Transcript Document
Prof. Dr. Ing. PUSZTAI Kalman ATM networks 1 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 2 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 3 ATM cell format 4 8 16 3 1 GFC VPI VCI Type CLP 8 HEC (CRC-8) 384 (48 bytes) Payload • 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 4 Segmentation And Reassembly (SAR) AAL AAL … … ATM ATM • 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 5 AAL5 SAR protocol < 64 KB 0– 47 bytes 16 16 32 Data Pad Reserved Len CRC-32 Padding CS-PDU trailer User data 48 bytes 48 bytes 48 bytes 6 ATM header Cell payload VPIs and VCIs Public network Network A Network B • 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 7 ATM in the LAN environment • A bit of history: ATM vs Ethernet • Major issue: how to implement broadcast and multicast in an ATM LAN? H4 H5 Ethernet links ATM links H6 H3 E2 E1 Ethernet switch H7 ATM switch AT M-attached host E3 H1 H2 8 ATL LAN Emulation (LANE) • Objective: make ATM network look like shared-media LAN (e.g., Ethernet) to higher-layer protocols Higher-layer protocols (IP, ARP, . . .) Signalling + LANE Ethernet-like interface AAL5 ATM PHY Host Higher-layer protocols (IP, ARP, . . .) Signalling + LANE AAL5 ATM PHY PHY Switch ATM PHY Host 9 LANE servers LES AT M network BUS Point-to-point VC Point-to-multipoint VC H1 H2 • 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 10