section5 - 5.0 HARDWARE/SOFTWARE SYSTEM RELIABILITY...

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5-1 5.0 HARDWARE/SOFTWARE SYSTEM RELIABILITY MODELING Reliability modeling of combined hardware and software systems is in many ways analogous to reliability modeling of purely hardware systems. Reliability block diagrams of system elements are developed and employed. Individual hardware platforms and the software assigned to those platforms are independent of other hardware/software platforms. Reliability block diagrams that accurately portray the interrelationship between the hardware platforms and the software executing on the platforms are developed and used in estimating reliability metrics. For complex structures, state diagrams are developed to accurately portray the unique interrelationships of the structure being modeled. This section provides the techniques applicable to the reliability modeling of combined hardware and software systems. For series systems, the process is straightforward. For complex, redundant, repairable systems, an abbreviated overview of the system modeling process is provided. Because these complex models are unique to the system being modeled, detailed procedures for developing these complex models cannot be provided. The overview provided is used to help the analyst identify those system properties which are unique to redundant combined hardware and software systems. The majority of this section is dedicated to describing the development of software failure rates that are a composite of the multiple processes that may be executing during any time period. 5.1 Basic Reliability Model . A basic reliability model for a hardware/software system can be prepared. However, the result is two reliability models; one for the software and one for the hardware elements of the system. The basic hardware reliability model consists of all hardware elements of the system in series so that the overall logistics support requirements for spares, maintenance personnel, training, etc., can be readily assessed based on the failure rates of the equipment. Individual software components (CSUs) as a rule do not fail independently and are not subject to wear out. Software components fail in association with the operational profile and must be modeled that way. Also, software components are not replaced independently like hardware units because they do not wear out and the unit itself is not independent. For hardware/software systems, the results of the basic software reliability model can be used to estimate the number and types of equipment that must be supplied when a software maintenance facility is required as a part of the contractual effort. 5.2 Mission Reliability Model . The mission reliability model is used to support allocations of system requirements to individual hardware and software elements and to support assessment of the compliance to requirements.
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5-2 Reliability modeling methods used to model combined HW/SW systems for the purposes of reliability estimation and allocation need to accurately assess the interdependence between individual software
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section5 - 5.0 HARDWARE/SOFTWARE SYSTEM RELIABILITY...

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