Optimal execution tests and system reliability

Automatic routine execution (REx) testing of telecommunications switching equipment is often performed to improve system reliability, and to check the operational status of communications equipment. For duplex equipment REx testing, at a minimum, periodically checks the inactive unit for problems. If problems are found, then repairs can be made before service is impacted. In addition to this clear benefit, there are also costs associated with testing. Equipment that is being extensively tested may not be in-service, and the testing itself can lead to two types of incorrect conclusions, conclusions that lead to either unnecessary repairs, or to a dangerous misperception that the equipment is working properly. The first type of incorrect conclusion is a false positive. The second is a false negative. Since there are both costs and benefits associated with REx testing, it is natural to conclude that REx tests should be run neither too frequently nor too infrequently. In this paper we develop a detailed reliability model of a generic duplex peripheral module. The term generic is used, since the intent is to provide a useful methodology with some results that are independent of specific equipment, provided by various manufacturers. The model shows that picking the correct REx testing frequency results in significant savings in downtime. The impact on system outages as a function of REx testing frequency is shown for a number of cases.