278 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH, 1954 



in which a first choice direct route for traffic would he provided with trunks 

 in such numbers as to force a predetermiried portion of the load to seek 

 another route where such residue or overflow would he carried at less cost 

 per unit and with little or no delay. The principles of alternate routing and 

 their application to interoffice tr unking systems are the subject of this paper. 



PRINCIPLES OF ALTERNATE ROUTING 



Broadly speaking, alternate routing of telephone traffic is a procedure 

 whereby a parcel of traffic is provided a second choice path to its destina- 

 tion which is also the second choice path for one or more other parcels 

 of traffic. The use in common of this second choice path by the several 

 parcels results in an overall requirement of trunk paths to the particular 

 destination which is less than would be the case if each parcel had sole 

 access to its own group of paths — and this is accomplished without 

 impairment of the average speed with which all of the traffic is handled. 



The familiar graded arrangements of trunk terminals in dial central 

 office switching systems are examples of the trunking efficiency gained 

 from alternate routing. It should be noted at once that the most efficient 

 way to handle traffic from one central office to another is by means of a 

 single group of trunks to which all traffic so destined has access. How- 

 ever, many such loads are so large that the number of trunks required 

 to handle them on a single group often exceeds the practical terminal 

 capacity of ordinary switching systems. Therefore, it becomes necessary 

 to present portions of such a load, each to an individual subgroup. For 

 example, assume that a busy-hour load of 334 calls of 100 seconds dura- 

 tion each (334 CCS) is to be carried at a probability of delay of one per 

 cent (Poisson) from office M to office N and that the equipment permits 

 access to a maximum of only ten trunk terminals for any one trunk 

 group. Since the number of trunks required for the load in question is 

 17.5 (18) in a single group the establishment of three individual sub- 

 groups is indicated. To maintain P.Ol service with three individual 

 subgroups requires 8.3 trunks in each, assuming equal division of the 

 load. The efficiency of each subgroup is 13.4 CCS per trunk which is 

 some 30 per cent less than that of the larger single group which is 19.1 

 CCS. 



Thanks to graded multiple arrangements,* such a heavy loss in effi- 

 ciency need not be accepted. Pursuing our example, consider the load in 

 question to be presented to an appropriate grade. This is illustrated 

 schematically in Fig. 1. 



* R. I. Wilkinson, The Interconnection of Telephone Systems — Graded Mul- 

 tiples, B. S. T. J., 10, Oct., 1931. 



