Software Design 2e - DIM

326Jackson System Development (JSD)Figure 15.12 The JSD model at the end of the modelling stage. The notation is intended toindicate many instances of entities 2 and 3.15.3.2 The network stageThe initial model phaseThis phase involves the designer in linking the entities defined in the first step, andbeginning the construction of the initial model of the system as a whole. Not surprisingly,as this initial model is concerned with modelling the interactions between thevarious entities, it produces a combination of SSDs and ESDs, with the added functionalityarising from the fact that the operations of this step are contained in the SSDs.The task of creating a model begins with the designer seeking to find the input thatis required to ‘trigger’ each action of an entity that has been identified in the first step.Each such input will be either:nnan input corresponding to an event that arises externally to the system – for example,the radar detects a new aircraft in the control zone;an input generated internally by the system – for example, when interest is addedto the bank account every six months.This step is concerned with identifying instances of the first group of inputs, while themodelling task involving the second group forms the subject of the next phase(through the interactive function step).The procedure for identifying the inputs is first to identify the external actions ofthe model, and then to determine how the corresponding event for each one of themwill be detected in the real (external) world. For example, for the ATC system, we cansee that a new aircraft entity will usually be instantiated when the radar detects a newsignal in the area being controlled, and so this external event can be linked with a particularaction of the entity (in this case, the ‘action’ of being created).A part of this task of adding inputs (and outputs) to the model processes involvesthe designer in choosing the forms that these will take (data stream or state vector).