ATM交换技术与BISDN原理

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1、ATM业务分类和ATM业务适配的思想ITU－T标准委员会根据以下三个参数对用户平面高层业务进行分类：Timingrelationbetweensourceanddestination(requiredornotrequired)Bitrate(constantorvariable)Connectionmode(connection-orientedorconnectionless)基于以上三个参数，用户高层业务可被划分成四类：ClassA:Connection-orientedconstantbitrate(BCR)service,suchascircuitemulation

2、andCBRvideoClassB:Connection-orientedvariablebitrate(VBR)servicewithatimingrelationbetweenthesourceandthedestination,suchasVBRvideoandaudioClassC:Connection-orientedVBRservicewithoutatimingrelationbetweenthesourceandthedestination,suchasconnection-orientedfile(I.e.,data)transferClassD:Connec

3、tionlessVBRservice,suchasLANdata1ATMAdaptationLayerTrafficTypesAAL1－ATMAdaptationLayerType1:CBRservice,whichallowsATMtoemulatevoiceorDSn(n=0,1,or3)traffic.举例：PCM　64kb/sVoice,H.261codedvideotransmission,etc.AAL2－ATMAdaptationLayerType2:VBRservicewithtimingrelationbetweenthesourceanddestination

4、-Understudying.举例：可变比特率分组传输，可变比特率视频编码MPEG-1/2etc.Rt-VBR/nrt-VBRAAL3/4－ATMAdaptationLayerType3/4:VBRservicewithouttimingrelationbetweensourceanddestination.举例：帧中继，X.25etc.AAL3是面向连接的业务，AAL4是无连接数据业务AAL5－ATMAdaptationlayerType5:Connectionlesstraffic.举例：IP，SMDSetc.是简化的AAL3/4(SEAL:SimpleandEfficien

5、tAdaptationLayer)2ATMITUATM/B-ISDNServiceClasses3ApplicationsofAALandATMLayerServiceCategory4高层、AAL层和ATM层5GenericAALProtocolSublayerModel6MappingofAAL、ATMSDU/PDU7SummaryofAdaptationLayerProcess8ATM适配层协议-AAL1面向的业务(classA)－信源与信宿之间有严格的定时要求－恒定比特率编码－面向连接方式业务举例：话音业务、电路仿真(如E1仿真)、恒定比特率的图象编码业务。AAL1向A

6、AL层用户提供四种层服务－以恒定比特率传送业务源的SDU，并在目的地以同样的比特率递交这些SDU－在源和目的地之间信送数据结构信息－根据需要，指示出AAL1本身无法恢复的丢失或错误信息9ATM适配层协议-AAL110AAL1的一般结构11AAL1SARsublayer12AAL1SARLayer:Concepts－SAR-PDU:48octets;PCI=firstoctet,remaining47octetsavailableforSAR-PDUPayload－PCI:4-bitSNand4-bitSC－SN=CSIbitand3-bitSC－SNP:3-bitCRCforSNfi

7、eldandanevenparitybitcalculatedoverresulting7-bitcodeword－SNsequencecounttodetectlossormisinsertionofcells－SNPprovidescorrectionofallsingleerrorsanddetectionofmultiple-biterrors:(1)SNprotectedbyG(x)=x3+x+1(2)Resulting7-bitcodeprot