Reliability and safety for hardware in computer systems have been sufficiently studied in recent years. On the other hand, a safety-related system (SRS) for software has not been assured with the proper method of calculating the software safety integrity level (SIL) in the functional safety standards, which is currently determined only by the number of development methods applied to practical safety-related system (SRSs). In this paper, we discuss quantitative assessment for it by applying quantitative measures based on software reliability growth models (SRGMs) that have been widely and successfully applied to practical software quality management activities. Based on a nonhomogeneous Poisson process (NHPP), the plausible methods of calculating software SIL in the functional safety standard are proposed. Further, we discuss the quantitative method for assuring software SIL based on the optimal release policies with the test cost during testing-environment and the risk cost after the software products will be released.
The Importance of Interoperability in Functional Safety Standards