used for television. Traditional television has an aspect ratio of 4:3, and DTV has a 16:9 aspect ratio, which more closely resembles human vision. ASTN (Automatic Switched Transport Network) ANSI specification ITU G.807, which describes a network that allows traffic paths to be set up automatically through a switched net- work. ASTN allows the user to specify the start point, endpoint, and bandwidth required. asynchronous (1) Occurring without a regular or predictable time relationship to a specified event. (2) In data communications, a method of transmission in which the bits representing a character are preceded by a start bit and followed by a stop bit, which are used to separate the characters and to synchronize the receiving with the transmitting station. It does not use a regu- lar time relationship between the sending and receiving devices. asynchronous transmission A transmission in which each information character, or some- times each word or small block, is individually synchronized, usually by the use of start and stop elements. Also called start-stop transmission. ATM (Asynchronous Transfer Mode) An international packet-switching standard uses a cell-based approach in which each packet of information features a uniform cell size of 53 bytes. ATM is a high-bandwidth, fast packet-switching and multiplexing technique that allows the seamless end-to-end transmission of voice, data, image, and video traffic. It is a high-capacity, low-latency switching fabric that is adaptable for multiservice and multirate connections and offers an architected approach to QoS. Glossary ATM layer ATM layer A layer that performs four main functions: multiplexing and demultiplexing of cells of different connections, translation at ATM switches and cross-connects, cell header extrac- tion or addition before or after a cell is delivered to or from the adaptation layer, and flow control. ATM physical layer A layer that is composed of two sublayers: Physical Medium, which supports pure medium-dependent functions, and Transmissi