THEORIES FOR TOLL TRAFFIC ENGINEERING IN THE U. S. A. 441 



calculated for each further connecting route will be recorded as part of 

 the offered load for consideration when the next higher switching center 

 is engineered. It is implicitly assumed that a call which has selected one 

 of the alternate route paths will be successful in finding the necessary 

 paths available from the distant switching point onward. This is not 

 quite true but is believed generally to be close enough for engineering 

 piu'poses, and permits ignoring the return attempt problem. 



6. NEW PROBLEMS IN THE ENGINEERING AND ADMINISTRATION OF INTER- 

 TOLL GROUPS RESULTING FROM ALTERNATE ROUTING 



With the greatly increased teamwork among groups of intertoll trunks 

 which supply overflow calls to an alternate route, an unexpected increase 

 or flurry in the offered load to any one can adversely affect the service to 

 all. The high efficiency of the alternate route networks also reduces their 

 overload carrying ability. Conversely, the influence of an underprovision 

 of paths in the final alternate route may be felt by many groups which 

 overflow to it. With non-alternate route arrangements only the single 

 groups having these flurries would be affected. 



Administratively, an alternate route trunk layout may well prove 

 easier to monitor day by day than a large number of separate and in- 

 dependent intertoll groups, since a close check on the service given on 

 the final routes only may be sufficient to insure that all customers are 

 being served satisfactorily. When rearrangements are indicated, how- 



SIMPLE PROGRESSIVE 



GRADED MULTIPLE GRADED MULTIPLE 



(a) (b) 



t t t t t t tt t t tl 



ILLUSTRATIVE INTERLOCAL AND INTERTOLL 

 ALTERNATE ROUTE TRUNKING ARRANGEMENT; 



(c) (d) 



t t t t t = ,-"" ^ 



tttl It ttl 1 t 



Fig. 10 — Graded multi])los .•nid altornaic route trunking nrrangeinoiits. 



