Synchronisation of a p.c.m. network using digital techniques

In a network of interconnected digital p.c.m. telephone exchanges, all the signals should be virtually in synchronism to facilitate interconnections between channels on different routes. However, to maintain the security of the system, it is also desirable for each station to have an independent master oscillator. There are several possible methods for effectively synchronising the signals in such a system. Apart from the effect of frequency differences, synchronism is also affected by changes in the propagation delays of the interconnecting junctions, caused by temperature variations. A system is described in which the phase of an incoming signal is allowed to drift in relation to the signal generated locally, until a given limit is reached. The limit may conveniently be related to the amount of storage provided for temperature compensation of the junction delay. When the limit is reached, a digit is either added to, or omitted from, one of the signals during a part of the signal not carrying intelligence, such as a frame-synchronising period, so as to reduce the degree of asynchronism. This system adds some complexity to both the exchange master timing circuits and the received-signal timing circuits, and also complicates the switching of calls through the exchange. The time relationships between the various signals in the network can be deduced mathematically, allowing the behaviour of a network with a given set of initial conditions to be computed. Paralysis of the system can be avoided by a proper choice of the amount of storage.