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Difference between Current and Required

During the early stages, the system's requirements are identified and the current business environment is modelled in terms of the processes carried out and the data structures involved. In stage 1, DFDs are used to produce detailed logical models of the current system. In stage 2, DFDs and LDSs are produced to support the final chosen option. The transition from stage 1 to stage 2 is a key part of SSADM. This is where we move from a logical model of the current system to a logical model of the required system, ie this is where the DFDs and LDSs have to be refined to cater for new/changed requirements.

So, our task is to carry out the following:

  • Document the current physical system
  • Convert this to the current logical system.

After we have converted it to its logical version, we can then derive our three views mentioned earlier.

To start the procedure off, we need to learn our first technique - a diagramming method called data flow modelling. This technique gives us a diagram of the system's operation, which is much easier to follow than the text description.

Data flow modelling is carried out in stages, or levels as they are known. We start with a top level that we call the Level 0 Data Flow Diagram (sometimes called the Context Data Flow Diagram). We then expand this into the Level 1 Data Flow Diagram and then further expand this into a set of Level 2 Data Flow Diagrams.

Next: DFD Symbols