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Because of the growing commercial, governmental, and scientific requirements for system availability, evaluating this factor has become increasingly important. This paper presents a unified approach to hardware and software availability of a system in the operational phase. The aim is to evaluate the availability in a given time interval, to show how to improve it, and to determine the probability that a specified level is met over the period. The inputs are the failure and repair rates of the system elements, and the functional relationship between them. Field tracking provides the failure and repair data, and Markov-chain techniques make it possible to construct, reduce, and solve the model. Availability is computed by the program package System Availability Estimator (SAVE). The model has been used and validated with actual field data for the IBM 3725 Communication Controller.
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