and ones (binary coding). Binary digits (bits) can be either zeros or ones and are typically grouped into "words" of various lengths-8 bit words are called bytes. Digital Access Cross DACS. A switch/multiplexer that permits DSO cross Connection System connection from one T-1 transmission facility to another. Digital Signal Hierarchy A TDM multiplexed hierarchy used in telephone networks. DSO, the lowest level of the hierarchy, is a single 64 Kbps channel. DS-1 (1.544 Mbps) is 24 DSOs. DS-2 (6.312 Mbps) is four DS-1 signals multiplexed together; DS-3 (44.736 Mbps) is seven DS-2 signals multiplexed together. At the top of the hierarchy is DS-4, which is six DS-3 signals and which requires a transmission system capable of handling a Glossary of Terms 274.176 Mbps signal. Digital Signal Processor See DSP. digital transmission The conveyance over a network of digital signals by means of a channel or channels that may assume in time any one of a defined set of discrete values or states. digitizing The process of sampling an analog signal SO that it can be represented by a series of bits. digitizing tablets Graphics systems used in conjunction with videoconferencing applications. Using a special stylus or electronic "pen" a meeting participant can write on the tablet and the message can be viewed by the distant end and, if desirable, stored on a PC. Photos and text can also be annotated electronically. These devices are unsettling to use, however, because no image appears on the tablet, thus it is difficult to orient the letters. direct broadcast satellite The use of satellite to broadcast directly to homes or businesses. Subscribers are obliged to purchase and install a satellite dish. DBS service originated in Japan, which is composed of many islands and which has a harsh geography that includes mountains, rivers, valleys and ridges that made it very difficult to plan and execute a terrestrial broadcasting CATV system. directional couplers In cable systems, multiple feeder cables are coupled with these devices to matc