Software Design 2e - DIM

324Jackson System Development (JSD)Figure 15.11The three-stage model of JSD as used in Sutcliffe (1988) and Cameron (1988a).nThe first two steps of the method as it is described in Figure 15.9 have been mergedin Figure 15.10. The change involved can be considered as being largely cosmetic,as the two steps are so closely related.n The function step of Figure 15.9 has evolved into two separate steps in Figure 15.10(the interactive function step and the information function step). We will examinethe roles of these in a little more detail later. This modification is a more significantdevelopment than the previous one, since it gives additional structure to the designprocess for a task that is generally seen as posing difficult problems for the designer.In Section 15.1 the JSD design process was also described in terms of the three-stageoutline subsequently used by both Sutcliffe (1988) and Cameron (1988a). These stagesdiffer slightly from the abstractions used in the other two forms, and the correspondingmapping of the design activities is shown in Figure 15.11. By comparing this withthe descriptions of the earlier forms, it can be seen that the modelling stage can beidentified as the first part of Jackson’s ‘specify model of reality’, while the networkstage comprises the latter part, together with Jackson’s ‘specify system functions’. (Theimplementation stage is common to all three.)Sutcliffe’s framework will be adopted for the outline description of the JSD processpart in this section, since this is the most highly evolved structural description.However, the choice of framework does not greatly affect the description of thedesigner’s activities, since these are based on the major steps of the method, which are,of course, essentially common to all the descriptions of the method.15.3.1 The modelling stageIn many ways, the role of this step corresponds (rather loosely) to the ‘analysis’ phaseof other methods such as SSA/SD, in that it is concerned with building up a ‘black box’model of the problem, rather than considering the form of a solution.