atm交換技術(shù)與bisdn原理(編輯修改稿)
2025-06-16 22:55 本頁(yè)面
【文章內(nèi)容簡(jiǎn)介】 4 －源與宿間無(wú)嚴(yán)格定時(shí)要求 －源與宿之間無(wú)嚴(yán)格定時(shí)要 對(duì)時(shí)延相對(duì)不敏感 求對(duì)時(shí)延相對(duì)不敏感 －可變比特率 －可變比特率 對(duì)數(shù)據(jù)丟失敏感 對(duì)數(shù)據(jù)丟失敏感 －面向連接型數(shù)據(jù) (或信令 ) －無(wú)連接型數(shù)據(jù) － Class C － Class D －如 －如 LAN Data 消息或信令 LAN Emulation 有多處共同之處 (差別不大 ) AAL3/4 19 (2)AAL3/4一般結(jié)構(gòu) －多增加 SSCS 子層功能： SSCS－ Service Specific Convergence Sublayer 業(yè)務(wù)特定匯聚子層 SAPUser Data (065,535Bytes)CPI(1B)Btag(1B)BAsize(1B)Information(065,535B)PAD(03B)AL(1B)Etag(1B)Length(1B)ST(2 bits)SN(4 bits)MID(10 bits)Payload(44 Bytes)LI(6 bits)CRC(10 bits)No SAP PointCS_PDUSAP_PDUHigh LayersCPCSSublayersSARSublayersHeaders(5B)Information Field(48 Bytes)ATM LayerATM cell20 AAL 3/4 general data unit ATM LayerSSCSCPCSAALSDUSSCSPDUHeaderSSCSPDUPayloadSSCSPDUTrailerSSCSPDUServicesspecificconvergencesublayer(SSCS)No SAP defined betwween SSCS and CPCSNo SAP defined betwween CPCS and SARAALSDUCPCSPDUHeaderCPCSPDUPayloadCPCSPDUTrailerCPCSPDUCommon Partconvergencesublayer(CPCS)SARPDUHeaderSARPDUSegmentationand reassembly(SAR)sublayerSARPDUPayloadSARPDUTrailerAALSAPATMSAPSARCellHeadersCell Information Field(cell payload)ATMSDUATMPDU=cellAALATM21 AAL 3/4 layers and PDUs UserPDUCSPDUHeaderCSPayloadCSPDUTrailerCellHeaderCell payloadSARPDUHeaderSARPDUPayloadSARPDUTrailerSARPDUHeaderSARPDUPayloadSARPDUTrailerCellHeaderCell payloadCSSARCPI Btag BAsize AL Etag LengthATM22 AAL 3/4 SARPDU ST SN MID SARPDU Payload LI CRCSARPDU header(2 bits) (4 bits) (10 bits) (44 Octets)(6 bits) (10 bits)SARPDU trailerSARPDUCRC:Cyclic redundancy SAR:Segmentation and ressemblyLI:Length indicator SN:Sequence numberMID:Multiplexing identifier ST:Segment typePDU:Protocol data unit23 ST：段類(lèi)型 (segment type),2bits SN：序號(hào) (Sequence Number),4bit SN: 0~15模 16計(jì)數(shù) ST 編碼 含義B O MC O MEOMSSM10000111開(kāi)始段中間段結(jié)束段單段報(bào)文AAL 3/4 SARPDU 24 MID:復(fù)用標(biāo)志 (Multiplexing Identifier),10bit 無(wú)連接數(shù)據(jù)通信時(shí)用戶復(fù)用， 210＝ 1024(個(gè)用戶 )， 實(shí)現(xiàn)同一 VCI(ATM層虛通路連接 )可支持多個(gè) AAL 層的連接。 － MID復(fù)用 ATM VCINetworkLAN Gateway LAN GatewayBACA B C永久/半永久Same QosMID0 MID1 MID2MID0 MID1 MID2MID:復(fù)用標(biāo)志 25 AAL3/4 functional model on the send side.[] AALSAPAALCEPMID=cSSCSMID=bMID=aSSCSSSCSSegmentation for MID cCPCSSARSegmentation for MID bCPCSSARSegmentation for MID aCPCSSARInterleaverATMCEPATMSAP ATMDATArequstCPCSUNITDATAinvokeCPCSUabortinvokeSARUNITDATAinvokeSARUabortinvokeSSCSCPCSSARAAL26 AAL3/4 functional model on the receive side.[] AALSAPAALCEPMID=cSSCSMID=bMID=aSSCSSSCSSegmentation for MID cCPCSSARSegmentation for MID bCPCSSARSegmentation for MID aCPCSSARDispatcherATMCEPATMSAP ATMDATArequstCPCSUNITDATAinvokeCPCSUabortinvokeSARUNITDATAinvokeSARUabortinvokeSSCSCPCSSARAAL27 ? LI:長(zhǎng)度指示 (Length Indicator),6 bit 指示 SARPDU Payload段中有效字節(jié)的長(zhǎng)度。 LI與段 類(lèi)型密切相關(guān)。
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