540 BELL SYSTEM TECHNICAL JOURNAL 



Purpose of Study 



Earlier information, based on data assembled in Cleveland in 1929 and 

 1930, was formulated as a series of relationships between varying degrees of 

 loading (in terms of busy hour per cent, use) on trunk groups of different 

 sizes and the overall speed of service. These relationships were set forth in a 

 table which was to be used as a guide to the trunk provision needed to ac- 

 complish a desired overall service result. 



The table also furnished the percent calls encountering an NC (no circuit) 

 condition but made no specific reference to the average duration of NC 

 although from the data shown it could be inferred and demonstrated that 

 other factors, such as operating method, operating and party delays, nor- 

 mally have a more pronounced influence on the total day overall speed of 

 service than the busy hour trunk provision. That being so, as changing 

 conditions since 1930 have affected these other factors, either in the direction 

 of faster or slower service, the relationships in terms of overall speed 

 of service shown in that table have become less valuable as engineering 

 guides. 



The purpose of the current study, therefore, was to improve the engineer- 

 ing and management tools used in determining the number and arrangement 

 of trunks required to attain faster toll service so that the investment in 

 facilities may be used as effectively as possible. 



Study Procedure 



The study was based on the premise that if the size of group, per cent, 

 usage and holding time are knowoi, the trunk speed can be determined and will 

 remain constant under that particular set of conditions. With this constant 

 known, it would then become possible to construct from analyses of overall 

 speed of service data for groups, offices, areas or networks the going relation- 

 ship between the trunk speed of service and the overall speed of service and 

 to predict with reasonable assurance the effect on the overall speed which 

 would be brought about by changes in the group sizes or traffic character- 

 istics. The effect of foreseen changes in operating method, force conditions 

 or the character of the toll traffic on the overall speed can be estimated sepa- 

 rately and taken into consideration in determining the basis of trunk pro- 

 vision. With such information available, trunks can be provided where 

 they will be most effective. This is especially important during periods of 

 major change such as the transition from war to peacetime conditions or 

 from the ringdown to the dial method of toll operation. 



The problem was therefore to determine the average delay in securing a 

 trunk with various sizes of groups at various levels of usage with a view to: 



Stating that portion of the overall speed of servdce which results from 

 inability to secure a circuit, and 



