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Identifying Safety Dependencies in Modular Computer Systems

The aviation industry has started to adopt complex distributed computing networks (known as Integrated Modular Avionics (IMA)). IMA raises a number of new issues for syste development. Firstly, IMA is designed to use logical partitioning of data, both to provide security of access to resources and also to support incremental change of one component with limited impact on other components. Secondly, an incremental approach to development is often needed, for example to purchase an operating system prior to application development. To support this there is a need for a modular approach to safety analysis which not only provides assurance of system safety, but also provides a breakdown of responsibilities across logical boundaries. However, current safety analysis techniques are not well suited to a modular approach, treating software as a monolithic entity. This paper presents a method used to perform safety analysis on the module support layer (MSL) of an IMA system as part of a modular approach. The approach takes the system context into account, using system level hazard analysis and component interaction diagrams to assess the impact and cause of failures occurring in each component. We discuss the advantages of this approach, such as potential support for incremental change. However, we also highlight some difficulties associated with the component based approach, particularly when considering the re-use of components such as the MSL.