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. >