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Asynchronous Transfer Mode (ATM) is a packet based switching and multiplexing technology. ATM is connection-oriented.

In ATM, the information is packed into fixed-size cells of relatively short length. Each cell has a length of 53 bytes (octets) of which 48 can be used for user information. These cells are identified and switched throughout the network by means of a label in the header. The term asynchronous refers to the fact that cells allocated to the same connection do not necessarily have to be transmitted in a regular interval. Cells are filled according to the actual demand. In ATM it is possible to allocate capacity on demand.

The header of an ATM cells contains a virtual path identifier (VPI), a virtual channel identifier (VCI) and an error detection field. Error detection is confided to the header. The user has to take care of its own error detection. The VPI and VCI define the fixed route all cells of a session take when transmitted through the network.

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