Dependability assessment for decentralized systems

Conventional dependability measures, such as reliability or availability, assume that the equipment characterized by the measure is either operational or has failed. This dichotomy does not hold for decentralized or distributed systems because these can operate in modes in which partial or degraded service is furnished. Whether a specific degraded mode should be counted as "operational" or "failed" is a subjective decision, but this decision can make a large difference in the dependability assessment. Examples show that this will affect not only numerical reporting, but also the selection of reliability improvements. A weighted averaging of the dependability measures obtained under various failure criteria is seen to be a workable method for reliability assessment that provides much more stable measures than can be obtained by selection of a single failure criterion.