Requirements Capture and System Design with Rational Statemate
Functional system design is based upon the principle of Functional Decomposition. Functional decomposition breaks down complex systems into a hierarchical structure of simpler parts. It ensures that the role of the identified sub-functions is clear and distinct from the other sub-functions.
Hierarchy Level 0 is the starting point for a Top-Down system approach. It defines the system context, i.e. the system boundary, the external data sources and sinks (“External / Environment Activities”), and the respective information flows.
Hierarchy Level 1 describes the system in terms of high-level functions or subsystems with their logical interfaces. They should be encapsulated and re-usable entities.
Hierarchy Level 2 is the functional decomposition of the respective functional modules or subsystems identified at hierarchy level 1. Starting at this level, state-based behavior might be added through layers of additional decomposition (see the following figure), describing the relationship between the identified functional blocks and respective system states.
Typical Rational Statemate Structure of Hierarchy Level 2 with State-based Behavior Embedded in the Control ActivityExperience shows that the system functionality should be completely defined at hierarchy level 2. This does not mean that the system is completely specified at this level. Rather, the structural outline of the system should be captured in the same way as a table of contents shows the outline of a document.
Typically after decomposition level 2 - latest after decomposition level 3 - the system design process (i.e. the definition of the high-level architecture) might be started.
The outlined top-down system approach is the most frequently used in systems design. But there are also approaches, which start at hierarchy level 2 (“Component-Level”) and then iteratively design the system bottom-up / top-down. An example for this is the feature-based design approach described in the Feature-based System Design Approach section.