FUNDAMENTAL PLANS FOR TOLL TELEPHONE PLANT 839 



tically all overflows from the hi^h usage jijroup.s during the heavy traflic 

 periods. The "high usage" atid "finar' gi'oups which could he used for 

 routing calls l)et\vc(Mi Ilihhing, Minn(>s(>(a and DaAciiporl , Iowa ai'e 

 shown by Fig. 5. 



Generalization of the Toll Switching Plan. 



The generalization of the arrangements discussed for the Chicago 

 region is illustrated in Fig. G. This shows diagi'ammatically all types of 

 switching points in two regions and also indicates the relative position 

 occupied by the National Center in the switching plan. On this chart, 

 the solid lines represent the "final groups" of trunks, and the dotted 

 lines represent "high usage" trunks. Examination of this chart will 

 indicate that the mechanical switching system need perform only rela- 

 tively simple toll switching operations at the toll centers. At other 

 points the system must attempt to complete the call over the most 

 favorable routes, in planned sequence, until the "final" route is selected. 



For example, from a given primary outlet such as POl on a call des- 

 tined for a toll center in the other region such as TC2, the switching 

 equipment would attempt to complete the call, in sequence over the 

 routes marked 1 to 6. 



Should Route 6, which is the "final" route, be selected because all of 

 the trirnks in the "high usage" groups marked 1 to 5 were busy at the 

 time, the switching equipment at the SC would in turn try routes marked 

 A, B, C, etc., in attempting to complete the call. A fairly complete 

 pattern of circuit groups is indicated in this illustration. Depending on 

 the relative locations of the points concerned and the traffic load re- 

 quirements, certain of the "high usage" groups shown may not exist. 

 It is expected, however, that most TC's will have high usage groups to 

 points other than their "home" PO's. Also each PO can be expected to 

 have high usage groups to sectional centers other than its "home" SC. 

 All regional centers will be interconnected with direct trunks, regardless 

 of geographical location. 



Control Switching Points 



Because of rapid and complex switching operations required by the 

 automatic equipment at PO's and higher order switching points, (SC's, 

 RC's and the NC) these switching centers are called Control Switching 

 Points (CSP's). 



As covered by a companion paper, the switching equipment required 

 at the CSP's is quite complex. This equipment must have a high degree 



