The aim of our work is to provide a quantified means helping in the definition of a new architecture for CAUTRA, a subset of the French Air Traffic Control system. In this paper we define a set of alternative architectures, give some elements for constructing their dependability models and compare their availability. Modeling is carried out following a modular and systematic approach, based on the derivation of block models at a high level of abstraction. In a second step, the blocks are replaced by their equivalent Generalized Stochastic Petri Nets to build up the detailed model of the architecture. The evaluations performed permit identification of a subset of architectures whose availability meets the dependability requirements and also identification of the best architecture among this subset.

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This paper addresses the dependability modeling of hardware and software fault-tolerant systems taking into account explicitly the interactions between the various components. It presents a framework for modeling these interactions based on Generalized Stochastic Petri Nets (GSPNs). The modeling approach is modular: the behavior of each component and each interaction is represented by its own GSPN, while the system model is obtained by composition of these GSPNs. The composition rules are defined and formalized through clear ident@cation of the interfaces between the component and the dependency nets. In addition to modularity, the formalism brings flexibility and re-usability. This approach is applied to a simple, but still representative, example.