Asynchronous Transfer Mode

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Asynchronous Transfer Mode (ATM) is a switching technique for telecommunication networks. It uses asynchronous time-division multiplexing,[1][2] and it encodes data into small, fixed-sized cells. This differs from networks such as the Internet or Ethernet LANs that use variable sized packets or frames. ATM provides data link layer services that run over OSI Layer 1 physical links. ATM has functional similarity with both circuit switched networking and small packet switched networking. This makes it a good choice for a network that must handle both traditional high-speed data traffic (e.g., file transfers), and real-time, low-latency content such as voice and video. ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins.[2] ATM is a core protocol used over the SONET/SDH backbone of the Integrated Services Digital Network (ISDN).

Contents

ATM concepts

Why cells

Consider a speech signal reduced to packets, and forced to share a link with bursty data traffic (traffic with some large data packets). No matter how small the speech packets could be made, they would always encounter full-size data packets, and under normal queuing conditions, might experience maximum queuing delays. That is why all packets, or "cells," should have the same small size. In addition the fixed cell structure means that ATM can be readily switched by hardware without the inherent delays introduced by software switched and routed frames.

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