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Text in bold is suggested wording and text is italics is commentary on the answer.

First, it is important to remember to answer each part of the question.  Make it clear which part of the question you are answering by using the question parts indicated (a, b, c and d). It is useful to start each part of your answer on a new page, allowing you to add extra detail after your initial answer. 

• Answer to part a
• Answer to part b

• Answer to part c

Part a

A data flow is a pipeline or stream of data which is produced by and/or consumed by a process or external entity.

A control flow is a message, signal or trigger which is sent from one process to another process. 

A control flow can either come a control process which is wanting to start or trigger a process or it can come from a process that has completed its task and is returning a control message back to a control process- for example, that it has completed its task satisfactorily or that it has had to abort its task because of errors.

A process or 'data transforming process' is a process which has a defined start and stop point, during which it consumes one or more incoming data flows and generates one or more outgoing data flows.

A process can be triggered by a control flow to initiate the process and, when complete, a return control flow may be generated.

A control process normally occurs once in a data flow diagram and is used to initiate the execution of data transforming processes in a DFD.

A control process initiates a data transforming process using a control flow.

Part b

Click here for the data flow diagram.

Click here for the state-transition diagram.

Part c

When creating any system model, analysts using the concept of 'abstraction' to deliberately omit certain detail in the early stages of analysis.

Therefore when initially creating a DFD, analysts will want to concentrate on 'what' a system must do, rather than 'how' it will do it.  Thus in an initial DFD model, only data flows and data transforming processes would be shown. 

It is only once a system has been understood (in terms of user need), that the analyst should start to think about 'how' it will undertake the required tasks, at which point control flows and control processes can be added.

When using the DFD technique, a number of models can be produced which describe different states of understanding of the system.

These models are:

• current physical specification
• current logical specification
• new logical specification
• new physical specification

Each model is a successor to the next.

The first two models are used to describe the current business situation, whilst the latter two describe the new situation.

The term 'physical' is used to describe 'how' a system will operate, whilst the term 'logical' is used to describe 'what' a system does/will do.

You would therefore only expect to see control flows and control processes in a new physical specification. 

It may be possible to also include control flows and control processes in a current physical specification, although the main aim is to get to a current logical specification and so there is no much point in describing how the old/current system works.

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