Asynchronous Transfer Mode (ATM) is a switching and multiplexing technology that operates using fixed-size cells of relatively short length. ATM is connection-oriented.

In the ATM system, information is organized into fixed-size cells, each consisting of 53 bytes (octets), with 48 bytes available for user data. These cells are labeled and switched throughout the network based on the information contained in their headers. The term “asynchronous” in ATM refers to the fact that cells assigned to the same connection do not need to be transmitted at regular intervals; instead, cells are transmitted as needed to accommodate the actual data demand. ATM allows for on-demand capacity allocation.

The header of an ATM cell contains key elements such as a Virtual Path Identifier (VPI), a Virtual Channel Identifier (VCI), and an error detection field. Error detection is handled within the header, while users are responsible for their own error detection. The VPI and VCI together define the predetermined route that all cells of a session follow when traversing the network.