Asynchronous Transfer Mode (ATM) is a cell relay, network and data link layer protocol which encodes data traffic into small (53 bytes; 48 bytes of data and 5 bytes of header information) fixed-sized cells.

This is instead of variable sized packets (sometimes known as frames) as in packet-switched networks (such as the Internet Protocol or Ethernet). ATM is a connection-oriented technology, in which a connection is established between the two endpoints before the actual data exchange begins.

ATM has proved very successful in the WAN scenario and numerous Telcos have implemented ATM in their wide-area network cores. Also many ADSL implementations use ATM. However, ATM has failed to gain wide use as a LAN technology, and its complexity has held back its full deployment as the single integrating network technology in the way that its inventors originally intended.

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