MEMORY REQUIREMENTS IN A TELEPHONE EXCHANGE 349 



the total memory by (6) would be 45" log .Y, while with all 2.V subscribers 

 in the same exchange -iS log 2N would be required. 



The saving just discussed is possible because of a group effect. There are 

 also statistics involving the calling habits of individual subscribers. A typical 

 subscriber may make ninety per cent of his calls to a particular small 

 number of individuals with the remaining ten per cent perhaps distributed 

 randomly among the other subscribers. This effect can also be used to 

 reduce memory requirements, although paper designs incorporating this 

 feature appear too complicated to be practical. 



Acknowledgment 



The writer is indebted to C. A. Lovell and B. D. Holbrook for some 

 suggestions incorporated in the paper. 



