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Unformatted text preview: ©Ian Sommerville 2000 Dependable systems specification Slide 1 Dependable Systems Specification • Processes and techniques for developing a specification for system availability, reliability, safety and security ©Ian Sommerville 2000 Dependable systems specification Slide 2 Functional and nonfunctional requirements • System functional requirements may be generated to define error checking and recovery facilities and features that provide protection against system failures. • Nonfunctional requirements may be generated to specify the required reliability and availability of the system. ©Ian Sommerville 2000 Dependable systems specification Slide 3 System reliability specification • Hardware reliability • What is the probability of a hardware component failing and how long does it take to repair that component? • Software reliability • How likely is it that a software component will produce an incorrect output. Software failures are different from hardware failures in that software does not wear out. It can continue in operation even after an incorrect result has been produced. • Operator reliability • How likely is it that the operator of a system will make an error? ©Ian Sommerville 2000 Dependable systems specification Slide 4 System reliability engineering • Subdiscipline of systems engineering that is concerned with making judgements on system reliability • It takes into account the probabilities of failure of different components in the system and their combinations • Consider a system with 2 components A and B where the probability of failure of A is P (A) and the probability of failure of B is P (B). ©Ian Sommerville 2000 Dependable systems specification Slide 5 Failure probabilities • If there are 2 components and the operation of the system depends on both of them then the probability of system failure is • P (S) = P (A) + P (B) • Therefore, as the number of components increase then the probability of system failure increases • If components are replicated then the probability of failure is • P (S) = P (A) n (all components must fail) ©Ian Sommerville 2000 Dependable systems specification Slide 6 Functional reliability requirements • A predefined range for all values that are input by the operator shall be defined and the system shall check that all operator inputs fall within this predefined range. • The system shall check all disks for bad blocks when it is initialised. • The system must use Nversion programming to implement the braking control system. • The system must be implemented in a safe subset of Ada and checked using static analysis ©Ian Sommerville 2000 Dependable systems specification Slide 7 • The required level of system reliability required should be expressed in quantitatively • Reliability is a dynamic system attribute reliability specifications related to the source code are meaningless....
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 Spring '09
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 Safety engineering, Dependable Systems, Sommerville, ©Ian Sommerville

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