Dependability of distributed control system fault tolerant units

We investigate two types of fault tolerant units (FTUs) suitable for dependable distributed control systems and numerically evaluate their reliability and mean time to failure (MTTF). A simple simulation-based methodology to numerically evaluate dependability functions of a wide variety of fault tolerant units is presented. The method is based on simulation of stochastic Petri nets. A set of 15 FTU configurations belonging to five groups is analyzed. Groups 1 and 2 belong to the node oriented category whereas groups 3 through 5 belong to the application oriented category. The methodology allows a quick and accurate evaluation of dependability functions of any distributed control system design in terms of the type of FTU (i.e., node or application), replicas per group, replicas per FTU, and shared replicas.