r with sudden change at the core-cladding interface. (See also graded temperature. For example the temperature coefficient of resist- index fibre.) ance is the change of resistance value with temperature. Steradian. A measure of solid angle. It is defined as the solid angle Terminated line. A transmission line having a far end resistance formed at the centre of a sphere by an area, equal to the square equal to the characteristic of the line SO that there are no signal of the radius, on the surface of the sphere. reflections. Stop bit. The last bit used in an asynchronous transmission to Ternary coding. A coding system which uses a three level code to indicate completion of the message. send more than one bit of information in a single symbol. Subrate. Usually refers to transmission rates below 64kbit/s. Thermal resistance. A measure of the resistance of a component to Supervisory signal. Signal which is used to control equipment on the flow of heat. One unit of thermal resistance is that which a line or to control and monitor the state of the line. causes a temperature difference of 1°C when 1W of heat flows Switched line. Usually refers to a public switched telephone line, through it. in which the path taken can vary for each use. Throughput. The maximum rate of transmitted data, which is re- Switching array. Usually refers to a matrix of switching elements ceived with no errors. forming part of an exchange switch. Tie line. A communication line connecting two or more sites, for Switchook. The name given to the hook or button on which a example through private branch exchanges or multiplexers, and telephone set rests when not in use. dedicated to their use. Synchronisation bits. The bits in a data transmission which are Time congestion. A measure of the proportion of time that all the used for element or network synchronisation. facilities of a system are busy, or congested. Synchronous. Usually refers to the transmission method in which Time expansion. The process of using more time slots within an data at the 