The efficacy of diverse redundancy against design error: some practical considerations

By Lorenzo Strigini, Peter Popov, and Michele Pizza;

ABSTRACT
Design faults are a serious problem for the safety and reliability of complex systems, especially when based on complex, hardware/software digital design. Redundancy with diversely-designed channels is a useful defence against them. However, it cannot be trusted to guarantee independence of failures between the channels. Exactly how much can be expected from diversity (and thus, for instance, which b factor is acceptable in a safety case for a dual-diverse protection system) is an open research problem. We present a simple model predicting the advantages given by diversity, under simplifying assumptions, and discuss which of the model's conclusions should be trusted to apply in the real world. We also discuss how this modelling approach can be applied to the spectrum of diversity options, from "functional diversity", as normally used in protection systems, to "design-only" diversity between functionally identical channels, as applied, for instance, in some avionics systems.

We outline the limits to what can be predicted given the information that one may currently hope to be available for a new piece of equipment, and which additional general information should be gathered to allow narrower estimates.

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