Protection of Service in the TD-2 Radio Relay System by Automatic Channel Switching

When communication systems fail, provision must be made to restore service as rapidly as possible. In systems providing only a few telephone circuits, restoral may be by manual rearrangement of the circuits affected so that they are made good on stand-by or alternate facilities. But in the case of systems carrying a large number of telephone channels or network television, the impact of a system failure is so great t h a t automatic arrangements must be provided to restore service much faster than is possible by manual operations. This need led to the development of the automatic protection switching system for the TD-2 radio relay system, 1 which has in recent years come to provide a large portion of the Bell System's long-haul telephone and television facilities. The TD-2 system provides six two-way radio channels, any one of which will carry several hundred telephone circuits or one television signal. It is used generally as a long-haul backbone route system, principally between major centers of population. Repeater stations are provided at intervals of about thirty miles, depending 011 considerations of terrain, and the number of repeaters between the terminal points is a function of course of their geographical separation. Frequency modulation is used, and the repeaters do not reduce the signal to its original amplitude modulation form, so that the base-band is not normally available at repeaters intermediate to the terminal points. 473