31 - Principles of switching systems

Abstract

The simplest form of telephone system consists of two terminations and a metallic connection between them. Within an exchange, more than one connection will be required at any one time but not all terminations will bewanting service at the same time. Practical systems are designed to provide a number of links that is less than the number of terminations determined on a statistical basis. A network must consist of terminations provided at the customers' premises, a transmission medium to connect them to the local exchange, and a means of signaling between the termination and the exchange. Each local public exchange provides the means for setting up calls between its own customers' lines. It is normally also connected to other local exchanges in the same area by junctions. For calls outside this area, junctions are provided to trunk exchanges that are in turn interconnected by trunk lines, enabling calls to be established to customers in other areas. Trunk lines are also provided to one or more international exchanges that enable calls to be established to customers in the national networks of other countries. In large areas, some local exchanges may be interconnected only by having calls switched through another local exchange, a technique known as tandem switching, which can be independent of the trunk switching hierarchy used for calls outside the area.