284 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH, 1954 



1 2 3 4 5 6 7 8 9 10 11 12 13 14 



TRUNK NUMBERS 



Fig. 3 — Distribution of offered load according to the Erlang B assumption, 

 lost -calls-cleared. 



(240-185) overflowed to the alternate route where they will be carried at 

 the rate of 28 CCS per trunk. The trunks which need be added to the 

 alternate route are therefore, (55 -h 28) or 1.96. 



The comparative costs of a number of alternate routing arrangements 

 in which the only variable is the number of trunks provided in the direct 

 high usage group are given in Table I. It is interesting to note that there 

 is no measurable difference in cost between the optimum value for the 

 number of HU trunks (7) and the cost of a 6 HU trunk arrangement. 

 And indeed had five trunks or eight trunks been provided in the HU 

 group under the stated conditions the additional cost above the most 

 economical arrangement would have been nominal. This phenomenon is 

 typical of many alternate routing triangles particularly when the offered 

 load is in excess of say, 200 CCS. The significance of this fact is its 

 effect upon the degree of accuracy with which cost ratios need to be 

 determined. For example, assume that 1.4, the cost ratio used in our 

 discussion, is precisely the right ratio for the facilities involved in the 

 ATN triangle but that an approximate computation had arrived at a 



