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THE BELL SYSTEM TECHNICAL JOURNAL, MARCH, 1954 



the groups to and from tandem previously described (Fig. 2, AT and TN) 

 for the local trunking situation. It should be noted here that, in the 

 interest of economy, it was proposed for planning to further compromise 

 with the lost-calls-cleared assumption by setting the speed objective on 

 each final group at P.03 rather than P.Ol. Theoretical considerations 

 indicate that the violence done to the Erlang B premise would not be 

 severe enough to invalidate a study of trunk requirements made on that 

 basis nor interfere substantially with the achievement of a high speed 

 trunking system. 



The switching plan requires that each switching office, i.e., the national 

 center, the regional and sectional centers and the primary outlets be 

 equipped with common control apparatus which would accept the num- 

 ber of digits (10) required by the nationwide numbering plan and direct 

 a call to its designation in prescribed order over a variety of possible 

 routes of which the high usage groups indicated on Fig. 5 are representa- 

 tive. (Alternate routes at TC's would be selected by operators.) The No. 

 4 A toll switching system* was designed to perform this function and 

 offices so ecjuipped were designated control switching points or CSP's. 



The CSP's were classified for transmission and traffic routing purposes 

 in order of their relative importance in the network: 



The national center (NC) has the unique function of switching traffic 

 on the final route between regional centers thus being capable of handling 

 traffic from any point in the country to any other point. In addition the 

 NC is itself a regional center serving a particular area of the country; 



TC1(>=- ^-^ TC2 



D 



A 



NC-NATIONAL CENTER 

 RC-REGIONAL CENTER 

 SC-SECTIONAL CENTER 

 PO-PRIMARY OUTLET 



O TC-ORDINARY TOLL CENTER 



FINAL GROUP 



HIGH USAGE GROUP 



Fig. 5 — Illustration of basic intertoll network for nationwide toll dialing 

 showing maximum final route linkage and typical high usage groups. 



* F. F. Shipley, Automatic Toll Switching Systems, Communications and 

 Electronics, No. 2, Sept., 1952. 



