Asynchronous transfer mode in UMTS

It is a telecommunications concept defined by ANSI and ITU standards for carriage of a complete range of user traffic including voice , data and video signals.

ATM was developed to meet the needs of the Broadband Integrated Services Digital Network, as defined in the late 1980 s, and designed to unify telecommunication and computer networks. It 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. The reference model for ATM approximately maps to the three lowest layers of the ISO-OSI reference model: network layer, data link layer and physical layer.  ATM is a core 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 IP.

ATM provides functionality that is similar to both circuit switching and packet switching networks: ATM uses asynchronous 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. ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins. These virtual circuits may be “permanent”, i.e. dedicated connections that are usually pre configured by the service provider, or “switched”, i.e. set up on a per-call basis using signalling and disconnected when the call is terminated.