Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by the American National Standards Institute and 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 provides functionality that uses features of circuit switching and packet switching networks by using asynchronous time-division multiplexing. In the 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) 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. 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of the public switched telephone network and in the Integrated Services Digital Network (ISDN) but has largely been superseded in favor of next-generation networks based on IP technology.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 analogue 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 (SDH) 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