Developments in long-distance telephone switching

Abstract

This paper gives an account of automatic methods for handling long-distance telephone connections. It is now possible for an operator to dial any subscriber in a national network without the help of any other operator.One of the most important factors in attaining this is the design of the signalling equipment which is provided on the long-distance lines. The paper describes the signalling methods which are now available, indicates the problems which arise, and analyses the various solutions. The need for uniformity of practice with regard to the frequencies used and the signals transmitted is emphasized, and the conclusion is reached that a 2-frequency system, preferably using frequencies of 600 and 750 cycles per sec., is probably most suitable for general adoption. It is claimed that future developments will be hampered to the minimum degree by such a system.Recent developments in long-distance switchboards are described.Automatic long-distance tandem exchanges are described. These exchanges replace the through positions on long-distance switchboards and enable a distant operator to build up indirect connections over a number of long-distance lines by dialling or key-sending. These exchanges also permit a local-area operator to control long-distance connections and thus enable outgoing long-distance operators to be dispensed with.A brief summary of existing operating methods is given and the effect of the new developments upon these is considered. An improvement of the order of 12 per cent in the earning power of long-distance lines is obtained. Substantial savings in operating costs are also realized owing to the elimination of incoming positions and an increase in the traffic-carrying capacity of outward positions. The line time required to obtain ringing tone or busy tone is around 14 seconds on direct connections as compared with 40 seconds by manual methods. On indirect connections the time saved is even greater. As a result, the originating operator can, if necessary, make several attempts to obtain a subscriber in less time than she previously took for one attempt.The problems which arise in introducing automatic switching methods into existing long-distance networks are considered and resultant changes in routing methods, cable lay-outs, and transmission lay-outs, for long-distance lines are described.Subscriber-to-subscriber dialling over long-distance lines with time and zone metering is already in operation, in several countries. Methods for realizing this and the precautions to be taken with a view to its ultimate introduction in less fully developed areas, are described.