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ATM Networks

Introduction ATM Cells Virtual-circuit switching. ATM Networks. ATM Multiplexing. Voice. Data packets. MUX. Wasted bandwidth. Images. TDM. 4 3 2 1 4 3 2 1 4 3 2 1. ATM. `. 4 3 1 3 2 2 1.

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ATM Networks

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  1. Introduction ATM Cells Virtual-circuit switching ATM Networks

  2. ATM Multiplexing Voice Data packets MUX Wasted bandwidth Images TDM 4 3 2 1 4 3 2 1 4 3 2 1 ATM ` 4 3 1 3 2 2 1

  3. Connection-oriented packet-switched network Used in both WAN and LAN settings Signaling (connection setup) Protocol: Q.2931 Specified by ATM forum Packets are called cells 5-byte header + 48-byte payload Commonly transmitted over SONET other physical layers possible Cell Switching (ATM)

  4. No Optimal Length if small: high header-to-data overhead if large: low utilization for small messages Fixed-Length Easier to Switch in Hardware simpler enables parallelism Variable vs Fixed-Length Packets

  5. Small Improves Queue behavior finer-grained pre-emption point for scheduling link maximum packet = 4KB link speed = 100Mbps transmission time = 4096 x 8/100 = 327.68us high priority packet may sit in the queue 327.68us in contrast, 53 x 8/100 = 4.24us for ATM near cut-through behavior two 4KB packets arrive at same time link idle for 327.68us while both arrive at end of 327.68us, still have 8KB to transmit in contrast, can transmit first cell after 4.24us at end of 327.68us, just over 4KB left in queue Big vs Small Packets

  6. Small Improves Latency (for voice) voice digitally encoded at 64KBps (8-bit samples at 8KHz) need full cell’s worth of samples before sending cell example: 1000-byte cells implies 125ms per cell (too long) smaller latency implies no need for echo cancellors ATM Compromise: 48 bytes = (32+64)/2 Big vs Small (cont)

  7. The ATM Cell 48 Bytes 5 Bytes Header Payload

  8. User-Network Interface (UNI) host-to-switch format GFC: Generic Flow Control (still being defined) VCI: Virtual Circuit Identifier VPI: Virtual Path Identifier Type: management, congestion control, AAL5 (later) CLPL Cell Loss Priority HEC: Header Error Check (CRC-8) Network-Network Interface (NNI) switch-to-switch format GFC becomes part of VPI field 4 16 3 1 8 8 384 (48 bytes) GFC VPI VCI Type CLP HEC (CRC-8) Payload Cell Format

  9. ATM Switching 1 1 voice 67 Switch … video 2 67 N 25 75 video 32 voice 5 25 1 32 67 data 3 39 32 3 39 32 video 61 data 6 … 61 2 67 … N video 75 N

  10. Virtual Paths in an ATM Network a VP3 VP5 a b ATM Sw 1 ATM Sw 2 b ATM DCC c ATM Sw 3 c d e VP2 VP1 d ATM Sw 4 e Sw = switch

  11. ATM Virtual Connections Virtual Paths Physical Link Virtual Channels

  12. Virtual circuit Switching Packet Packet

  13. VC Setup Delays t Connect request 1 3 2 CC t Release 3 CR 1 2 CC t Connect confirm 1 2 CR 3 t Figure 7.19

  14. VC Signaling Connect request Connect request Connect request SW 1 SW n SW 2 … Connect confirm Connect confirm

  15. VC Routing Table Output port Next identifier Identifier 12 44 13 Entry for packets with identifier 15 15 15 23 27 13 16 58 7 34

  16. Broadband ISDN reference model Management plane Plane management Control plane User plane Layer management Higher layers Higher layers ATM adaptation layer ATM layer Physical layer

  17. User Plane Layers User information User information AAL AAL ATM ATM ATM ATM PHY PHY PHY PHY … End system End system Network

  18. Private ATM network Private UNI X X Private NNI Public ATM network A Public UNI X X X NNI X Public UNI B-ICI Public ATM network B X Public UNI X X ATM Network Interfaces

  19. Non-negotiable parameters: characteristics of networks Cell error ratio Cell misinsertion ratio: undetected header errors. Severely-errored cell block ratio: Bursty errors Negotiable parameters: User can ask during setup Cell loss ratio Cell transfer delay Cell delay variation ATM QOS Parameters

