Context diagrams & Data Flow Diagrams

A context diagram is a graphical representation of a system which must only use one process to represent the entire system and deliberately does not go into defining all the processes so as to prevent people getting bogged down in complex details at an early stage. There are only three symbols used in a context diagram:

• A circle to represent the system in terms of a single process. There will never be more than a single process in any context diagram,
• arrows to represent data flow,
• and a rectangle to represent any external entities affecting the system. There can be numerous external entities.

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Systems analysts draw data flow diagrams in several nested layers. A single process Context diagram can be expanded to show a more detailed data flow diagram. In the next level DFD the single process of the Context Diagram is broken down into several main processes and must include storage which previously was lumped inside the single process of the Context Diagram. Each successive layer is systematically broken down into more processes. This proces can continue until the required level of detail is found.

A data flow diagram is the next level up in detail from a context diagram. It will show processes, inputs, outputs and storage. The DFD shows the data flow between the processes within a system. A DFD can become as detailed as the user requires. The DFD will become increasingly detailed as the levels go higher. However each level will tend to focus on extending one or more of the processes rather than the entire system. The IPT course will only use level 0 (context diagram) DFD and a level 1 DFD.

The first level DFD shows the main processes within the system. Each of these processes can be broken into further processes.