1108 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



service than by an arrangement which must care for the peaks of each 

 office separately. The non-coincidence of peaks of traffic of different 

 types of offices permits economies both on trunks to tandem and trunks 

 from tandem. For example, assume that a given office completes calls 

 via tandem to some offices which have a morning busy hour and to 

 others which have an evening busy hour. Then the group to tandem 

 must provide capacity to handle the traffic for the busier hour of the 

 two, but this capacity need care only for the peak traffic to part of the 

 destinations. If individual direct groups had been provided instead of a 

 common group to tandem, each group would have required capacity for 

 its own peak, regardless of when it occurred. The common group to 

 tandem therefore benefits by the noncoincidence of the peaks. A corre- 

 sponding situation also occurs on trunks from tandem. Each group com- 

 pletes calls to a given destination from a number of originating offices 

 whose peak hours may not coincide, and hence groups from tandem 

 derive economies similar to those of the incoming groups to tandem. 



Tandems are also required for alternate routing. Alternate routing is 

 an arrangement to provide trunking economies by using a limited num- 

 ber of direct trunks for the traffic between two offices, and permitting 

 the calls which do not find an available direct trunk to overflow to one 

 or more tandems in succession. Because of the ability to load the direct 

 circuits very heavily and yet provide good service by taking the overflow 

 from and to a number of offices through a common tandem point, sub- 

 stantial economies are possible. Automatic alternate routing is practical 

 only with common control systems. Common controls are needed to 



DIRECT TRUNKS ONLY TANDEM TRUNKS ONLY 



(90 groups) (20 groups) 



Fig. 7 — Reduction of tlu! nuinljeiol' trunk groups by tlie use of a tandem office. 



