August 1997 SD206: Systems Design (Question 2) Suggested Solutions

2.	(a) Describe the five main aspects of structured design.		[5]
	One mark should be awarded for each of the following points:
		Structured design uses a definition of the problem to guide the definition of the solution.
		Structured design seeks to conquer the complexity of large systems by means if partitioning the system into 'black boxes', and by organising the black boxes into hierarchies suitable for computer implementation.
		Structured design uses tools, especially graphical ones, to render systems readily understandable.
		Structured design offers a set of strategies for developing a design solution from a well-defined statement of a problem.
		Structured design offers a set of criteria for evaluating the quality of a given solution with respect to the problem to be solved.

	(b) Describe the relationship between data flow diagrams and structure charts.		[3]
	A data flow diagram is a data flow analysis technique which uses symbols to create a pictorial representation of data movement within a given system.		[1]
	Structure charts are derived from a levelled set of data flow diagrams.		[1]
	A structure chart provides a detailed view of a low-level process on a data flow diagram.		[1]

	(c) Describe, with examples, six major steps which are required to transform a data flow diagram into a structure chart.		[12]
	1/2 mark should be awarded for correctly identifying each step. A further mark should be awarded for a satisfying elaboration of each step. In each case, 1/2 mark should be awarded for a well-chosen example.
		Step 1: Perform transaction analysis. Transactions are formed by partitioning the data flow diagram into small units. Transaction analysis is the technique of determining the transaction types of a system and using them as the units of design.
		Step 2: Determine the central transform. The central transform is the portion of the data flow diagram which contains the essential functions of the system, and is independent of any specific implementation of the input and output. The best way of determining the central transform is to identify the centre of the data flow diagram by removing its afferent and efferent branches.
		Step 3: Produce a first-cut structure chart. When the first-cut structure chart is being drafted, it is a good idea to visualise the algorithms for each module. If any of the modules appear complicated, then a different central transform should be identified.
		Step 4: Add modules and show data flow, Modules that perform the central functions are added. Each module should represent one self-contained function and be labelled with a descriptive name.
		Step 5: Write module details. The structure chart depicts the overall design of the program to be coded and tested, by showing all of the modules and the relationships between them. Details of the modules are specified in an algorithmic language such as pseudocode or structured English.
		Step 6: Conduct a structure walkthrough. After the module details are specified, a structured walkthrough should be conducted to determine if the structure chart is correctly designed.