ATM, which stands for asynchronous transfer mode, is a digital network transport technology based on a 53-byte cell. In an ATM network, all voice, data and video is digitized and assembled into cells that have a 5-byte header and 48-byte payload. ATM is more efficient than circuit-switched networks, in which separate channels are generally reserved for each voice, data and video "conversation," because packets of all types can be intermixed in the same virtual "pipe." Addressing and other information in the packet header ensures each packet goes to its proper destination and in the correct sequence. This allows for a more efficient use of network bandwidth.

Also see frame relay
ATM Net Resources
Links to more detailed info and articles.