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The No. 2 Electronic Switching System has been planned with dependability, maintainability, and operational ease representing a major portion of the total system design concept. The major characteristics that fall under these headings are redundancy, trouble detection, trouble recovery, diagnosis, repair, man-machine interface, data protection, and operational and administrative procedures for growth and change. All these factors play a large role in both the hardware and software portions of the system design. This article reviews the highlights in each of these areas and describes how they interact in the overall system across both the control complex and peripheral system areas.

The No. 1 electronic switching system has a much higher concentration of control than previous telephone systems. This makes the task of providing continuous telephone service more challenging than ever. It is important that troubles be detected almost instantaneously, before many calls are affected. The main subjects of this article are (1) a scheme for duplicating control equipment and (2) a description of circuit and program facilities for taking advantage of this duplication to detect, to automatically recover from, and to analyze troubles.

This paper briefly outlines the Australian system of maintenance for airborne radio equipment, and gives in more detail the results of a recent investigation into the reasons for, and incidence of, unscheduled removals of units. The probability of unscheduled removal, derived from the investigation data, was used as a criterion for the assessment of the optimum period between scheduled overhauls. It was found that, in many instances, the probability of failure is greatest immediately after overhaul and decreases with time. Comparisons of the performance of various types of units are given.