InfoSource

ATM is a high-speed, low-delay multiplexing and switching technology which supports and integrates voice, video and data. Information is transmitted in cells. Each ATM cell is 53 bytes long comprising a 5-byte header and a 48-byte payload. The cell header has a Virtual Path Identifier (VPI) and a Virtual Circuit Identifier (VCI). The ATM switches use these identifiers to relay traffic through the high-speed network.

Because of its simplified approach, ATM allows for multi-gigabit transfer and switching rates. Although it resides on top of the physical layer of the conventional layered model, it does not require the use of a specific physical layer protocol.

ATM Description

• Knits local and wide-area networks and services into a seamless whole
• Billing possible on per-cell basis
• Scalable - Works at different speeds and on different media
• Open-ended growth path - Not locked to any physical medium or speed
• Example:
  • Cell size: 53 bytes
  • Speed: 155.52 Mbits/s, 622.08 Mbits/s

Why ATM-based LANs?

• Switched-based networks give full bandwidth over an interface for both multicast and 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

Advantages of ATM

• Flexible bandwidth allocation
• Simple routing due to connection oriented technology
• High bandwidth utilization due to statistical multiplexing
• i.e. ``Central Limit Theorem'' ensures peak deviates little from average (requires several activemessage streams). Deviation proportional to square root of number of streams
• Potential QOS (Quality Of Service) guarantees

ATM Benefits

• One Network-ATM will provide a single network for all traffic types-voice, data, video. ATM allows for the integration of networks improving efficiency and manageability.
  
• Enables new applications-Due to its high speed and the integration of traffic types, ATM will enable the creation and expansion of new applications such as multimedia to the desktop.
  
• Compatibility-Because ATM is not based on a specific type of physical transport, it is compatible with currently deployed physical networks. ATM can be transported over twisted pair, coax and fiber optics.
  
• Incremental Migration-Efforts within the standards organizations and the ATM Forum continue to assure that embedded networks will be able to gain the benefits of ATM incrementally-upgrading portions of the network based on new application requirements and business needs.
  
• Simplified Network Management-ATM is evolving into a standard technology for local, campus/backbone and public and private wide area services. This uniformity is intended to simplify network management by using the same technology for all levels of the network.

Disadvantages of ATM

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

ATM Forum

The ATM Forum was started in October of 1991 by a consortium of four computer and telecommunication vendors. Since its inception, it has seen unprecedented growth, and today (as of June 1994), has over 500 members. Today's membership is made up of network equipment providers, semiconductor manufacturers, service providers, carriers and, most recently, end users.

The Forum is not a Standards body. The ATM Forum is a consortium of companies that writes ATM specifications to accelerate the definition of ATM technology. These specifications are then passed up to ITU-T (Formerly the CCITT) for approval. The ITU-T standard body fully recognizes the ATM Forum as a credible working group.