State-Event-Fault-Trees - A Safety Analysis Model for Software Controlled Systems

Safety models for software-controlled systems should be intuitive, compositional and have the expressive power to model both software and hardware behaviour. Moreover, they should provide quantitative results for failure or hazard probabilities. Fault Trees are an accepted and intuitive model for safety analysis, but they are incapable of expressing state dependencies or temporal order of events. We propose to enrich Fault Trees with State/Event semantics. We use a graphical notation that is similar to Statecharts. Our model subsumes deterministic state machines that are suited to describe software behaviour and Markov Chains that model probabilistic failures. We allow exponentially distributed probabilistic events, deterministic delays and triggered events. The model is compositional and joins components by ports. Quantitative evaluation is achieved by translating the component models to Deterministic and Stochastic Petri Nets (DSPNs) and using an existing tool for analysis. We introduce the model and the analysis procedure and provide a small case study of a fire alarm system, completed by an outlook on our tool project ESSaRel.

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