The problem of modeling a system's reliability and availability with respect to the various classes of faults (physical and design, internal and external) which may affect the service delivered to its users is addressed. Hardware and software models are currently exceptions in spite of the user's requirements; these requirements are expressed in terms of failures independently of their sources, i.e., the various classes of faults. The causes of this situation are analyzed; it is shown that there is no theoretical impediment to deriving such models, and that the classical reliability theory can be generalized in order to cover both hardware and software viewpoints that are X-Ware.
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