ng also covering the group of shielded copper pairs. This type of cabling offers improved protection from interference from external sources. SCTP (Stream Control Transmission Protocol) A transport protocol that supports mul- tistreaming and multihoming. SDH (Synchronous Digital Hierarchy) An ITU-T standard for digital broadband commu- nications. SDLC (Synchronous Data Link Control) An IBM data link-layer protocol associated with SNA. In contrast to BSC, SDLC provides for full-duplex transmission and is more efficient. (Session Description Protocol) A protocol used to describe and encode the capabili- ties of multimedia session participants. This description is then used to negotiate the characteris- tics of the session so that all the devices can participate. (software-defined radio) A forthcoming application of multiradio technology that is a collection of hardware and software technologies that enable reconfigurable system architec- tures for wireless networks and user terminals. An SDR device can become a cellular phone, a GPS receiver, an amateur packet radio, or any other sort of radio transmitting or receiving device. SDSL (Symmetrical [or Single-Line] DSL) A symmetrical service that has a maximum loop length of 18,000 feet (5.5 km) and is deployed as a single twisted-pair cable. It can be deployed in various capacities, in multiples of 64Kbps, up to a maximum of 2Mbps in each direction. SDTV (standard-definition television) SDTV (standard-definition television) A DTV format that provides a picture quality sim- ilar to that of DVD. SECAM (Systeme Electronique Couleur Avec Memoire [Sequential Color with Memory]) A TV system used in France and the former French colonies, as well as in parts of the Middle East. Russia and the former Soviet-allied countries use a modified form of SECAM. There are two versions of SECAM: SECAM vertical and SECAM horizontal. See also PAL and NTSC. secondary channel A low-speed channel established on a four-wire circuit over which diag- nostics or control information is passed. Us