Tandem Operation in the Bell System 



By F. M. BRONSON 



Tandem operation is becoming of increasing importance in the Bell 

 System. The operating and service features of the different types, and the 

 conditions under which each type is used, are outlined. Charts are included 

 showing, schematically, typical trunking arrangements in the various 

 systems. The increasing use of tandem operation on traffic handled at toll 

 boards is discussed. 



THERE are 14,000,000 telephones in the Bell System, served from 

 6,800 central offices. Means must be provided to permit any 

 one of these telephones to be connected to any of the others. There- 

 fore, 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 

 [iV X (iV — l)/2], or more than 23 million, groups of two-way cir- 

 cuits, most of which would carry little or no traffic. To keep the num- 

 ber of circuit groups within reasonable limits and to obtain reasonable 

 circuit efficiency, direct circuits are provided only between offices hav- 

 ing a sufficient community of interest to justify them. Connections 

 between the others are obtained, as required, through switching opera- 

 tions 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. 



To facilitate the interconnection of central offices in areas having 

 large volumes of local and short haul toll traffic, switching arrange- 

 ments designed particularly for this purpose are frequently provided. 

 These are known as tandem arrangements and, for the purpose of this 

 paper, may be more specifically defined as facilities for the inter- 

 mediate switching of traffic between central offices other than those 

 facilities involving the use of outward, inward and through toll switch- 

 boards and of local switchboards which interconnect trunks of the 



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