ed transmission from a satellite system which draining its battery. covers a relatively small area on the Earth's surface, enabling Tandem exchange. US name for the exchange which connects frequency re-use between beams. together several local exchanges. Spread spectrum. Signal coding technique which spreads the sig- Telco. Although the term strictly stands for a telephone central nal energy over a wide bandwidth, so making it hard to detect office, it is more commonly used to refer to a telephone com- or jam. pany. Standby time. Time for which a mobile can be on standby without Telecommunications watchdog. Regulatory body set up within a draining its battery. country, usually after its telecommunications operations have Standstill agreement. Agreement amongst ETSI members not to been opened to competition. develop conflicting standards, for a defined period, once ETSI Teleconferencing. Linking together groups of people at different has started work on a standard. geographical locations, with audio and video. Start stop transmission. Asynchronous transmission sytem which Telemetry. Control and signalling information which is usually uses a bit at the beginning of the message (start) and a second communicated over the transmission line. bit at the end (stop) to indicate completion of the message. Teletex. A data transmission standard, typically operating at Statistical multiplexer. A multiplexer, generally operating with 1200bit/s, which is an upgrade on the slower telex standard. asynchronous data, which enables several channels to utilise Teletext. Broadcast of graphics or text for reception by low cost the same transmission link. Equipment connected to the link terminals, such as television sets. transmit data only when they have information to send. Telex. Switched message service, for text transmission, using Step index fibre. An optical fibre construction which has a uni- Baudot-coded data. form refractive index throughout the core and cladding, but a Temperature coefficient. The rate of change of a paramete