Asynchronous Transfer Mode (ATM) is, according to the ATM Forum, "a telecommunications concept defined by ANSI and ITU (formerly CCITT) standards for carriage of a complete range of user traffic, including voice, data, and video signals".[1] ATM was developed to meet the needs of the Broadband Integrated Services Digital Network, as defined in the late 1980s,[2] and designed to unify telecommunication and computer networks. It was designed for a network that must handle both traditional high-throughput data traffic (e.g., file transfers), and real-time, low-latency content such as voice and video. The reference model for ATM approximately maps to the three lowest layers of the ISO-OSI reference model: network layer, data link layer, and physical layer.[3] ATM is a core protocol used over the SONET/SDH backbone of the public switched telephone network (PSTN) and Integrated Services Digital Network (ISDN), but its use is declining in favour of all IP.

ATM provides functionality that is similar to both circuit switching and packet switching networks: ATM uses asynchronous time-division multiplexing,[4][5] and encodes data into small, fixed-sized packets (ISO-OSI frames) called cells. This differs from approaches such as the Internet Protocol or Ethernet that use variable sized packets and frames. ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins.[5] These virtual circuits may be “permanent”, i.e. dedicated connections that are usually preconfigured by the service provider, or “switched”, i.e. set up on a per-call basis using signalling and disconnected when the call is terminated.

IBM Turboways ATM 155 PCI network interface card

Marconi ForeRunnerLE 25 ATM PCI network interface card

Contents [hide]
1 Layer 2 – Datagrams 1.1 Cell size
1.2 The structure of an ATM cell
1.3 Cells in practice
1.4 Why virtual circuits?
1.5 Using cells and virtual circuits for traffic engineering 1.5.1 Traffic policing
1.5.2 Traffic shaping

1.6 Types of virtual circuits and paths
1.7 Virtual circuit routing
1.8 Call admission and connection establishment
1.9 Reference model

2 Deployment
3 Wireless ATM or Mobile ATM
4 See also
5 Notes
6 References
7 External links

Layer 2 – Datagrams[edit]

In the ISO-OSI reference model data link layer (layer 2), the basic transfer units are generically called frames. In ATM these frames are of a fixed (53 octets or bytes) length and specifically called "cells".

Cell size[edit]

If a speech signal is reduced to packets, and it is forced to share a link with bursty data traffic (traffic with some large data packets) then no matter how small the speech packets could be made, they would always encounter full-size data packets. Under normal queuing conditions the cells