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Abstract. Today, component-based software and system development has gained considerable attention and is wide-spread. Components and/or modules manifest the well-proven engineering principle of divide and conquer. Formal approaches have helped to provide a theoretical foundation to component-orientation. There is a broad range of system architectures in practice today in terms of layered distributed open systems (as for example known in telecommunications). However, the mathematical formalisms developed so far are rarely applied to these practical architectures. In fact, the notion of distribution and stacked communication layers is hardly understood precisely and not supported by architecture description languages. This article addresses this gap. The formal conditions of distribution and layering are investigated and a mathematical model for layered distributed systems is presented. Communication refinement and so-called complex connectors turn out to be vital concepts which are not addressed in today’s modeling languages. In addition to that, two important design approaches are derived for the architectural design of complex communication systems: A node-centric and a network-centric design approach. Both can significantly improve the design process.

Computer-based counseling systems in health care play an important role in the toolset available for medical doctors to inform, motivate and challenge their patients according to a well-defined therapeutic goal. The design, development and implementation of such systems require close collaboration between users, i.e. patients, and developers. While this is true of any software development process, it can be particularly challenging in the health counseling field, where there are multiple specialties and extremely heterogeneous user groups. In order to facilitate a structured design approach for counseling systems in health care, we developed (a) an iterative three-staged specification process, which enables early involvement of potential users in the development process, and (b) a specification language, which enables an author to consistently describe and define user interfaces and interaction designs in a stepwise manner. Due to the formal nature of our specifications, our implementation has some unique features, like early execution of prototypes, automated system generation and verification capabilities.

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