An Introduction to ATM (Asynchronous Transfer Mode) Networks

The standards for ATM were first developed in the mid 1980s. The goal was to design a single networking strategy that could transport real-time video and audio as well as image files, text and email.

ATM (Asynchronous Transfer Mode) has been proposed as an enabling network technology to support broadband integrated services. It was designed to provide a single platform for the transmission of voice, video and data at specified quality of service and at speeds varing from fractional T1 to Gbps.

Currently voice, data and video are transported by different networks. Voice is tranported by the public telephone network, and data by a variety of packet-switched networks. Video is transported by networks based on coaxial cables, satellites and radio waves. ATM is designed to integrate all these services together.

However, ATM itself is not a complete, stand-alone networking standard; rather, ATM defines a common layer of interoperability called the ATM layer, on which various services ranging from telephony and video conferencing to TCP/IP data networking and multimedia can be delivered.

The ATM layer defines a common format used for switching and multiplexing bit streams from one end of an ATM network to another. The ATM layer then uses the hardware facilities of lower layers to deliver the bits across individual links in a network.

A variety of such physical layers have been defined, most of which are based on existing standards in order to maximally leverage existing technologies and installed bases.

Technically, ATM is a cell relay, packet switching network and data link layer protocol which encodes data traffic into small fixed-sized cells (53 bytes; 48 bytes of data and 5 bytes of header information).

ATM provides data link layer services that run over Layer 1 links. This differs from other technologies based on packet-switched networks(such as IP or Ethernet), in which variable sized packets (known as frames when referencing Layer 2) are used.

ATM is a connection-oriented technology, in which a logical connection is established between the two endpoints before the actual data exchange begins.

ATM has proven very successful in the WAN scenario and numerous telecommunication providers have implemented ATM in their wide-area network cores. Many ADSL implementations also use ATM. However, ATM has failed to gain wide use an a LAN technology, and its complexity has held back its full deployment as the single integrating network technology in the way that its inventors original intended.

Why? The reason is that there will always be both brand-new and obsolescent link-layer technologies, particularly in the LAN area, not all of them fit neatly into the synchronous optical networking model for which ATM was designed.

Therefore, a protocol is needed to provide a unifying layer over both ATM and non-ATM link layers, as ATM itself cannot fit that role. IP already does that; therefore, there is often no point in implementing ATM at the network layer.

In the early 1990s, ATM was once posed to replace Ethernet and IP networks. But Ethernet made a dramatic come-back when it was defined to run at 100Mbps and later on at 1Gbps. As a result, ATM lost the battle to the "desktop".