Inherent Load-Balancing in Step-By-Step Switching Systems
M. M. Buchner, S. R. Neal · Bell System Technical Journal · 1971
Questions have been raised over the years regarding the validity of the traffic-engineering tables used by the Bell System and others for the graded-multiple trunk groups within the step-by-step switching system. The tables indicate that the capacity of a graded multiple is increased when it is imbedded in the step-by-step system, the increase becoming larger as the number of switches connected to the graded multiple decreases. The increase in capacity supposedly occurs because of a “finite-source effect.” In this paper, we investigate in detail the flow of traffic through the step-by-step system to determine the validity of the tables. By a combination of simulation and analysis, we show that the increase in capacity arises not from a finite-source effect but from an inherent load-balancing that results from the clever manner in which the switches and trunks are interconnected. We conclude that, when used for engineering in the presence of day-to-day variations, the tables are adequate when the gradings are used with the large selector groups. For the minimum number of selectors, the tables estimate fairly accurately the capacity of the larger gradings, slightly overestimate the capacity for the medium-sized gradings, and overestimate the capacity of some of the smaller gradings by about 20 percent.