Stages of Creating a DFD

The normal stages are

1. Model the current system as it is presently implemented. It is rare that a system is designed to do something entirely new. Normally the intention is

to make an existing system efficient by automating it
to streamline more than one system by integrating them
to replace a sequence of tasks with a new process

The DFD drawn at the end of this stage is called the Current Physical System.

The main characteristics of a Physical DFD are that a user will recognise how a system currently operates, warts and all. It should not attempt to improve the system, although it may document problems.

2. The essential activities of the system are extracted. What is now left is just the data, information stores and starts and destinations. What is removed is physical things like any IT used, the media on which the data is recorded and stored, and who does what. The idea is to focus attention on what is being done to the data and free the mind from such considerations as whether the data is paper-based or on an electronic file; whether the data is stored in a filing cabinet or on a disk; whether data is transmitted electronically or by post, and so on.

The DFD remaining is called the Current Logical System.

3. The current logical system is examined alongside a list of prioritised requirements and redesigned to satisfy those requirements subject to any resource constraints told to the designer by the person commissioning the system.

The new DFD is called the Required Logical System.

To move to a logical DFD you do things like:

Get rid of references to physical implementation

Remove references to 'Who', 'When', 'Where', and 'How' a job is done or Data is stored
Rename Data Stores so that the name reflects the content
Remove processes that reflect how a person does their job

Make minor improvements in the logic of stored data

Combine references to data stores using replicated data
Remove data stores which are only used because of inefficiencies in the present system

4. Eventually the new system takes physical shape according to factors like the existing IT platform it will have to operate on, the data model adopted by the organisation and organisational procedures favoured within the corporate culture.