re both ends of a trunk are seized at the same ing exchange. time. Glossary of terms 62/25 Gold plating. Adding of non-essential features. Usually occurs in Hertz. A measure of frequency. One Hertz is equal to one cycle per a cost plus situation where the supplier adds 'bells and whistles' second. in order to increase the price. Holding time. The time for which a circuit is held in a busy state. Graded index fibre. An optical fibre in which the refractive index Hot standby. Situation where identical equipment is kept in a varies gradually throughout the cladding and core, and there is ready state, but is not used, so that it can switch over into active no sudden change at the core-cladding interface. (See also step service if the main equipment fails. index fibre.) Hub polling. A polling technique in which the poll, or permission Grade of service. Measure of the efficiency of a telephone sys- to transmit data, passes from one station to the next rather than tem. Defined as the proportion of call attempts made during the between a central station and other stations. See also roll call busy hour (or specified overload conditions) which fail. polling. Ground. Usually an electrical connection which eventually con- Hub site. Site, usually in a distributed network such as CATV, nects to the earth. Often loosely used to define a common point where processing occurs prior to distribution. in a circuit. Hub station. Usually refers to the central sation on Earth which is Ground start. A method used in telephony signalling where the used to control a group of VSATs. receiver detects the grounding of a circuit at the sender's end. Hybrid. In telephony it usually refers to the ciruit which interfaces Ground station. Refers to the equipment located on the Earth and between a two wire and a four wire transmission circuit. directly associated with a satellite system. Hybrid. A mixture of two technologies or services. For example a Group delay distortion. See delay distortion. hybrid transmission system may consist of private cir