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This part of the POSD presentation analyses what is meant by the phrase, "representing the structure of systems". It thereby provides an introduction to any notation for representing the structure of systems, including POSD. As the examples in this section are simple, the notation used is not POSD, but is a simple derivative of existing notations. However the ideas presented carry over to POSD.
Click here to see how the composition structure of a simple system may be represented graphically. The example system is a supply process for the construction industry. Its component systems are human roles and the information and artifacts that they manipulate and share. For other examples the component systems might be software systems, hardware systems, and data structures.
One problem with the composition structure, shown above, is that it is not obvious which components in the structure are significant and which are insignificant. Click here for examples of what is meant by this statement.
The significance or insignificance of a component depends on the view taken of the structure of the system and this in turn depends on the view taken of the purpose of the system. Click here to see just three different structural views of the example system reflecting three different views of the purpose of the system.
Each structural view of a system can be represented by a hierarchical decomposition of that system. Click here to see the hierarchical decompositions representing the three structural views introduced above.
Why are hierarchical structures needed as well as the original composition structure? The first answer to this question is that the structure of a very complex system cannot be represented in one (composition structure) diagram (as this would be far too large and convoluted) and can only be captured in a structured set of diagrams. The second answer is that each hierarchical structure gives information not in the original composition structure. Structural information appearing high in the hierarchy can be assumed to be more important than that appearing lower down.
last revision: 21st June 1997