THE INTERCONNECTION OF TELEPHONE SYSTEMS 



559 



of service is negligibly small in comparison with the effect of subgroup 

 load variations. 



Practical Graded Multiple Engineering 

 In the Bell System, engineering of equipment and lines is done on 

 the basis of the grade of service desired in the busy hours over a con- 

 siderable period of time. The theoretical formula used, then, is the one 

 giving, for those ranges in which satisfactory service is being rendered, 

 an approximate relationship between the average total load carried 

 and the proportion of lost calls expected over a number of busy hours. 

 This, rather than some less conservative plan which would simulate the 

 losses encountered only in a particular busy hour. The use of the 

 "Half Gain" efficiency curves, therefore, is justified from a considera- 

 tion of Fig. 16, which illustrates how^ closely that theoretical expression 



20 



30 



40 50 60 70 

 TOTAL AVERAGE 



Fig. 23 — Multiple capacity curves, P = 0.01. 



approximates the actual conditions which maintain over a considerable 

 range of trunk combinations and load values in two typical graded 

 multiple installations.^^ 



In practice, of course, the load to be carried rarely comes out 

 exactly equal to that which a given symmetrical trunk arrangement 



"Some five years ago it was appreciated that "Full Gain" was not likely to be 

 attained. The value of "Half Gain" was then arbitrarily selected as the graded 

 engineering basis in the Bell System until field studies of the efficiency could be com- 

 pleted. The merit of this estimate is attested by the material presented m this paper. 



