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Prof. Dr. Ing. PUSZTAI Kalman

Prof. Dr. Ing. PUSZTAI Kalman. ATM networks. ATM overview. Promising technology in early 90s (why?) Connection-oriented (virtual circuits) Signalling protocol: Q.2931 Support for QoS (reserve bandwidth and buffer space for each VC at switches) Fixed-size packets (cells): 53 bytes.

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Prof. Dr. Ing. PUSZTAI Kalman

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  1. Prof. Dr. Ing. PUSZTAI Kalman ATM networks

  2. ATM overview • Promising technology in early 90s (why?) • Connection-oriented (virtual circuits) • Signalling protocol: Q.2931 • Support for QoS (reserve bandwidth and buffer space for each VC at switches) • Fixed-size packets (cells): 53 bytes

  3. Cells and cell size • Pros and cons of variable-sized packets • Bandwidth efficiency (no zero padding) • Hardware complexity • Benefits of fixed-size packets (cells) • Switch processing is same for all cells • Small cells reduce “preemption delay” • Trade-offs in choosing cell size • Bandwidth efficiency & cell processing speed • ATM: 48B, 5B header, 53B total

  4. ATM cell format 4 16 3 1 8 8 384 (48 bytes) • VPI & VCI: identify virtual-circuit • Type bits: • Explicit Forward Congestion Indication • User signalling bit (used by AAL5: shows last cell of a packet) • Cell Loss Priority bit GFC VPI VCI Type CLP HEC (CRC-8) Payload

  5. Segmentation And Reassembly (SAR) AAL AAL • Also referred to as “fragmentation and reassembly” (in the IP context) • SAR is performed by ATM Adaptation Layer (AAL) • AAL attaches an additional header to the packet needed for reassembly at the receiver • Five different AAL protocols were defined; AAL5 was the most popular in data networks … … A TM A TM

  6. AAL5 SAR protocol Padding CS-PDU User data trailer 48 bytes 48 bytes 48 bytes ATM header Cell payload

  7. VPIs and VCIs • ATM connection identifiers: 8b VPI + 16b VCI • Two-level hierarchy: • Virtual paths are used between major network nodes • Virtual circuits are multiplexed in same virtual paths • Objective: backbone switches need smaller VC table

  8. ATM in the LAN environment • A bit of history: ATM vs Ethernet • Major issue: how to implement broadcast and multicast in an ATM LAN?

  9. ATL LAN Emulation (LANE) • Objective: make ATM network look like shared-media LAN (e.g., Ethernet) to higher-layer protocols Higher-layer Higher-layer protocols protocols (IP, ARP, . . .) (IP, ARP, . . .) Ethernet-like interface Signalling Signalling + LANE + LANE AAL5 AAL5 ATM ATM ATM PHY PHY PHY PHY Host Switch Host

  10. LANE servers • LECS: LAN Emulation Configuration Server • Used to first connect an ATM host to a LANE network • LES: LAN Emulation Server • Keeps track of ATM and MAC addresses for each host • BUS: Broadcast & Unknown Server • Maintains point-to-multipoint VC to all registered clients

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