Hierarchical Modelling of Complex Control Systems: Dependability Analysis of a Railway Interlocking

A. Bondavalli, M. Nelli, L. Simoncini, G.  Mongardi

This paper reports an experience made in building a model and analysing the dependability of an actual railway station interlocking control system. Despite our analysis has been restricted to the Safety Nucleus subsystem, mastering complexity and size required a considerable effort. We identified a modelling strategy, based on a modular, hierarchical decomposition allowing to use different methods and tools for modelling at the various level of the hierarchy. This multi-layered modelling methodology led to an accurate representation of the system behaviour and allowed us (i) to keep under control the size of the mod-els within the different levels to be easily managed by the automatic tools, (ii) to make changes in the model in a very easy and cheap way. The paper con-tains also examples of the extensive analyses performed regarding the sensitivity of the dependability measures to variations of critical parameters and
towards the validation of the assumptions made.
Keywords: computer based interlocking systems, analytical modelling and evaluation, hierarchical modelling methodology, unsafety, reliability, availability, sensitivity analysis


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