in the ISO OSI model responsible for Redundancy checking. Atechnique used for error control on trans- setting up and breaking links between communicating ele- mitted signals, in which additional data is appended to the ments. signal, which is subsequently discarded at the receiving termi- Shannon channel capacity theorem. Theorem which specifies the maximum transmission rate for data over a communication Redundant equipment. Equipment which is arranged such that if a channel, with very low probablity of loss of any information. failure occurs in a working part a standby part can take over the Shelf life. The time for which an equipment or component can be function. stored without any of its parameters degrading significantly. Reframe time. The time between the receipt of a frame alignment Shielding. Protecting interference to equipment from an external signal and the time when alignment occurs. field. Usually refers to protection against electromagnetic radi- Refresh rate. The rate at which data is refreshed, for example in a ation. video display, or a dynamic memory. Shortest routing. Routing which is performed between two Regional centre. Refers to a US Class 1 telephone office which points using the shortest path between them. Usually this is also connects together several sections of the country. linked to the lowest cost (called least cost routing). Repeater. A device used in a transmission link which receives a Shortest path spanning tree. An information tree of all the shortest signal, amplifies and retimes it, and then retransmits it along the paths from a transmitting node to all receiving nodes, or from line. Also called a regenerative repeater. a receiving node to all transmitting nodes. Used in determining Reservation. Usually refers to the process, within a multiple access shortest path routing. scheme, where a device wishing to transmit data first reserves Short haul. Transmission of signals over relatively short distances. part of the capacity of the transmission medium. Example is See also long ha