The Functionality Description diagram is based on the concept of Data Flow Diagrams (DFDs). A Data Flow Diagram shows where the data comes from, where it goes, where it is stored, and what happens to it along the way. The concept is that there are the only four things that can happen to data.
According to the MODAF specification, in the Functionality Description diagram, you do these things:
•Model the data flows that are input into (consumed by) and output by (produced by) each resource.
•Make sure that a resource’s required inputs are all satisfied; in other words, the functional connectivity is complete.
•Develop the functional decomposition to an appropriate level of detail.
The following image shows an example of a Functionality Description diagram.
In the NSV-4 Functionality Description diagram, you describe the flow of data between functions, the allocation of functions to resources, and the relationship of functions to operational activities. The NSV-4 diagrams that you build should model both the present process and the future process, depending upon the projects needs.
The NSV-4 Functionality Description diagram starts with a Context diagram. A Context diagram is the top-most NSV-4 diagram in a structured decomposition of diagrams that you build. The Context diagram is used to specify the overall task of the function that you are modeling. You place one Function symbol on the diagram, draw the externals and data that the function interfaces with, and create a child diagram to show more detail about the system.
