method of error correction in transmitted data, callers to leave messages for other users of the system. Also used with some types of modems. known as phone mail. Truncation. Usually refers to the effect where the signal is filtered Voice print. Voice recognition technique where the voice print of to such an extent that too much of the wanted signal is also a caller is taken during an enrolment session and then used to removed, making the signal unintelligible. authenticate the caller's identity. Turnaround time. The time, in a half duplex transmission system, Volatile store. A memory device whose content is lost if the power to reverse the direction of transmission (e.g. from receive to supply is disconnected. transmit of a signal.) Voltage coefficient. The rate of change of a parameter with volt- Trunk line. Long distance transmission lines which connect age. For example the voltage coefficient of resistance is the together several primary trunk switching centres. (Also called change of resistance value with temperature. group switching centres in the UK and a Class 4 office in the USA.) Twisted pair. Refers to a cable consisting of a pair of wires which are twisted around each other, so as to minimise pick up of unwanted signals from other circuits. Wait on busy. A telephone facility in which a call is held when the Two wire circuit. A circuit consisting of a single pair of wires and called party is busy, and is then put through when the number is free. able to carry simultaneous traffic in each direction. Two way trunk. A trunk circuit which can be seized at either end. White noise. Type of interfering noise caused by movement of electrons. Wideband. Usually refers to a band of frequencies greater than Unattended mode. Any equipment (e.g. answerphone) designed voice. Transmission speeds often exceed 2Mbit/s. to operate without manual intervention. Window. A flow control system used in packet switching systems. Unbalanced to ground. A two wire transmission line in which the The size of the window indicates the