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

2. 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. 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. 4 ATM cell format 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 GFCHEC (CRC-8) 41631 8 VPIVCICLPTypePayload 384 (48 bytes)8

5. 5 Segmentation And Reassembly (SAR) 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 AAL ATM AAL ATM ……

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

7. 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. 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. 9 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 AAL5 ATM PHY ATM PHY Higher-layer protocols (IP, ARP,...) Signalling + LANE AAL5 ATM HostSwitchHost PHY Ethernet-like interface

10. 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