive,synchronizationandinterfacingareinsufficient.Itisnecessarytoimposealayerofcontrolthatprovidesfunctionssuchasflowcontrol,errordetection,anderrorcontrol.Thislayerisknownasdatalinkcontrolprotocol.Flowcontrolenabl

3、esareceivertoregulatetheflowofdatafromasendersothatthereceiver’sbuffersdonotoverflow.Errordetectionisperformedbycalculatinganerror-detectingcodethatisafunctionofthebitsbeingtransmitted.Thecodeisappendedtothetransmittedbits.Thereceivercalculatesthecodebasedontheincomingbit

4、sandcomparesittotheincomingcodetocheckforerrors.Indatalinkcontrol,errorcontrolisachievedbyretransmissionofdamagedframes.2Background在分层协议（特别是OSI协议）体系中，包含一个数据链路层。为什么需要数据链路层Becauseofthepossibilityoftransmissionerrors,andbecausethereceiverofdatamayneedtoregulatetherateatwhich

5、dataarrives.Itisnecessarytoimpose（施加）alayerofcontrolineachcommunicatingdevicethatprovidesfunctionssuchasflowcontrol,errordetection,anderrorcontrol.Thislayerofcontrolisknowasadatalinkcontrolprotocol.数据链路层：实现实体之间链路的建立，维护和拆除，形成可用的连接。数据链路：在使用数据链路控制协议时，系统之间的传输媒体称为数据链路。3数据链路控制的

6、主要功能线路规程解决现在时刻该谁发送的问题流量控制负责调整某个时间间隙中可以发送多少数据。使接收方能够调整来自发送方的数据流，以防止接收方出现溢出。差错控制检测和纠正传输错误。差错检测的实现：依靠差错检测码的计算实现，它是传输比特的一个函数。差错检测码通常附加在传输的数据位之后。接收方根据接收到的比特计算出差错检测码，然后将其与收到的差错检测码比较，以检查是否有差错。差错控制的实现：通过重传损坏帧来实现。4数据链路控制的要求和目标Framesynchronization:Dataaresentinblockscalledframes.Th

7、ebeginningandendofeachframemustberecognizable.Flowcontrol:Thesendingstationmustnotsendframesataratefasterthanthereceivingstationcanabsorbthem.Errorcontrol:Biterrorsintroducedbythetransmissionsystemshouldbecorrected.Addressing:Onamultipointline,suchasalocalareanetwork(LAN)

8、,theidentityofthetwostationsinvolvedinatransmissionmustbespecified.Controlanddataonsamelink:Itis