TRI'XK TiEQT'IRF.MF.XTS T\ ALTERNATE ROT'TIXO NETWORKS 283 



is a function of the number of trunks provided to handle a gi\cn load 

 on a lost-calls-cleared basis. 



Referring once more to Fig. 2, let us assume that the ratio of tlie 

 cost of an incremental path in ATN to the cost of an incremental path 

 ill AN is 1.4. It may then be stated that it costs 1.4 times as much to 

 handle traffic on the alternate route as on the direct route. The cost 

 ratio is computed on the basis of values of incremental trunks because 

 the ultimate question in the economical separation of load between a 

 direct and an alternate route is whether one more trunk should be added 

 to the direct route or should more trunk capacity be provided in the 

 alternate route to handle a marginal portion of the total load. Let us 

 assume further that the offered load, A to N, is 240 CCS and that the 

 efficiency of incremental trunks in the alternate route is 28 CCS per 

 trunk. 



With these three factors, the offered load, the cost ratio and the effi- 

 ciency of incremental trunks in the alternate route, the most economical 

 arrangement of trunks for carrying traffic from A to N may now be 

 determined. The first step is to make sure that any trunk in the direct 

 (HU) group will carry load at a cost per CCS equal to or less than the 

 cost per CCS which is characteristic of the incremental trunks in the 

 alternate route. Since the last trunk in a high usage group carries the 

 least traffic, as previously discussed, the significant comparison is the 

 ratio of the load carried by a trunk added to the alternate route to the 

 load carried by the last trunk in the high usage group. The numerator 

 of that ratio is 28 (CCS) and the denominator could be any one of 14 

 values shown on Curve A of Fig. 3, depending upon the number of 

 trunks provided. If that ratio is made equal to the cost ratio (ATN/AN) 

 there will be determined a value of load to be carried by a last trunk 

 which in turn will determine the most economical number of trunks for 

 the direct high usage group. This value is referred to as the "economic 

 CCS" of the problem and is determined as follows: 



1 4 28 



Cost ratio -^ = Efficiency ratio -z^ 



1.0 ^ X 



X= 20, the economic CCS 



On Curve A it will be seen that the sixth trunk will carry 22.5, the 

 seventh, 19.6 and the eighth, 16.4 CCS. Since the loading of the seventh 

 trunk is closest to the economic CCS just computed, the conclusion is 

 that seven trunks should be pro\'ided in the high usage group for the 

 minimum overall cost of handling traffic from A to N. Since the seven 

 trunks as a group will carry 185 CCS (Curve B), there will be 55 CCS 



