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Asynchronous Transfer Mode
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".〔ATM Forum, The User Network Interface (UNI), v. 3.1, ISBN 0-13-393828-X, Prentice Hall PTR, 1995, page 2.〕 ATM was developed to meet the needs of the Broadband Integrated Services Digital Network, as defined in the late 1980s,〔 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.〔McDysan, David E. and Spohn, Darrel L., ''ATM : Theory and Application'', ISBN 0-07-060362-6, McGraw-Hill series on computer communications, 1995, page 563.〕 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,〔(【引用サイトリンク】title=Recommendation I.150, B-ISDN asynchronous transfer mode functional characteristics )〕〔McDysan (1999), p. 287.〕 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.〔 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.
ATM eventually became dominated by Internet Protocol (IP) only technology (and Wireless or Mobile ATM never got any foothold).
==Layer 2 – Datagrams==
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".
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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