Reliability and maintenance of electronic exchanges

A review is made of the basic requirements relating to reliability of telephone exchanges. The terms `quality¿ and `continuity of service¿ are introduced as criteria for expressing the reliability quantitively. Adequate reliability must be obtained despite the existence of faulty components and the system must be designed to be fault tolerant. It is estimated that a component failure rate not exceeding 0.05% per annum is necessary. The system can be made fault tolerant by using a multiplicity of similar transmission paths and functional sections. This multiplicity can either be in the form of redundant equipment or may consist of several alternative paths and sections continuously in use. Examples of redundancy techniques to be used in the Highgate Wood exchange are given. A brief analysis of the relative merits of various methods of obtaining fault tolerance and their relation to the permissible repair time is made.