An Approach to Modeling Software Safety in Safety-Critical Systems

Software for safety-critical systems has to deal with the hazards identified by safety analysis in order to make the system safe, risk-free and fail-safe. Software safety is a composite of many factors. Problem statement: Existing software quality models like McCall’s and Boehm’s and ISO 9126 were inadequate in addressing the software safety issues of real time safety-critical embedded systems. At present there does not exist any standard framework that comprehensively addresses the Factors, Criteria and Metrics (FCM) approach of the quality models in respect of software safety. Approach: We proposed a new model for software safety based on the McCall’s software quality model that specifically identifies the criteria corresponding to software safety in safety critical applications. The criteria in the proposed software safety model pertains to system hazard analysis, completeness of requirements, identification of software-related safety-critical requirements, safety-constraints based design, run-time issues management and software safety-critical testing. Results: This model was applied to a prototype safety-critical software-based Railroad Crossing Control System (RCCS). The results showed that all critical operations were safe and risk-free, capable of handling contingency situations. Conclusion: Development of a safety-critical system based on our proposed software safety model significantly enhanced the safe operation of the overall system.