The Activity-Dependent Failure Intensity of SPC Systems--Some Empirical Results

This paper presents the results of an investigation of the relationship between the occurence intensity of different failures in the fault-tolerant control system of three SPC (stored-program control) telephone exchanges and various activity dependent factors in the real time environment of the exchanges. The analysis is based on 475, 439, and 243 failure registrations during 85, 71, and 64 months of operation, respectively. In order to get quantitative estimates of the influence of the different explanatory functions a detailed statistical analysis has been carried out. This analysis is based on a nonhomogeneous Poisson process model where the following functions are included in the failure intensity: call-arrival rate, operational activity, routine testing program activity, time elapsed since start of operation. The maximum likelihood estimates of the corresponding parameters and their covariances are determined. The effect of several of the factots turns out to be significant.