Improved digital telephone and switching system employing time division

Abstract

A telephone system is disclosed in which local groups of customers each have a respective concentrator. The concentrators are connected to two main switching centers by pulse code modulation highways. Four highways from each of eight concentrators are shown and each highway from a concentrator is terminated on a different group control, there are two group controls in each main switching center. Each concentrator contains a line unit, a switching network, a signalling unit, auxiliary units, a pulse code modulation multiplexor and control logic. The line unit detects the customer calling condition; the switching network is a two stage analogue crosspoint switch connected from customers' lines to the signalling unit; the signalling unit provides an analogue/digital interface; the auxiliary units provide special facilities such as coin fee checking; the pulse code modulation multiplexor passes data to and from the main switching centers; and the control logic controls the operation of the concentrator in accordance with instructions received from a main switching center. The main switching centers instruct the concentrators on the allocation of pcm channels to customers and the interconnection of two customers is performed by an interconnection of channels within a switching center. The switching network of each concentrator is made up of a number of sections and information is sent on each highway as to which sections are being addressed on the other highways of the concentrator. The system includes an arrangement to avoid setting up the same connection at once on two highways.