. The Bell System technical journal . se Chicago half-hourly observations, however, that under or-dinary conditions the reaction of holes-in-the-multiple upon the grade THE INTERCONNECTION OF TELEPHONE SYSTEMS 559 of service is negligibly small in comparison with the effect of subgroupload variations. Practical Graded Multiple EngineeringIn the Bell System, engineering of equipment and lines is done onthe 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 onegiving, for those ranges in which satisfactory servi
. The Bell System technical journal . se Chicago half-hourly observations, however, that under or-dinary conditions the reaction of holes-in-the-multiple upon the grade THE INTERCONNECTION OF TELEPHONE SYSTEMS 559 of service is negligibly small in comparison with the effect of subgroupload variations. Practical Graded Multiple EngineeringIn the Bell System, engineering of equipment and lines is done onthe 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 onegiving, for those ranges in which satisfactory service is being rendered,an approximate relationship between the average total load carriedand the proportion of lost calls expected over a number of busy , rather than some less conservative plan which would simulate thelosses encountered only in a particular busy hour. The use of theHalf 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 70TOTAL AVERAGE Fig. 23—Multiple capacity curves, P = approximates the actual conditions which maintain over a considerablerange of trunk combinations and load values in two typical gradedmultiple installations.^^ In practice, of course, the load to be carried rarely comes outexactly equal to that which a given symmetrical trunk arrangement Some five years ago it was appreciated that Full Gain was not likely to beattained. The value of Half Gain was then arbitrarily selected as the gradedengineering 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. 560 BELL SYSTEM TECHNICAL JOURNAL TABLE VI Graded Multiple Trunk Capacity Tables P = .01—Terminal Assignment = 20 Equiv- Number Equiv- Number alent of Total Ind. Common alent of Total Ind. Common 100 Ind. Trunks Trunks Trunks 100 Ind. Trunks Trunks Trunks Calls Groups
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