The full stature of the crossbar tandem switching system

CROSSBAR tandem is a relatively young switching system. It first went into service in 1941. Its original role was well defined although, in retrospect, severely limited; it was to take over in large metropolitan areas the functions of the panel-type office selector tandem system, even as the no. 1 crossbar system1 took over from the local panel central office. However, even at that early date, its latent potentialities for a broader field of usefulness were appreciated. This appreciation could not then be implemented because during World War II development of telephone switching facilities at Bell Telephone Laboratories came to a near standstill. The needs of national defense transformed switching development engineers almost overnight into designers and teachers of radar, sonar, etc. True, there followed the great postwar surge of switching development activities but these, in turn, involved new systems urgently needed to start customer and operator nation-wide dialing.1 This period saw the development of the no. 5 crossbar local switching system,2 the automatic message accounting (AMA) system,3 and the 4A crossbar long-distance switching system.4 In view of this tremendous development program of new systems, the available development talent had to be practically rationed, and so only a moderate amount of effort could be applied to the crossbar tandem system to increase its scope of usefulness. The outcome of this effort and the basic description of the crossbar tandem system were presented in 1950.5 The closing paragraph of this 1950 paper was the following statement: “An extensive development program is now in progress to incorporate new features which will still further enhance the usefulness of crossbar tandem in the rapid expansion of automatic switching in the Bell Telephone System.” By now, this extensive development program referred to is nearing completion, and it is the purpose of the present paper to highlight the significant new features embodied in the 1956 design version of the crossbar tandem system.