Publisher Summary This chapter provides an introduction to asynchronous transfer mode (ATM), which is a high-speed switching network architecture created in the late 1980s/early 1990s. ATM was designed expressly to combine the delivery of a wide range of services over a single network. ATM is a sophisticated, multi-speed network environment that provides a variety of complex network services for applications requiring various types of network solutions. It can be used to carry data, voice and video, separately or simultaneously over the same network path, and is one of the most complex communications technologies available today for public or private network infrastructures. ATM can be used in various local area networks (LANs), metropolitan area networks (MANs), and wide area networks (WANs), all at the same time if needed. Using terminology developed in previous chapters, ATM might also be considered a “hyphenated” protocol — it is connection-oriented, full-duplex, point-to-point, and cell-switched. ATM uses fixed-length cells, which contain exactly 53 bytes, 48 bytes for user data, and 5 bytes for overhead. As a result, ATM is sometimes referred to as “cell relay,” which dates back to the late 1960s. The concept of cell relay is predicated on time domain multiplexing and packet switching.