ATM uses a fixed-length cell of 53 bytes. It is capable of carrying any type of data. Even data with protocol headers from other networks can be carried in an ATM cell (or segmented into several ATM cells).

The concept is that small cells introduce less delay. Devices waiting to transmit do not have to wait as long if cells are short. Smart cells can be processed more quickly than large cells, reducing the processing at network nodes. The small cell size was a compromise between the telephone and cable industries and the data industry. While large cell sizes are better for data, small cells work better for voice and video.

There are various "planes" supported in ATM. Think of each plan as a level of communications. The user plane transmits users data from one endpoint to another. This plane is also responsible for multiplexing among different connectinos (using the Virtual Path Identifier / Virtual Channel Identifier, or VPI/VCI) and for cell rate decoupling, cell discrimination, and traffic shaping. The user plane performs selective cell prioritization using the CLP field in the ATM cell header.

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