 the wanted signal. system which incorporates a vestige of the deleted sideband Transmobile. A mobile telephone which can be removed from the along with an amount of the carrier energy. vehicle in which it is normally operated, and carried as a hand portable. Video text. Usually refers to an interactive communication appli- cation in which a user can communicate with a remote database Transmultiplexer Equipment used for converting between signals obtained from frequency division multiplexers and time divi- using a low cost video terminal. sion multiplexers. Also known as transmux. Virtual circuit. Network operation which gives the user the im- Transponder. Satellite based equipment which receives an up-link pression of having an end to end connection although, as in a signal at a given frequency, amplifies it and translated it into packet switched service, this is not the case. another frequency, and then retransmits it as a down-link signal. Virtual private circuit. The facility provided by a public telephone Transport layer. The layer in the ISO OSI model which is respon- operator of using a public line as a private circuit for a private sible for the optimum use of network resources. company. Transverse parity check. A form of parity checking on groups of Visible arc. The arc of the geostationary orbit of a satellite during bits within a transmitted frame. which the satellite can be seen from its earth station. Transversal equiliser. Equaliser used to remove intersymbol inter- Vocoder. A component which digitises voice in terms of its ampli- ference. tude, pitch, and voicing. Treaty of Rome. The treaty, adopted in March 1957, which created Voice grade line. A transmission line designed to handle low the European Community. speed information (300Hz to 3400Hz) as required for voice Tree. A network topology, often used in CATV, where there is applications. This is also known as a voice circuit. only one route between any two nodes. Voice mail. Voice processing application which allows telephone Trellis coding. A 