section6 - 6.0 RELIABILITY ALLOCATION Reliability...

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6-1 6.0 RELIABILITY ALLOCATION Reliability Allocation deals with the setting of reliability goals for individual subsystems such that a specified reliability goal is met and the hardware and software subsystem goals are well- balanced among themselves. Well-balanced usually refers to approximate relative equality of development time, difficulty, risk, or to the minimization of overall development cost. 6.1 System Reliability Allocation . Reliability allocations for hardware/software systems can be started as soon as the system reliability models have been created. The initial values allocated to the system itself should either be the specified values for the various reliability metrics for the system, or a set of reliability values which are marginally more difficult to achieve than the specified values. Reliability values that are slightly more aggressive than the required values are sometimes allocated to the system to allow for later system functionality growth and to allow those parts of the system which cannot achieve their allocated values to be given some additional reliability margin later in the design process. The apportionment of reliability values between the various subsystems and elements can be made on the basis of complexity, criticality, estimated achievable reliability, or any other factors considered appropriate by the analyst making the allocation. The procedures provided for allocation of software failure rates can be applied to both hardware and system elements provided the user recognizes that these elements typically operate concurrently. System-level allocations are successively decomposed using the reliability model(s) until an appropriate set of reliability measures has been apportioned to each hardware and hardware/software component of the system. 6.2 Hardware Reliability Allocation . The allocation of reliability values to lower-tiered hardware elements is a continuation of the allocation process begun at the system level. The system level hardware reliability models are used to successively apportion the required reliability measures among the various individual pieces of hardware and from the hardware equipment level to the various internal elements. For existing hardware items, the reliability allocations used should be based on the reliability performance of previously produced equipment. Reliability allocations to internal elements of existing hardware are not typically performed. Hardware equipment level allocations are further allocated to various internal elements within the equipment. 6.3 Software Reliability Allocation . The first step in the allocation process is to describe the system configuration (system reliability model). Next, trial component reliability allocations are selected, using best engineering judgment. Compute system reliability for this set of component reliability values. Compare the result against the goal. Adjust component reliability values to move system reliability toward the
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6-2 goal, and component reliability values toward better balance.
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section6 - 6.0 RELIABILITY ALLOCATION Reliability...

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