Evaluating FTRE´s for dependability measures in fault tolerant systems

In order to analyze dependability measures in a fault tolerant system, we generally consider a nonstate space or a state space type model. A fault tree with repeated events (FTRE´s) presents an important strategy for the nonstate space model. The paper deals with a conservative assessment to complex fault tree models, henceforth called CRAFT, to obtain an approximate analysis of the FTRE´s. It is a noncutset, direct, bottom-up approach. It uses failure probability or failure rate as input and determines a bound on the probability of occurrence of the TOP event. CRAFT generalizes the concept of a cutting heuristic that obtains the signal probabilities for testability measurement in logic circuits. The method is efficient and solves coherent and noncoherent FTRE´s having AND, OR, XOR, and NOT gates. In addition, CRAFT considers M/N priority AND, and two types of functional dependency, namely OR and AND types. Examples such as the Cm* architecture and a fault-tolerant software based on recovery block concept are used to illustrate the approach. The paper also provides a comparison with approaches such as SHARPE, HARP, and FTC