 used to select one signal or channel from amongst many signals or channels. twisted pair Two insulated copper wires twisted at regular intervals and normally covered by a protective outer sheath composed of PVC. U-Interface The U-Interface carries ISDN formatted signals over a single pair of wires between a subscriber's location and a telephone company's central office. User datagram protocol; an unreliable connectionless transport protocol that sends un-sequenced packets across a packet-switched network. UDP is defined in RFC 768. Ultra High Frequency. The spectrum band that occupies frequencies from 470 to 890 MHz, within which television channels 14 through 83, are transmitted. CATV systems cannot transport these frequencies SO they are converted for the purposes of cable transmission. Glossary of Terms User Location Service. A white-pages directory that allows users of Internet videoconferencing and personal conferencing tools to refresh IP addresses dynamically and to determine who is logged on and available to have a conference. unicast Application of conferencing, usually over packet- switched networks, where only one user or site receives data. Contrast this with multicast in which data is received by more than one user or site. up-link The portion of a communications link used for the transmission of signals from a satellite earth station to satellite transponders in space. upstream transmission The ability for a CATV subscriber to communicate interactively with the head-end or a hub-site. Universal Serial Bus is a hardware interface for peripherals such as keyboards, mice, scanners, and telephony devices. USB supports MPEG-1 and MPEG- 2 digital video. USB allows for 1-127 attached devices, and allows devices to use its full 12 Mbps bandwidth or a 1.5 Mbps subchannel (it also allows for hot- swapping). Low-power devices that might normally require a separate AC adapter can be powered through the cable because the USB bus distributes 0.5 amps of power through each port. Unshielded Twisted Pair. An ITU-T 