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442 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



ever, the determination of the proper place to take action, and the de 

 sirable extent, may sometimes be difficult to determine. Suitable traffic 

 measuring devices must be provided with these latter problems in mind 

 For engineering purposes, it will be highly desirable: 



(1) To be able to estimate the load-service relationships with any 

 specified loads offered to a particular intertoll alternate routing network; 

 and 



(2) To know the day-to-day busy hour variations in the various 

 groups' offered loads during the busy season, so that the general grade of 

 service given to customers can be estimated. 



The balance of this paper will review the studies which have been made 

 in the Bell System toward a practicable method for predicting the grade 

 of service given in an alternate route network under any given loads. 

 Analyses of the day-to-day load variations and their effects on customer 

 dialing service are currently being made, and will be reported upon later. 



?; 



7. LOAD-SERVICE RELATIONSHIPS IN ALTERNATE ROUTE SYSTEMS 



In their simplest form, alternate route systems appear as symmetrical 

 graded multiples, as shown in Fig. 10(a) and 10(b). Patterns such as 

 these have long been used in local automatic systems to partially over- 

 come the trunking efficiency limitations imposed by limited access 

 switches. The traffic capacity of these arrangements has been the sub- 

 ject of much study by theory and "throwdowns" (simulated traffic 

 studies) both in the United States and abroad. Field trials have sub- 

 stantiated the essential accuracy of the trunking tables which have 

 resulted. 



In toll alternate route systems as contemplated in America, however, 

 there will seldom be the symmetry of pattern found in local graded 

 multiples, nor does maximum switch size generally produce serious 

 limitation on the access. The ''legs" or first-choice trunk groups will vary 

 widely in size; likewise the number of such groups overflowing calls 

 jointly to an alternate route may cover a considerable range. In all cases 

 a given group, whether or not a link of an alternate route, will have one 

 or more parcels of traffic for which it is the first-choice route. [See the 

 right-hand parcel of offered traffic on Fig. 10(c).] Often this first routed 

 traffic will Ijc the bulk of the load offered to the group, which also serves 

 as an alternate I'oute for other traffic. 



The simplest of the approximate formulas developed for solving the 

 local graded multiple problems are hopelessly unwieldy when applied 

 to such arrangements as shown in Fig. 10(d). Likewise it is impracticable i 



