74 



BELL SYSTEM TECHNICAL JOURNAL 



when X calls, at least 20 of the other subscribers will have called in 

 the two minutes immediately preceding, and therefore X fails to get a 

 trunk immediately. In other words, we may consider that on the 

 average one in every thousand calls is lost. 



In the problem just considered, a known number of subscribers' 

 lines have had a known number of trunks assigned to them and we 

 have inquired the probability that any subscriber would fail to find 

 an idle trunk. It is frequently desirable to change the statement of 



12 3 591 



591 Lines 



6^ 



m 



Line Switches 



k 



35 



2 



Fipst Selectors 



sa 



± 



Fig. 3 



Second 

 Selectors 



^ Group of 

 — ^18 Trunks 

 — ► fon one of 



^10 Levels 



the problem slightly. For instance: given a known number of sub- 

 scribers' lines and having decided upon a desirable value of the prob- 

 ability P, we may inquire the number of trunks which must be as- 

 signed. It is evident that in this problem we would enter the table 

 knowing the value 8.96 and .001, and would find corresponding to 

 these, the number 20 as representing the size of the required group 

 of trunks. 



Problem II 



Referring to Fig. 3 consider a group of 591 subscribers' lines each 

 equipped with a 35 point line switch giving access to first selectors. 

 We will suppose for illustration that each first selector has 10 levels 



