Tandem Operation in the Bell System

n p H E R E are 14,000,000 telephones in the Bell System, served from 6,800 central offices. Means must be provided to permit a n y one of these telephones to be connected to any of the others. Therefore, facilities must be provided for interconnecting all of the 6,800 central offices. Obviously it would be impracticable to provide direct circuits from each central office to all of the others; this would require IN X (N -- l)/2], or more t h a n 23 million, groups of two-way circuits, most of which would carry little or no traffic. To keep the number of circuit groups within reasonable limits and to obtain reasonable circuit efficiency, direct circuits are provided only between offices having a sufficient community of interest to justify them. Connections between the others are obtained, as required, through switching operations performed at one or more intermediate points. The 14,000,000 telephones referred to originate 75,000,000 daily calls, the great bulk of which, of course, are local calls dialed direct by customers or handled at local manual switchboards. There are, however, about 1,500,000 short haul station-to-station toll calls which, because of the close community of interest between the cities involved, are also handled by local operators by methods essentially similar to those used on local calls. Obviously, these are largely concentrated in sections of the country having greatest population densities, such as in the New York City, Boston, and San Francisco metropolitan areas.