480 



THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



Table XIII — Grade of Service on the System 



Total load submitted 



Total load overflowing 



Proportion of load not served 



Theory 



Observed 



475 erlangs 

 3.20 erlangs 

 0.00674 



34,001 calls 

 200 calls 

 0.00588 



compare the proportions of all traffic offered the system which failed to 

 find a trunk immediately. See Tables XII and XIII. 



After these several and varied combinations of offered and overflowed 

 loads to a system of one direct and three alternate routes it is seen that 'i 

 the final prediction of amount of load finally lost beyond the ST trunks 

 is gratifyingly close to that actually observed in the throwdown. The 

 prediction of the system grade of service is, of course, correspondingly 

 good. 



It is interesting in this connection to examine also the proportions I 

 overflowing the ST group when summarized by parcels contributed from 

 the several OST groups. The individual losses are shown on Fig. 36; they 

 appear well in line with the variation one would expect from group to 

 group with the moderate numbers of calls which progressed this far 

 through the multiple. 



0.4 



0.3 



octr 

 o^ 0.2 



So 

 a ^0.1 



,-THEORY =0.21 

 ._>. 



• • 



--AVG OBSD = 0.19 



12 4 6 8 10 12 14 16 18 20 

 OST GROUP NUMBER 



Fig. 36 — Overflow calls on third alternate (ST) route. 





