Asynchronous transfer mode (ATM) is a switching technique used by telecommunication networks that uses asynchronous time-division multiplexing to encode data into small, fixed-sized cells. This is different from Ethernet or internet, which use variable packet sizes for data or frames.

ATM is the core protocol used over the synchronous optical network (SONET) backbone of the integrated digital services network (ISDN).

The ATM provides data link layer services that run on the OSI's Layer 1 physical links. It functions much like small-packet switched and circuit-switched networks, which makes it ideal for real-rime, low-latency data such as VoIP and video, as well as for high-throughput data traffic like file transfers.

ATM services generally have four different bit rate choices:

Available Bit Rate: Provides a guaranteed minimum capacity but data can be bursted to higher capacities when network traffic is minimal.

Constant Bit Rate: Specifies a fixed bit rate so that data is sent in a steady stream. This is analogous to a leased line.

Unspecified Bit Rate: Doesn’t guarantee any throughput level and is used for applications such as file transfers that can tolerate delays.

Variable Bit Rate (VBR): Provides a specified throughput, but data is not sent evenly. This makes it a popular choice for voice and videoconferencing.

Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by ANSI and ITU (formerly CCITT) for digital transmission of multiple types of traffic, including telephony (voice), data, and video signals in one network without the use of separate overlay networks. ATM was developed to meet the needs of the Broadband Integrated Services Digital Network, as defined in the late 1980s, and designed to integrate telecommunication networks. It can handle both traditional high-throughput data traffic and real-time, low-latency content such as voice and video. ATM provides functionality that uses features of circuit switching and packet switching networks. It uses asynchronous time-division multiplexing.

In the OSI reference model data link layer (layer 2), the basic transfer units are generically called frames. In ATM these frames are of a fixed (53 octets or bytes) length and specifically called cells. This differs from approaches such as IP or Ethernet that use variable sized packets or frames. ATM uses a connection-oriented model in which a virtual circuit must be established between two endpoints before the data exchange begins.[5] These virtual circuits may be either 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 signaling and disconnected when the call is terminated.

The ATM network reference model approximately maps to the three lowest layers of the OSI model: physical layer, data link layer, and network layer.[6] ATM is a core protocol used in the SONET/SDH backbone of the public switched telephone network (PSTN) and in the Integrated Services Digital Network (ISDN), but has largely been superseded in favor of next-generation networks based on Internet Protocol (IP) technology, while wireless and mobile ATM never established a significant foothold.