Use case modeling is widely used for specifying the functional requirements of software systems. This chapter describes how use case modeling can be applied to real-time embedded systems from both a systems engineering and a software engineering perspective. With use case modeling, the system is viewed as a black box, so that only the external characteristics of the system are considered. Both functional and nonfunctional requirements need to be described for embedded systems. Functional requirements address the functionality that the system needs to provide. Nonfunctional requirements, sometimes referred to as quality attributes, address quality of service goals for the system, which are particularly important for real-time embedded systems. Although use case modeling is typically only used for specifying functional requirements, this chapter describes how it can be extended to specify nonfunctional requirements. Several examples of use case modeling for embedded systems are given in this chapter.

Section 6.1 gives an overview of use case modeling. Section 6.2 then describes actors and their role in use case modeling from both systems engineering and software engineering perspectives. The important topic of how to identify use cases is covered in Section 6.3. Section 6.4 describes how to document use cases. Section 6.5 describes how to specify nonfunctional requirements, which is particularly important for real-time embedded systems. Section 6.6 gives detailed examples of use case descriptions from both systems engineering and software engineering perspectives. Section 6.7 then describes use case relationships; modeling with the include relationship is described in Section 6.8; modeling with the extend relationship is described in Section 6.9. Finally, use case packages for structuring large use case models are described in Section 6.10.

USE CASES

In the use case modeling approach, functional requirements are defined in terms of actors, which are external to the system, and use cases. A use case defines a sequence of interactions between one or more actors and the system. The use case model describes the functional requirements of the system in terms of the actors and use cases. In particular, the use case model considers the system as a black box and describes the interactions between the actor(s) and the system in a narrative textual form consisting of actor inputs and system responses.