Asynchronous Transfer Mode

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Asynchronous Transfer Mode (ATM) is a packet switching protocol that encodes data into small fixed-sized cells (cell relay) and provides data link layer services that run over OSI Layer 1 physical links. This differs from other technologies based on packet-switched networks (such as the Internet Protocol or Ethernet), in which variable sized packets (known as frames when referencing Layer 2) are used. ATM exposes properties from both circuit switched and small packet switched networking, making it suitable for wide area data networking as well as real-time media transport. ATM uses a connection-oriented model and establishes a virtual circuit between two endpoints before the actual data exchange begins.

ATM Traffic Engineering

Another key ATM concept involves the traffic contract. When an ATM circuit is set up each switch is informed of the traffic class of the connection.

ATM traffic contracts form part of the mechanism by which "Quality of Service" (QoS) is ensured. There are four basic types (and several variants) which each have a set of parameters describing the connection.

  1. CBR - Constant bit rate: a Peak Cell Rate (PCR) is specified, which is constant.
  2. VBR - Variable bit rate: an average cell rate is specified, which can peak at a certain level for a maximum interval before being problematic.
  3. ABR - Available bit rate: a minimum guaranteed rate is specified.
  4. UBR - Unspecified bit rate: traffic is allocated to all remaining transmission capacity.

VBR has real-time and non-real-time variants, and serves for "bursty" traffic. Non-real-time is usually abbreviated to vbr-nrt.

Most traffic classes also introduce the concept of Cell Delay Variation Tolerance (CDVT), which defines the "clumping" of cells in time.

ATM Encapsulations

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