Appendix A:
An ATM Primer

Asynchronous Transfer Mode (ATM) is a type of digital packet-switching technology designed to relay and route network traffic by means of an address contained within the packet. ATM uses very short, fixed-length packets referred to as cells.

ATM cells have a fixed length of 53 bytes. By using fixed-length cells, the information can be transported in a predictable manner. This predictability accommodates different traffic types on the same network—for example, voice, data, and video.

The ATM cell is broken into two main sections, the header and the payload. The header (5 bytes) is the addressing mechanism and is significant for networking purposes as it defines how the cell is to be delivered. The payload (48 bytes) is the portion that carries the actual information—either voice, data, or video. (The payload is also referred to as the user information field.) An ATM cell is shown below.