Asynchronous Transfer Mode

Short for Asynchronous Transfer Mode, a network technology based on transferring data in cells or packets of a fixed size. The cell used with ATM is relatively small compared to units used with older technologies. The small, constant cell size allows ATM equipment to transmit video, audio, and computer data over the same network, and assure that no single type of data hogs the line.

Some people think that ATM holds the answer to the Internet bandwidth problem, but others are skeptical. ATM creates a fixed channel, or route, between two points whenever data transfer begins. This differs from TCP/IP, in which messages are divided into packets and each packet can take a different route from source to destination. This difference makes it easier to track and bill data usage across an ATM network, but it makes it less adaptable to sudden surges in network traffic.

When purchasing ATM service, you generally have a choice of four different types of service:

# constant bit rate (CBR):

specifies a fixed bit rate so that data is sent in a steady stream. This is analogous to a leased line.

# variable bit rate (VBR):

provides a specified throughput capacity but data is not sent evenly. This is a popular choice for voice and videoconferencing data.

# available bit rate (ABR):

provides a guaranteed minimum capacity but allows data to be bursted at higher capacities when the network is free.

# unspecified bit rate (UBR):

does not guarantee any throughput levels. This is used for applications, such as file transfer, that can tolerate delays.