to a digital communications facility, converting between incompatible digital formats. DSUs can be considered modem replacements in digital networks. DTE (data terminal equipment) Equipment (including any type of computer terminal, such as a PC, as well as printers, hosts, front-end processors, multiplexers, and LAN intercon- nection devices such as routers) that transmits data between two points without error. Its main responsibilities are to transmit and receive information and to perform error control. The DTE generally supports the end-user applications program, data files, and databases. DTH (direct to home) A satellite system that can transmit DTV signals directly to individ- ual homes. DTMF (dual-tone multifrequency) signaling The basis for operation of pushbutton telephone sets. DTMF is a method of signaling in which a matrix combination of two frequen- cies, each from a group of four, is used to transmit numerical address information. The two groups of four frequencies are (a) 697Hz, 770Hz, 852Hz, and 941Hz, and (b) 1,209Hz, 1,336Hz, 1,477Hz, and 1,633Hz. (digital terrestrial television) An implementation of digital technology to provide greater number of channels and/or better quality of picture (via HDTV) and sound through a conventional antenna instead of a satellite dish or cable connection. DTV (digital TV) Television sent over a digital network. DTV is nearly immune to interfer- ence and degradation, and it can display a much better range of colors than can analog television. Dual Stack A technique for enabling IPv4-to-IPv6 interworking and coexistence that allows IPv4 and IPv6 to exist in the same host. duplex duplex Communications in which data can be transmitted between two stations in both directions at the same time, with the use of a four-wire circuit. It is the same as full-duplex. duplex circuit A four-wire circuit used for transmission in both directions at the same time. It can be called full-duplex to distinguish it from half-duplex. duplex signaling A signaling system that occupies the 