ATM : Main Points

Table of Contents

• ATM - an overview
• Technical description of ATM
• Advantages of ATM
• Disadvantages of ATM
• Why ATM-based LANs?

• Proposed Telecommunication Standard for Broadband ISDN
• Best effort delivery system
• Integrates Voice, Video and Data
• Uses short fixed length packets called cells
• Potential to remove performance bottlenecks in todays LANs and WANs

• ATM is a connection-oriented, packet-like switching and multiplexing principle
• Three layers in the B-ISDN protocol reference model
  • physical layer
    • various transmission media
    • kilobits per second to gigabits per second
  • ATM Layer
    • short fixed-length cells (53 bytes - 48 bytes payload)
    • multiplex logical channels within a physical channel
    • fixed length cells very-high-speed switching hardware
    • problems with traffic management
  • ATM adaptation layer
    • AAL Type 1 --- constant bit rate services
    • AAL Type 2 --- variable bit rate services
    • AAL Type 3/4 --- connectionless services/data protocols
    • AAL Type 5 --- high-speed data protocol

• Flexible bandwidth allocation (bandwidth on demand)
• Simple routing due to connection oriented technology - Every cell with the same source and destination travels over the same route
• High bandwidth utilization due to statistical multiplexing
  i.e. ``Central Limit Theorem'' ensures peak deviates little from average (requires several active message streams). Deviation proportional to square root of number of streams
• Potential QOS (Quality Of Service) guarantees

• Overhead of cell header (5 bytes per cell)
• Complex mechanisms for achieveing QoS
• Congestion may cause cell losses

• Switched-based networks give full bandwidth over an interface
  • multicast
  • broadcast
• Scalable multi-switch networks --- performance of bridging (direct network links) with scalability of routing (interpret addresses with selective forwarding)
• Resource reservation with ATM
  • guaranteed bandwidth
  • quality of service
  • but still require protocols (TCP) to guarentee data integrity
• Transparent support for common protocols
  • use IP protocol stack
  • use native ATM adaptation layer
• Standardized upgrade path
  • 140-155 Mbps today (TAXI, OC-3)
  • 622 Mbps (OC-12)
  • 2.5 Gbps (OC-48)
• Match speed to applications