Vol.3 No.3 2010
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Research paper : A methodology for improving reliability of complex systems (A. Katoh et al.)−199−Synthesiology - English edition Vol.3 No.3 (2010) divided as follows:a. The function for decomposing a system specification into component specifications and interface specifications among components;b. The function for verifying whether the cooperative behavior of the component specifications and interface specifications among components are consistent.The system design mehod which satisfies the function “a” is selected among the system developing methods. In general, system designing is a work for defining a system specification by analyzing the user’s needs, and defining specifications for functions of the components which compose the system, realization means of the components, and relationship among the components, based on the system specification. The representative method of system designing other than architectural design method includes structured analysis and structured design (SA/SD) method[13] Term 12. SA/SD method is a design method where a system is decomposed into components by focusing on data flows of the system. In SA/SD method, the system is designed by focusing on the data such as business information rather than the functions and processings, because the data is stable against changes in requirements or a technological evolution. This allows to construct systems with maintainability and expandability. However, since SA/SD method is developed primarily for technological systems such as information systems, it does not deal with control flows or processing timing[14]. Therefore, it is inappropriate for designing anything other than information systems such as embedded systems. On the other hand, architectural design method requires more efforts compared to the specific design method such as focusing on the data as in the aforementioned example, because the procedures and tasks specific to a certain designing are not defined. However, architectural design method is a general design method which is not dependent on some specific technological systems where the process for defining functions and realization means of the system are defined. Therefore, taking into account the research objective of achieving a methodology which is not specific to particular technological systems, we select the architectural design method as the system design method which satisfies function “a”. Also, the representative systems engineering standards which defines architectural design method include ISO 15288[9] Term 13, ANSI/EIA 632[10] Term 14, and IEEE 1220[11] Term 15. While ISO 15288 can be applied to the entire system lifecycle process from the conceptualizing phase to the dismantling phase, the tasks and procedures of architectural designing are not finely defined. While ANSI/EIA 632 can be applied widely to the system lifecycle process from the conceptualizing phase to the transition to operation phase, the tasks and procedures for architectural designing are not finely defined. On the other hand, although IEEE 1220 limits the range of application from the system requirement analysis phase to the system test phase, the tasks and procedures for architectural designing are finely defined. Therefore, we select architectural design method defined by IEEE 1220 for our methodology.The system verification method which satisfies function “b”. is selected among the system development methods. In general, system verification is a work for verifying whether a developed system satisfies the system specification or not. The representative system verification methods other than model checking include test methodTerm 16 and simulation method[15] Term 17. Test method is a verification method for verifying behavior of actual products against the test cases. While it can verify the actual behavior of actual products, it is difficult to extract all of the cases which may occur and to verify the behavior in all possible cases. Simulation method is a verification method where a target to be verified and peripheral environment of the target is simulated as models on a computer, and behavior of the models is verified against the test cases. While it can verify the behavior of the target in the early phase of system development when actual products and peripheral environment do not exist, it is difficult to extract all of the cases which may occur and to verify the behavior in all possible cases, as in the test method. On the other hand, although model checking can only verify state transitions of a verification target, it can verify whether the properties to be satisfied are valid or not for all state transitions exhaustively. If there is a deadlockTerm 18 in state transitions of a system, fatal accidents may occur during the system operation. Therefore, we select model checking for our methodology.Next, architectural design method defined in IEEE 1220 and model checking are described in detail.3.1 Architectural design method in IEEE 1220Figure 2 shows the architectural design process. Architectural designing is composed of functional designingTerm 19 and physical designingTerm 20. Functional designing is a work where functions defined as a system specification are decomposed and refined, and performances defined as the system specification are allocated to the decomposed and refined functions. Physical designing is a work where system components are specified, and the functions and performances decomposed and refined in functional designing are allocated to the components. The outputs of architectural designing are component specifications and interface specifications among components.Figure 3 shows the process of functional designing defined in IEEE 1220. The process of functional designing is defined in IEEE 1220 chapter 6 section 3 Functional analysisTerm 21. Figure 4 shows the process of physical designing defined in IEEE 1220. The process of physical designing is defined in IEEE 1220 chapter 6 section 5 SynthesisTerm 22. By conducting the tasks according to the numbers in Figs. 3 and 4, it is

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