Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by the American National Standards Institute and International Telecommunication Union Telecommunication Standardization Sector (ITU-T, formerly CCITT) for digital transmission of multiple types of traffic. 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 telephony (voice) and video. ATM is a cell switching technology, providing functionality that combines features of circuit switching and packet switching networks by using asynchronous time-division multiplexing. ATM was seen in the 1990s as a competitor to Ethernet and networks carrying IP traffic, as it was faster and, unlike Ethernet, designed with quality-of-service in mind, but it fell out of favor once Ethernet reached speeds of 1 gigabit per second. In the Open Systems Interconnection (OSI) reference model data link layer (layer 2), the basic transfer units are called frames. In ATM, these frames are of a fixed length (53 octets) called cells. This differs from approaches such as Internet Protocol (IP) (OSI layer 3) or Ethernet (also layer 2) 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. These virtual circuits may be either permanent (dedicated connections that are usually preconfigured by the service provider), or switched (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. ATM is a core protocol used in the synchronous optical networking and synchronous digital hierarchy (SONET/SDH) backbone of the public switched telephone network and in the Integrated Services Digital Network (ISDN) but has largely been superseded in favor of next-generation networks based on IP technology. Wireless and mobile ATM never established a significant foothold.
Protocol architecture To minimize queuing delay and packet delay variation (PDV), all ATM cells are the same small size. Reduction of PDV is particularly important when carrying voice traffic, because the conversion of digitized voice into an analog audio signal is an inherently real-time process. The decoder needs an evenly spaced stream of data items. At the time of the design of ATM, 155 Mbit/s synchronous digital hierarchy with 135 Mbit/s payload was considered a fast optical network link, and many plesiochronous digital hierarchy links in the digital network were considerably slower, ranging from 1.544 to 45 Mbit/s in the US, and 2 to 34 Mbit/s in Europe. At 155 Mbit/s, a typical full-length 1,500 byte Ethernet frame would take 77.42 μs to transmit. On a lower-speed 1.544 Mbit/s T1 line, the same packet would take up to 7.8 milliseconds. A queuing delay induced by several such data packets might exceed the figure of 7.8 ms several times over. This was considered unacceptable for speech traffic. The design of ATM aimed for a low-jitter network interface. Cells were introduced to provide short queuing delays while continuing to support datagram traffic. ATM broke up all data packets and voice streams into 48-byte pieces, adding a 5-byte routing header to each one so that they could be reassembled later. Being 1/30th the size reduced cell contention jitter by the same factor of 30. The choice of 48 bytes was political rather than technical. When the CCITT (now ITU-T) was standardizing ATM, parties from the United States wanted a 64-byte payload because this was felt to be a good compromise between larger payloads optimized for data transmission and shorter payloads optimized for real-time applications like voice. Parties from Europe wanted 32-byte payloads because the small size (4 ms of voice data) would avoid the need for echo cancellation on domestic voice calls. The United States, due to its larger size, already had echo cancellers widely deployed. Most of the European parties eventually came around to the arguments made by the Americans, but France and a few others held out for a shorter cell length. 48 bytes was chosen as a compromise, despite having all the disadvantages of both proposals and the additional inconvenience of not being a power of two in size. 5-byte headers were chosen because it was thought that 10% of the payload was the maximum price to pay for routing information.
Cell structure An ATM cell consists of a 5-byte header and a 48-byte payload. ATM defines two different cell formats: user–network interface (UNI) and network–network interface (NNI). Most ATM links use UNI cell format.
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