TRUNK REQUIREMENTS IN ALTERNATE ROUTING NETWORKS 280 



Each regional center (RC) homes on, ie., has a final group to the NC, 

 and has one or more sectional centers homing upon it; 



Each sectional center (SC) homes on an RC (or the NC) and lias one 

 or more primary outlets homing upon it; 



Each primary outlet (PO) homes on an RC, NC or SC and has one or 

 more ordinal y toll centers homing upon it; and 



Each ordinary toll center (TC) is so called because it performs no 

 through switching function but merely serves as the connecting point 

 between the intertoll network and local central offices or tributaries. 



Thus each toll center (and in the generic sense this phrase includes all 

 CSP's as well as ordinary'' toll centers) was classified with respect to the 

 area served: the TC serving a group of local offices or tributaries, the 

 PO serving a group of TC's, the SC serving a group of PO's, the RC 

 serving a group of SC's and lastly the NC serving all the RC's. Under 

 this arrangement any toll center could home on another of higher rank 

 or classification. Thus a TC or PO, for example, could home upon an 

 RC if so dictated by geographic and economic considerations. Before 

 proceeding with a detailed study of trunk requirements the classifica- 

 tion of toll centers and the homing relationships had first to be estab- 

 lished. This was done in a manner which reflected the known densities 

 and flow of traffic between the larger cities and the relative cost of final 

 routes which in turn reflected the differences in lengths of haul to one 

 CSP as opposed to another, etc. With the classification and homing of 

 each toll center established it was possible to trace the final route, the 

 route of last resort, between any two toll centers in the entire system. 

 Thus was the stage set for determining the location of and number of 

 trunks to be provided in high usage groups whose function would be to 

 move traffic more economically by direct connection between points 

 than could be done by following the final route. 



It is apparent at once from the illustration in Fig. 5 that the problem of 

 (Uitermining the most economical alternate for a given HU group is 

 different from that encountered in the interlocal situation of Fig. 2 in- 

 asmuch as the latter had only one intermediate switching point in each 

 final route. A further difference not specifically indicated is that intertoll 

 trunk groups handle traffic in both directions whereas interlocal trunk 

 groups handle traffic in only one direction, i.e., there are separate out- 

 ward and inward groups between any pair of local offices. There are 

 special circumstances under which one-way intertoll groups also are 

 estabUshcd but these may l)e ignored for purposes of our discussion. 



While this paper is not specifically concerned with the transmission 

 aspects of an alternate routing network some mention should be made 



