Cell relay

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{system, computer, user}

In computer networking, cell relay refers to a method of statistically multiplexing small fixed-length packets, called "cells", to transport data between computers or kinds of network equipment. It is an unreliable, connection-oriented packet switched data communications protocol.

Cell relay transmission rates usually are between 56 kbit/s and several gigabits per second. ATM, a particularly popular form of cell relay, is most commonly used for home DSL connections, which often runs between 128 kbit/s and 1.544 Mbit/s (DS1), and for high-speed backbone connections (OC-3 and faster).

Cell relay protocols have neither flow control nor error correction capability, are information-content independent, and correspond only to layers one and two of the OSI Reference Model.

Cell relay can be used for delay- and jitter-sensitive traffic such as voice and video.

Cell relay systems break variable-length user packets into groups of fixed-length cells, that add addressing and verification information. Frame length is fixed in hardware, based on time delay and user packet-length considerations. One user data message may be segmented over many cells.

Cell relay statems may also carry bitstream-based data such as PDH traffic, by breaking it into streams of cells, with a lightweight synchronization and clock recovery shim. Thus cell relay systems may potentially carry any combination of stream-based and packet-based data. This is a form of statistical time division multiplexing.

Cell relay is an implementation of fast packet-switching technology that is used in connection-oriented broadband integrated services digital networks (B-ISDN, and its better-known supporting technology ATM) and connectionless IEEE 802.6 switched multi-megabit data service (SMDS).

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