on Convergence, which converts the ATM cell stream into bits to be transported over the physical medium. ATM reassembly Restructuring of data units from information contained in cells. ATM segmentation Parsing of the information units of the higher layers into ATM cells. ATSC (Advanced Television Systems Committee) An organization that establishes vol- untary technical standards for advanced television systems, including DTV. attenuation A decrease in the power of a received signal due to loss through lines, equip- ment, or other transmission devices. It is usually measured in decibels. audible ringing tone A tone received by the calling telephone indicating that the called telephone is being rung (formerly called ringback tone). audio frequencies Frequencies that correspond to those that can be heard by the human ear (usually 30Hz to 20,000Hz). (attachment unit interface) The connector used to attach a device to an Ethernet transceiver. authentication A technique that enables the receiver to automatically identify and reject messages that have been altered deliberately or by channel errors. Authentication also can be used to provide positive identification of the sender of a message. autonomous system A collection of TCP/IP gateways and networks that fall under one administrative entity and cooperate closely to propagate network reachability (and routing) information among themselves, using an interior gateway protocol. Sometimes abbreviated AS. B8ZS (bipolar 8-zero substitution) An encoding method used on T-1 circuits (in the United States and Japan) that inserts two successive ones of the same voltage into a signal when- ever eight consecutive zeros are transmitted. B8ZS is not compatible with older AMI equipment. In Europe, E-1 uses the encoding scheme HDB3 instead of B8ZS. Also called binary 8-zero sub- stitution, clear channel, and clear 64. B (bidirectional) frame The part of the MPEG video compression process in which both past and future pictures/frames are used as references. B frames typically result i