290 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH, 1954 



of that subject since it has an important bearing upon how traffic is 

 routed. It is apparent that transmission standards for trunks connecting 

 each class of office with any other class had to be so determined that a 

 conversation between any two telephones in the system could be car- 

 ried on within a specified limit of over-all transmission loss. For example, 

 the possibility of a call being completed over an 8-link final route dictated 

 the transmission tolerances to which each of the 8 links should be de- 

 signed. Similarly, transmission equivalents for the various HU trunks 

 over which calls would by-pass the final route had also to be determined. 

 It became necessary, therefore, to devise a routing pattern which would 

 assure that neither too many links nor the wrong classes of links would 

 be connected in tandem. This interdependence of the transmission and 

 routing aspects led to the adoption of the rule that traffic should be so 

 routed that "when a call fails to find an idle path in the high usage net- 

 work at any switching point (CSP) it can always be offered to the final 

 network at that point." In other words, any CSP used as the intermedi- 

 ate switching point in the alternate route of a high usage group must 

 lie in the final route between the terminal offices of the high usage group. 

 This was the corner-stone of the routing pattern. 



The Location of High Usage Groups 



Having established the classification and homing arrangements of all 

 toll centers and the basic routing pattern the next step in engineering 

 the nationwide intertoll network was to determine between what points 

 there should be high usage groups. It should be mentioned here that 

 the following will describe in essential detail the procedures actually 

 used by the Bell System in preparing the first coordinated nationwide 

 estimate of intertoll trunk requirements under a plan deliberately de- 

 signed to take advantage of the speed and economy possibilities of the 

 alternate routing principles already described. The study, which was 

 completed in 1951, involved a look into the future of about 10 years and 

 was predicated on operator dialing of toll calls. Customer toll dialing 

 which is foremost at the present time in Bell System planning may re- 

 quire some further modification of the general toll switching plan but the 

 procedures about to be described \v\\\ be substantially unchanged. 



A high usage group is established between any two toll centers when 

 the traffic between them in both directions is sufficient in volume to make 

 such a group more economical then any other route available within the 

 routing pattern. In the earlier discussion of the principles involved in 

 determining what portion of a given load should be carried by a direct 

 route it was shown that the ratio of the efficiencies and the ratio of the 



