Asynchronous Transfer Mode Networks

Asynchronous transfer mode, or ATM, is a network transfer technique capable of supporting a wide variety of multimedia applications with diverse service and performance requirements. It supports traffic bandwidths ranging from a few kilobits per second (e.g., a text terminal) to several hundred megabits per second (e.g., high-definition video) and traffic types ranging from continuous, fixed-rate traffic (e.g., traditional telephony and file transfer) to highly bursty traffic (e.g., interactive data and video).

Because of its support for such a wide range of traffic, ATM was designated by the telecommunication standardization sector of the International Telecommunications Union (ITU-T, formerly CCITT) as the multiplexing and switching technique for Broadband, or high-speed, ISDN (B-ISDN) (1).

ATM is a form of packet-switching technology. That is, ATM networks transmit their information in small, fixed- length packets called cells, each of which contains 48 octets (or bytes) of data and 5 octets of header information. The small, fixed cell size was chosen to facilitate the rapid processing of packets in hardware and to minimize the amount of time required to fill a single packet. This is particularly important for real-time applications such as voice and video that require short packetization delays.

ATM is also connection-oriented. In other words, a virtual circuit must be established before a call can take place, where a call is defined as the transfer of information between two or more endpoints. The establishment of a virtual circuit entails the initiation of a signaling process, during which a route is selected according to the call’s quality of service requirements, connection identifiers at each switch on the route are established, and network resources such as bandwidth and buffer space may be reserved for the connection.

Another important characteristic of ATM is that its network functions are typically implemented in hardware. With the introduction of high-speed fiber optic transmission lines, the communication bottleneck has shifted from the communication links to the processing at switching nodes and at terminal equipment. Hardware implementation is necessary to overcome this bottleneck because it minimizes the cell-processing overhead, thereby allowing the network to match link rates on the order of gigabits per second.

Finally, as its name indicates, ATM is asynchronous. Time is slotted into cell-sized intervals, and slots are assigned to calls in an asynchronous, demand-based manner. Because slots are allocated to calls on demand, ATM can easily accommodate traffic whose bit rate fluctuates over time. Moreover, in ATM, no bandwidth is consumed unless information is actually transmitted. ATM also gains bandwidth efficiency by being able to multiplex bursty traffic sources statistically. Because bursty traffic does not require continuous allocation of the bandwidth at its peak rate, statistical multiplexing allows a large number of bursty sources to share the network’s bandwidth.

Since its birth in the mid-1980s, ATM has been fortified by a number of robust standards and realized by a significant number of network equipment manufacturers. International standards-making bodies such as the ITU and independent consortia like the ATM Forum have developed a significant body of standards and implementation agreements for ATM (1,4). As networks and network services continue to evolve toward greater speeds and diversities, ATM will undoubtedly continue to proliferate.