Overview#Asynchronous Transfer Mode (ATM) was designed for a network that must handle both traditional high-throughput data traffic (e.g., file transfers), and real-time, low-latency content such as voice and video.
Asynchronous Transfer Mode approximately maps to the three lowest layers of the OSI-Model:protocol used over the SONET/SDH backbone of the Public Switched Telephone Network (PSTN) and Integrated Services Digital Network (ISDN), but its use is declining in favour of all Internet Protocol Suite.
Asynchronous Transfer Mode uses Asynchronous Operation time-division multiplexing, and encodes data into small, fixed-sized packets (ISO-OSI frames) called cells. This differs from approaches such as the Internet Protocol or Ethernet that use variable sized packets and frames. Asynchronous Transfer Mode uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the actual Communication begins. These virtual circuits may be "permanent", i.e. dedicated connections that are usually preconfigured by the service provider, or "switched", i.e. set up on a per-call basis using signalling and disconnected when the call is terminated.