Inherent Load-Balancing in Step-by-Step Switching Systems

The graded-multiple trunk groups that connect the successive switching stages of the step-by-step switching system* are commonly engineered according to a set of tables furnished by American Telephone and Telegraph Company. 1 Because the tables, which were prepared some years ago, are based upon certain approximations and a good deal of engineering judgment and because the arguments used to * It is assumed that the reader is familiar with the step-by-step system. For those who are not, a summary of the system is given in Appendix A. 135 136 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , J A N U A R Y 1971 justify the tables are not convincing, their validity has been questioned^ Specifically, the tables indicate that the traffic capacity of a graded multiple is increased when it is imbedded in the step-by-step system and that the increase becomes more significant as the number of switches connected to the grading decreases. The common "explanations" for the increase in capacity usually relate in some imprecise manner to a "finite-source effect." Several common incorrect arguments 2-4 that relate to the capacity of the graded multiples in the step-by-step system are summarized in Appendix C. It is i m p o r t a n t to realize t h e n a t u r e of these arguments