486 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



It is clear from the above theory, throwdowns, and actual observa- 

 tion that there are certain areas where the service differences given first 

 routed and high usage trunk overflow parcels of traffic are significant. 

 In Section 8, where practical engineering methods are discussed, curves 

 are presented which permit recognition of this fact in the determination 

 of final trunk requirements. 



7.6 Load on Each Trunk, Particularly the Last Trunk, in a Non-Slipped 

 Alternate Route 



In the engineering of alternate route systems it is necessary to deter- 

 mine the point at which to limit a high usage group of trunks and send 

 the overflow traffic via an alternate route. This is an economic problem 

 whose solution requires an estimate of the load which will be carried on 



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O FIRST ROUTED TRAFFIC (NUMBERS INDICATE PER 

 CENT OF TOTAL WHICH IS FIRST ROUTED) 



• OVERFLOW TRAFFIC FROM 7 HIGH USAGE GROUPS 



60 70 80 90 100 110 120 



A'= ESTIMATED OFFERED LOAD TO PITTSBURGH IN ERLANGS (INCLUDING RETRIALS) 



Fig. 39 — Comparison of losses on final route (Newark to Pittsburgh) for high 

 usage overflow and first routed traffic. 



