The Bell System Technical Journal 



Vol. XXV October, 1946 No. 4 



A Study of the Delays Encountered by Toll Operators in 

 Obtaining an Idle Trunk 



By S. C. RAPPLEYE 



THE aim of the Bell System is to give the fastest possible toll service 

 consistent with costs. The aim of the Intertoll Trunk Engineer is to 

 provide the proper number of trunks in each group to obtain that objective. 

 His problem is to gauge the effect of his work on the overall speed of service. 



Overall speed of toll service is the elapsed interval from the filing of a call 

 until conversation starts or until there is a definite report about the called 

 party. This overall speed includes the operating time or interval required 

 for the operators to establish the connection; the subscriber time or interval 

 required for the calling party to give the details of the call, for the called 

 party to answer his telephone, etc.; and the circuit delay time or interval of 

 waiting for a trunk to become idle. This last factor may be termed the 

 trunk speed interval. 



The proportion of this trunk speed to the overall speed is an important 

 factor in determining the number of trunks to be provided. If it is a large 

 proportion of the total, a marked improvement may be expected as a result 

 of providing more trunks. Conversely, if the trunk speed is a small pro- 

 portion of the total, the improvement to be expected as a result of providing 

 more trunks will be small also, with a diminishing rate of improvement until 

 the trunk speed ceases to be a factor. The trunk speed in turn depends upon 

 three factors: 



Group size — number of trunks to the called city or in the direction of the 

 called city. 



The per cent, usage— the degree to which the trunks are kept busy in 

 carrying the load offered. 



Holding time — the length of time that a trunk is in use each time it is used. 



Since the trunk speed depends in part on the per cent, usage, it follows 

 that this interval will be longer in the busiest hour when the usage is greatest 

 and will be shorter in the hours which are less busy. Consequently, the 

 trunk speed inter\^al over the total day will be much less than in the busiest 

 hour. 



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