Relationships between Common Graphical Representations in System Engineering

Most systems engineers today use graphical representations of a system to communicate its functional and data requirements. The most commonly used representations are the Function Flow Block Diagram (FFBD), Data Flow Diagram (DFD), N2 Chart, IDEF0 Diagram, Use Case, Sequence Diagram, Enhanced Function Flow Block Diagram, and Behavior Diagram. This paper discusses the characteristics of each and shows how they are related. When analyzed in the context of specifying functional control and data modeling, it appears that the FFBD and DFD representations are limiting, special cases of the Enhanced Function Flow Diagram and Behavior Diagram. The N2 Chart has the same capability as the DFD, with a more formal format. The IDEF0 is essentially an N2 Chart with some control definition (no constructs) capability. The IDEF0 has the capability to indicate the allocation of functions to system components. Thus, the Enhanced Function Flow Diagram and the Behavior Diagram features comprise a “parent/unified” set of graphical system representations. To achieve the same level of specification completeness, you would have to use an integrated set of the FFBD and one of the data models or augment the FFBD with a graphical representation of the data model, as was done at TRW (then called Function Sequence Diagrams).

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