ingly important with the growth of the Internet because many users do not have fast enough access to download large multimedia files quickly. With streaming media, the client browser or plug-in can start displaying the data before the entire file has been transmitted. STS (Synchronous Transport Signal) The basic building block of SONET. The basic build- ing block signaling rate for a SONET transmission medium is STS-1, which is 51.8Mbps. The signal is composed of 8,000fps, with 810 8-bit bytes per frame (8 bits/byte X 810 bytes/frame X 8,000fps = 51.840Mbps). A SONET STS-1 frame contains more than 30 times the data of a T-1 frame. Other STS rates are STS-3, STS-12, STS-48, STS-192, and STS-768. subnet mask A configuration feature of a router that is used to select the portion of a 32-bit TCP/IP node address that refers to the LAN and to select the portion that refers to the node on the LAN. subnetting A technique used to impose a hierarchy on IP addresses that supports a form of area routing. subrate Transmission speeds below DS-0/64Kbps. subsplit To split the frequencies in a cable so that the lower frequencies are used for one pur- pose and the higher for another. Subsplitting is used when two cables are not used but there is a need to simulate a dual-cable system. subsystem A part of a system that performs defined functions. subvoice grade channel A channel with bandwidth narrower than that of voice-grade channels. Such channels are usually subchannels of a voice-grade line. superphone A feature-rich new-generation mobile phone that offers applications such as music, graphics, games, and videos. supervision The process of detecting a change of state between idle and busy conditions on a circuit. supervisory signal A signal that handles the on-hook/off-hook condition. SVC (switched virtual circuit) A virtual connection that is set up on a call-by-call basis. switched digital access A dialup option in which facilities are allocated based on demand rather than being associated with a specific customer all the tim