Reliability estimation of hierarchical software systems

The reliability evaluation of hardware systems is usually well integrated into the design process. Because it is done early, this reliability evaluation is useful in making design decisions. Software Reliability Evaluation (SRE), on the other hand, has been mostly conducted after development has been completed (Ref. 4), therefore offering little or no input into the design. This paper proposes a simple approach to estimate the reliability of software systems that are composed of a hierarchy of modules. Furthermore, the proposed approach takes advantage of the familiar reliability block diagram (RBD) methodology (Refs 3-4), which has long been used for hardware reliability estimation. This paper demonstrates how RBD can be used to represent hierarchical software systems. Using this technique early in the development process can offer system designers the opportunity to conduct sensitivity analysis that combines both hardware and software components. Criticality, as defined by Aggarwal (Ref. 1) and refined by Roman (Ref. 9), is also shown to be useful in the proposed SIZE approach. Examples are given to show how criticality makes the SIZE more realistic than the use of RBD alone.