A Hierarchical Requirements Reference Model

Abstract

Requirements reference models provide a conceptual framework for discussing and reasoning about system development artifacts such as requirements, assumptions and designs. In practice, complex systems are naturally constructed in hierarchies in which design choices at one level of abstraction influence the requirements that flow down to the subsequent lower levels. Well-known reference models such as Parnas’ Four-variable model, Jackson’s World-Machine model, and the WRSPM model of Gunter et al., can be seen as capturing on view of this decomposition—the one in which the solution (software or machine) emerges as a single component. This view cannot uniformly accommodate the requirements flow-down in the multi-component, multi-level hierarchies of complex systems. This paper introduces a hierarchical requirements reference model that provides a conceptual framework to represent and reason about the artifacts of a hierarchically composed system. This new model provides a simplified and consistent structuring mechanism for representing all layers of the system hierarchy. Formal compositional verification of a hierarchically organized medical device controller is used as case example to illustrate the application of the reference model.