TRUNK KEQITIREMENTS IN ALTERNATE ROUTING NETWORKS 279 



'J'lu^ capacity of this particular grade is precisely 334 CCS and its 

 ctiicicncy is 1().7 CCS per trunk or only 12 per cent below the single 

 trunk group efficiency. Here the common trunks serve as an alternate 

 route for such portions of the loads a, b, and c, respectively, as can not 

 be handled by the ti\-e trunks which are individual to each. Because it is 

 not likely that a, b, and c will overflow equal amounts of traffic at the 

 same time, the common trunks are kept busy by a more or less com- 

 plementarj^ pattern of greater and lesser overflows at any given moment, 

 emerging from the three subgroups. It is this action of the overflows, 

 amply substantiated by experience, which accounts for the efficiency 

 of graded multiples. Looked at another way, it may be said that all 

 trunks above five in each subgroup of the split-multiple case have been 

 pooled for the common use of all subgroups and that in so doing, it is 

 possible to reduce the number of pooled trunks from 9.9 to five without 



GRADED TRUNK MULTIPLE 



INDIVIDUAL- 



20 TRUNKS 

 TO OFFICE N 



OFFICE M 



Fig. 1 — Typical graded arrangement of 20 trunks on 10 terminals. 



impairing the speed of service. This very brief discussion of graded mul- 

 tiples serves merely to point out by familiar example, some of the poten- 

 tialities of the alternate routing principle in the economical handling of 

 telephone traffic. 



ALTERNATE ROUTING IN LOCAL INTEROFFICE TRUNK NETWORKS 



The effectiveness of alternate routing as illustrated by its action in 

 graded multiples suggests the possibility of improving the efficiency of 

 trunking between central offices by arranging the offices themselves in a 

 sort of grade. Let us carry the analogy as far as practicable and assume 

 that the loads a, b, and c in Fig. 1 are now emanating from central 

 offices A, B, and C and .still destined for office N. Let us assume that 

 A, B, and C are typical offices in a multi-office city which has a tandem 

 office, T, and further, that every office in the city has a group to and a 

 group incoming from the tandem office. Fig. 2 illustrates these condi- 

 tions with respect to offices A, B, C, and N and for simplicity indicates 



