Last-Trunk-Usage Measurements in Step-by-Step Switching Systems

Because of physical limitations, the trunks that connect the successive switching stages in a step-by-step switching system are arranged in sets of partial-access patterns called graded multiples (also called gradings, graded subgroups, or subgroups). The Bell System uses standard patterns for the gradings; a representative configuration is shown in Fig. 1. A call arriving hunts across the 10 trunks marked as heavy lines to find an idle trunk; if these 10 are busy, the call is blocked with a reorder tone returned to the customer. Details of the traffic flow through the system are given in Ref. 1. A long-standing problem in step-bv-step offices has been the difficulty in detecting traffic congestion in the graded multiples, since detailed measurements are not obtained. 1553 Fig. 1--Typical step-by-step subgroups showing limited trunk access for an arriving call.