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Cell Switching (ATM)

Cell Switching (ATM). 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.

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Cell Switching (ATM)

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  1. Cell Switching (ATM) • 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

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

  3. Big vs Small Packets • 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 wait in the queue for 327.68us • in contrast, wait is only 53 x 8/100 = 4.24us for ATM • near cut-through behavior • two 4KB packets arrive at same time on different links • output link idle for 327.68us while both arrive • at end of 327.68us, still have 8KB to transmit • in contrast, with small cells, can transmit first cell after only 4.24us • at end of 327.68us, slightly over 4KB left in queue

  4. Big vs Small (cont) • 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 (packetization latency) • example: 1000-byte cells implies 125ms per cell (too long) • smaller packetization latency implies no need for echo cancellers in phone system. • ATM Compromise: 48 bytes = (32+64)/2

  5. 4 16 3 1 8 8 384 (48 bytes) GFC VPI VCI Type CLP HEC (CRC-8) Payload Cell Format • 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) • CLP Cell Loss Priority • HEC: Header Error Check (CRC-8) • Network-Network Interface (NNI) • switch-to-switch format • GFC becomes part of VPI field

  6. Virtual Paths/Virtual Circuits • A “Virtual Path” is established between A and B • Switches within each Network A and B uses all of the 24 bit VPID/VCID. • Switches in the Public Network only look at the VPID • Remap (change the VPID) at each hop • Do not change nor consider the VCID field. • Thus, switches in the public network see only one connection • Saves a lot in terms of signaling and state information at the switch

  7. Segmentation and Reassembly • 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

  8. AAL Sublayers • Convergence Sublayer • It adds a header/trailer to the user’s data message, before breaking it into cells. • A “message” of any layer in ATM is known as a “protocol data unit”. • Hence, the output of the CS is a CS-PDU • Segmentation and Reassembly Sublayer • Grabs a CS-PDU and breaks it into pieces (SARS-PDUs)

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

  10. AAL 3/4 Segmentation CS-PDU CS-PDU User data header trailer 44 bytes 44 bytes 44 bytes 44 bytes SAR header SAR trailer ATM header Cell payload Padding

  11. Cell Format • Type • BOM: beginning of message • COM: continuation of message • EOM end of message • SEQ: sequence of number • MID: message id • Multiple CS-PDU can be multiplexed together • Length: number of bytes of PDU in this cell SAR trailer SAR header

  12. Padding CS-PDU User data trailer 48 bytes 48 bytes 48 bytes ATM header Cell payload AAL 5 Segmentation

  13. End of PDU Detection • Recall that the ATM header has a type field • One of these bits is defined as follows in AAL5 • If cell does not carry the end of the PDU, bit = 0 • If cell does carry the end of the PDU, bit = 1

  14. AAL5 • 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

  15. True beauty of ATM • In my opinion, it is not the cell size nor the use of virtual circuits (which we knew before anyway) • The beauty lies in its hierarchical structure • Its addressing and routing is very hierarchical and flexible. • It allows a GLOBAL ATM network to be built • The Internet should learn about the design of ATM

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