cuits over Groupe Special Mobile. CEPT group which developed the pan- some routes and public switched circuits over others, using a European digital cellular telephone standard. 123 channels, mixture of microwave for the private lines and copper for 200kHz separation in frequency band 890-915MHz for mobile public lines. to base and 935-960 for base to mobile. Guard band. A band of frequencies which is kept unused in order to separate the different communicating channels and SO pre- vent interference between adjacent channels. Idle character. Usually refers to a control character which is in- serted into the transmitted frame when there is no information available to be sent. Half duplex. Transmission system in which communications OC- Impulse hit. Voltage surges which cause errors in data trans- curs in both directions but in only one direction at a time. mission. Half echo suppressor. Echo suppressor in which speech signals on Impulse noise. Intermittent noise which consists of short duration one path control the suppression or loss in the other path, but pulses of high value. not vice versa. In-band signalling. Use of frequencies within the speech band Half life. The time to reach half the original value of a parameter. (300Hz to 3400HZ for signalling. For example the half life of a LED is the time for the luminance Index of refraction. It is a measure of a medium's optical capa- level to fall to half its original value. bilities. It is stated as the ratio of the velocity of light in vacuum Half transponder operation. Technique for the transmission of two to the velocity of light in the medium. television signals over a 36MHz transponder, using a lower Intercepted calls. Calls which are intercepted and sent to an alter- power and deviation. native destination, such as an answering machine, an operator, Hamming code. A code use for checking and correcting errors in or a pre-recorded message. transmitted data. Interconnection charge. Fee paid by one operator to another for Hamming distance. It is the number of bi