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THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 25.0 



t/h = DELAY IN MULTIPLES OF AVERAGE HOLDING TIME 



Fig. 12 — Delayed traffic served in random order, exponential holding times* 

 c = 5. 



Example No. 2 



How many operators will be required at a department store telephone 

 order desk to handle 225 calls per hour with an average delay not 

 longer than half a minute, and with no more than 20 per cent of the 

 calls delayed over 1 minute? Assume the average operator work time 

 per call is 100 seconds, and waiting calls are handled in indiscriminate 

 order. 



>So/w^ton. The load to be carried is a = (225)(100)/3600 = 6.25 erlangs. 

 The average delay, i, is not to exceed 30/100 = 0.3 holding time. Read- 

 ing on Fig. 19, opposite an ordinate of 0.3 we select several trial values 

 of trunks (operators) c, versus occupancy a, and form Table I, cal- 

 culating the last column from the first two: 



