A case study of decomposing functional requirements using scenarios

Potential users of a proposed system sometimes tend not to formulate functional requirements on that system. Rather they define requirements on the compound system that includes the proposed software/hardware as well as other objects and even people such as the users themselves. Since such requirements do not make explicit what exactly the proposed system is supposed to do, they are insufficient for developing it. We present a case study that shows how functional requirements can be successfully decomposed using scenarios. According to our approach, for each of the functional requirements on the compound system, a corresponding scenario is developed. The scenario descriptions include steps to be performed by the proposed system. For each of these steps one or more functions are developed that will enable that system to perform these steps. These functions correspond to functional requirements on the proposed system. As a consequence of our case study, we propose to use this approach for decomposing functional requirements whenever they are requirements on the compound system rather than the system that is to be built.