Asynchronous Transfer Mode (ATM) has evolved to be a standard for fixed-length cell switching. Within the specifications are the length and format of the ATM cell, adaptation layer functions, and signalling. Cells from multiple sources and multiple destinations are asynchronously multiplexed between multiple packet switches.

Every circuit on each link of the network is identified by unique integer fields called the Virtual Path Identifier (VPI) and Virtual Circuit Identifier (VCI). ATM switches are responsible for switching cells between ports, buffering cells, translating VPI/VCI's, guaranteeing Quality of Service (QOS), connection set-up, and connection tear-down.

The structure of the ATM cell is depicted above. Each cell is 53 bytes long, with 5 bytes reserved for the packet header and 48 bytes reserved for the payload. The GFC field exists only for the User-Network Interface (UNI), and can be used for primitive cell flow control at the endpoints of the network.

The VPI field identifies multiple circuits destined for the same endpoint, greatly reducing the number of entries in the translation table of each intermediate switch and minimizing the call-setup delay. The length of the VPI field is eight bits for UNI and twelve bits for a Network-Network Interface (NNI). The combination of the VPI and VCI fields uniquely identifies each of the possible 2^(12+16)=268 million channels which may be asynchronously transmitted across a shared link.

The HEC uses a CRC to ensure that an error has not corrupted the header. If the header is corrupted, the cell is immediately dropped.

The user's actual data are transmitted in the payload.