A Dependability Measure for Degradable Computing Systems

• This paper deals with the problem of finding a comprehensive dependabilitymeasure or figure of merit for computing systems. Dependability is a term usedfor a general description of a systems trustworthiness in non-quantitative terms. Itis commonly described by a number of aspects, like reliability, availability, safetyand security. Quantitative measures are conveniently used for e.g. reliability andavailability, but are rare for security.However, it is felt that a more general measure of a system’s dependability wouldbe of great interest and could be used for system evaluations, design trade-offs etc.In order to achieve this, we adopt a generalized view that facilitates a recompilationof the dependability aspects into fewer and more general qualities. Key issues forthe generalization are the concepts of degradability and service. A degraded serviceis the result of the discontinuation of one or several subservices, yielding a systemthat operates on a reduced service level.A vectorized measure based on Markov processes is suggested, and mathematicaldefinitions are given. The measure describes the expected time a system willbe operating at a certain service level, and also the probability that this level bereached. By means of applying the concept of reward rate to each service level, aneven more simplified figure of merit can be calculated.Normally, when making reliability calculations, an assumption of exponentialfailure rates for system components is made. Sometimes this assumption is notrealistic and we outline how phase-type distributions can be used to cope with thissituation.Finally, two different schemes for the calculation of the measure is given. First,a hierarchical procedure feasible for small systems and calculations by hand is presented.Second, a general procedure based on matrix calculus is given. This procedureis suitable for complicated systems. It is also general in the sense that it maybe used for measures extended to repairable systems.

Ämnesord

NATURVETENSKAP  -- Data- och informationsvetenskap -- Datorteknik (hsv//swe)

NATURAL SCIENCES  -- Computer and Information Sciences -- Computer Engineering (hsv//eng)

NATURVETENSKAP  -- Data- och informationsvetenskap -- Datavetenskap (hsv//swe)

NATURAL SCIENCES  -- Computer and Information Sciences -- Computer Sciences (hsv//eng)

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