Fault-Free Refinements for Interface Automata

A refinement preorder for a model of concurrent systems should be compositional (i.e. a precongruence for parallel composition) and should not introduce faults into a fault-free specification. Arguably, the coarsest such precongruence is the optimal refinement preorder. For the model of interface automata, faults are communication errors in the form of unexpected inputs. The respective optimal preorder has been characterized as the inclusion of two trace sets. Here, we extend this result by regarding also quiescence (quiescence and divergence resp.) as faults. The latter preorder is coarser, i.e. better, than an earlier preorder regarding errors, quiescence and divergence. We also present conjunction operators for our settings, avoiding flaws that can be found in the literature.