Introduction of p.c.m.-junction links and switching into a dispersed sectionalised switching system

The advantages are examined of a sectionalised main-/dependent-exchange complex for a local-exchange area, together with the particular suitability of a p.c.m. switching section in the main exchange. The dependent exchanges, which may be linked to the main exchange by audio or p.c.m. junctions, use a spatial system of concentration with reed-relay crosspoints, and operate as remote reed-electronic exchanges (REX) controlled by the main exchange. The dependent exchange therefore has all the advantages and facilities normally conferred on a REX type of main exchange. The progressive introduction of p.c.m. switching into the main exchange can be accommodated as p.c.m.-linked dependents are added; all share the main exchange control. Ultimately, a complete turnover to p.c.m. may be achieved at the central exchange. The general principles of p.c.m. switching are discussed, and the desirability of frame alignment is explained. A method of channel-slot changing is proposed, appropriate to small junction groups, and a variable-delay slot changer is described. The problems of time-slot alignment and area synchronisation are considered. A detailed description of the p.c.m. main-/dependent-exchange complex is given, with particular regard to various types of connection. The REX main/dependent arrangement is immediately suitable for exploitation, initially using audio junctions and, later, incorporating p.c.m. for junctions and switching at the main exchange. Codec equipment will be used at the interface between the audio and p.c.m. portions of the main exchange, and to replace links and junction relay sets at the dependent.