Asynchronous Transfer Mode (ATM) is, according to the ATM Forum, "a telecommunications concept defined by ANSI and ITU (formerly CCITT) standards for carriage of a complete range of user traffic, including voice, data, and video signals," and is designed to unify telecommunication and computer networks. The reference model for ATM approximately maps to the three lowest layers of the ISO-OSI reference model: data link layer, network layer, and physical layer. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that use variable sized packets or frames.
Back in the 1990’s ATM was the premier way to carry data traffic, the major advantage was the ability have a QoS in order to carry voice and video. Due to the overhead ATM service was not practical on a T-1 level (nearly one quarter of the circuit). ATM is still offered by a few carriers, most have phased out the service and replaced it with MPLS (Multi-Protocol Label Switching)