  20. Constant bit rate (CBR): Rigorous timing requirements, voice, circuit emulation. Real-time Variable Bit Rate: VBR traffic such as video. Non-real-time VBR: Bursty data sources, no rigorous timing requirments Available bit rate (ABR): Sources that can adapt to available bandwidth in the network. Low cell loss ratio. Unspecified bit rate (UBR): No QOS guarantees. ATM Service Categories

  21. AAL Sublayers Service Specific Convergence Sublayer Convergence Sublayer Common Part AAL Layer Segmentation and Reassembly Sublayer

  22. ATM Adaptation Layer (AAL) AAL 1 and 2 designed for applications that need guaranteed rate (e.g., voice, video) AAL 3/4 designed for packet data AAL 5 is an alternative standard for packet data AAL AAL … … A TM A TM Segmentation and Reassembly

  23. 47 47 47 1 47 1 47 1 47 H H H 5 48 5 48 5 48 AAL1 Process User data stream Higher layer … b1 b2 b3 Convergence sublayer CS PDUs SAR PDUs SAR sublayer H H H ATM Cells ATM layer For Constant rate transfers, e.g., 64kbps PCM voice call

  24. AAL1 PDUs (a) SAR PDU header SNP Seq. Count CSI 1 bit 3 bits 4 bits (b) CS PDU with pointer in structured data transfer 47 Bytes AAL 1 Pointer 46 Bytes 1 Byte

  25. Application scenario for AAL2 AAL 2 Mobile switching office ATM cells Low bit rate Short voice packets Originally, for variable bit rate (VBR) applications with end-to-end delay requirements, e.g., compressed video. Now, BW-efficient transfer of low-bit-rate short-packet traffic with low-delay requirements. Allows multiplexing of multiple users on a single ATM connection, e.g., cell phone traffic, compressed audio.

  26. P1 P2 P3 1 47 1 47 H H AAL2 Process Higher layer This example assumes 24 byte packets Service specific convergence sublayer Assume null Common part convergence sublayer Add 3-byte header to each user packet H 3 24 3 24 3 24 SAR sublayer Segment into SAR PDUs PAD H H ATM layer 5 48 5 48

  27. 2 44 2 2 44 2 2 44 2 AAL 3/4 Process Higher layer Information User message Service specific convergence sublayer Assume null Pad message to multiple of 4 bytes. Add header and trailer. Common part convergence sublayer H Information T PAD 4 4 Each SAR-PDU consists of 2-byte header, 2-byte trailer, and 44-byte payload. SAR sublayer … … ATM layer AAL3 for connection-oriented bursty data with low loss, no delay requirements. AAL4 for connection-less data transfers.

  28. (a) CPCS-PDU format Trailer Header AL Etag Length CPI Btag BASize Pad CPCS - PDU Payload 1 1 2 1 - 65,535 0-3 1 1 2 (bytes) (bytes) (bytes) (b) SAR PDU format Trailer (2 bytes) Header (2 bytes) LI CRC ST SN MID SAR - PDU Payload 2 4 10 44 6 10 (bits) (bytes) (bits) AAL 3/4 CPCS and SAR formats

  29. P2 P1 Higher layer Assume two packets from different users Service specific convergence sublayer MID = a MID = b Common part convergence and SAR sublayers Each packet is segmented separately. SAR PDUs identified by MID. CPCS SAR CPCS SAR  SPDUA2 SPDUA1  SPDUB2 SPDUB1 Interleaver Interleaved cells Cells from two packets are interleaved. ATM layer Multiplexing in AAL 3/4

  30. Convergence Sublayer Protocol Data Unit (CS-PDU) CPI: commerce part indicator (version field) Btag/Etag:beginning and ending tag BAsize: hint on amount of buffer space to allocate Length: size of whole PDU AAL 3/4

  31. Type BOM: beginning of message COM: continuation of message EOM end of message SEQ: sequence of number MID: message id Length: number of bytes of PDU in this cell Cell Format

  32. CS-PDU Format pad so trailer always falls at end of ATM cell Length: size of PDU (data only) CRC-32 (detects missing or misordered cells) Cell Format end-of-PDU bit in Type field of ATM header A more efficient alternative to AAL3/4 message and stream modes assured and nonassured delivery AAL5

  33. AAL 5 Process Higher layer Information Service specific convergence sublayer Assume null Common part convergence sublayer Information T PAD SAR sublayer … 48 (0) 48 (0) 48 (1) … ATM layer PTI = 1 PTI = 0 PTI = 0

  34. AAL 5 PDU UU CPI Length CRC Pad Information 0 - 65,535 0-47 1 1 2 4 (bytes) (bytes)

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