Combining Various Solution Techniques for Dynamic Fault Tree Analysis of Computer Systems

Fault trees provide a graphical and logical framework for analyzing the reliability of systems. A fault tree provides a conceptually simple modeling framework to represent the system-level interactions between component reliabilities. Dynamic fault trees have been shown particularly useful for reliability analysis of embedded computer systems. Dynamic fault trees are a superset of traditional (static) fault trees in that additional gates are used to model sequential behavior. DIFtree [1] is our fault tree methodology for the analysis of dynamic fault trees, effectively combining the best static fault tree solution technique (Binary Decision Diagrams) with Markov solution techniques for dynamic fault trees. DIFtree includes advanced techniques for modeling coverage; coverage modeling has been shown to be critical to the analysis of fault tolerant computer systems. DIFtree is based on a divideand-conquer technique for modularizing the system level fault tree into independent sub-trees; different solution techniques can be used for sub-trees. In this paper we extend the DIFtree analysis capability to model several different distributions of time to failure, including fixed probabilities (no time component), exponential (constant hazard rate), Weibull (time varying hazard rate), and log normal. Our approach extends both the BDD and Markov analytical approaches and incorporates simulation as well. An example analysis illustrates our approach.