ATM  is a cell-based, link layer switching and multiplexing technology designed to be used as a general-purpose, connection-oriented transfer mode for a wide range of services. Cells in ATM are of fixed size with a header length of 5 bytes and a payload of 48 bytes. Cells are mapped into a physical transmission path, usually within SONET frames which are normally transmitted over fibre-optic connections.
ATM virtual connections may operate at either a constant or variable bit rate, and each cell in the network contains addressing information that establishes a virtual connection from source to destination. All cells are transferred, in sequence, over this virtual connection. ATM provides for both permanent and switched virtual connections.
ATM handles both connection-oriented and connectionless traffic through adaptation layers (AAL), of which four have been specified:
|AAL-1||for Constant Bit-Rate (CBR) traffic.|
|AAL-2||for Variable Bit-Rate (VBR) packetized audio and video traffic.|
|AAL-3/4||for VBR traffic in both connection-oriented and connectionless mode.|
|AAL-5||a lightweight AAL layer for VBR traffic.|