Software Design 2e - DIM

142Some design representationsFigure 7.11An STD describing the behaviour of an aircraft in an air traffic control zone.Use of the STDA major role for the STD is in modelling problem entities: although it can have a rolein building models of solutions, this is less significant. It is also particularly useful inmodelling some of the real-time needs of a system, hence its popularity in real-timevariants of design methods.The STD is a useful modelling tool, but as with all diagrammatical forms, it haslimitations. As the example of the vi editor has shown, if there are a lot of transitionsbetween a small number of states, the diagram can become very tangled. Similarly,large and complex systems result in large and complex diagrams, as the form does notlead directly to any form of hierarchical layering of diagrams. In the next section wedescribe another state-oriented form of description that overcomes the problem oflayering by adding a hierarchy to the states themselves.