286 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH, 1954 



group, a group to an ofRce selector tandem, a group to tandem No. 1 and 

 finally a group to tandem No. 2 in .succession in its cjuest for an idle 

 path to a given office. It is not the pin-pose here to explore the intricacies 

 of the multi-alternate route system but rather to point out that the basic 

 principles descrilsed for separating a load between a first choice direct 

 route and a single final alternate route on an economic basis were applied 

 successfully to the multi-alternate route arrangements. A thorough anal- 

 ysis of one of the trials disclosed that there was a saving of approximately 

 20 per cent in the number of interoffice trunks of all types as compared 

 with what would have been required without the use of alternate routes. 

 Another result of the alternate routing system was an improvement in 

 speed of service, since whatever portion of a load was carried by a high 

 usage group was carried without delay and only the portion carried on 

 the final alternate route was subject to about a 2 per cent delay. If, for 

 example, 75 per cent of an item was carried without delay and the re- 

 mainder at P.02 (two P.Ol groups in tandem) the average delay for the 

 whole load would be P. 005 which is one half of the average per cent delay 

 considered "normal" for a direct group without an alternate route. 



ALTERNATE ROUTING IN INTERTOLL TRUNK NETWORKS 



The successful application of alternate routing to local interoffice 

 trunking led to investigation of the practicability of applying similar 

 techniques to trunking between toll offices. The problem was different 

 in a number of respects from that of the local interoffice system. 



First, the physical size of the toll network covering the United States 

 and Canada involved some 2,500 toll centers and lengths of haul between 

 toll centers varying between 20 and 3,000 miles. From this it was evident 

 that many first routes would not be direct but would involve one or more 

 intermediate switching points thus complicating the problem of deter- 

 mining relative costs of first and alternate routes. 



Second, the development of an alternate routing system for intertoll 

 trunking would have to be predicated on the existence of a slow speed 

 (high delay) trunking system. Whereas the aim of alternate routing 

 within metropolitan areas was to make an already high speed trunking 

 system cheaper the aim in toll would be to make a slow speed system a 

 high speed system at little or no additional cost. 



Third, new toll office switching facilities of a high degree of mechani- 

 cal intelligence would be rcfjuired. 



Fourth, improvement in transmission and signaling design of toll 

 circuits would be required to accommodate more links in tandem. 



And fifth, an entirely new procedure for the correlation and routing 



