INFORMATION SYSTEMS MODELLING

System Design

                The basic information model consists of three parts as shown below and the data dictionary. The external models represent the user views of the system. These are brought together into a single global conceptual model as illustrated below. The conceptual model does not take into consideration how the data and processes will be implemented on the computer. This is accomplished through the internal model (shown as 'schemas' and 'programs' in the diagram below)

Stages in IS modeling

As mentioned earlier, we do not design the internal model right away. A conceptual information model is first formulated and this is then translated into an internal model. Also we need to develop two conceptual models

(a) A conceptual data model and

(b) A conceptual process model.

These are then checked for consistency and, any necessary modifications are made to arrive at the data processing model. This is the first level information model. (the data model, the process model and the data processing model) otherwise called first level design.

Second level design is concerned with adapting the first level design to the specific requirements of the processing required. It may be that the first level design consists of so many relations that it would not achieve an acceptable level of performance if it were implemented in that form. Hence the model may need 'flexing' to accommodate the demands of the functions which the information system may support. The result of this is the second level design information model.

The final stage is the third level design where the model is mapped into the internal schemas required by the software being used to implement the system.

The process of moving form first to second to third level design may seem rather long but it is important for a number of reasons. The first level will provide a transaction-independent view of the information content of the system. However it will still be enterprise-dependent because the data model will only reflect the things of interest to that enterprise. The second level design is enterprise or transaction dependent, but is still independent of hardware and software. Hence the second level can be used to evaluate alternative implementations and assist in the selection of hardware and software. Third level design is enterprise, transaction and software dependent.

The process of design is shown in the diagram below. IT is important to note that this process does not equate the design of the entire information system. In particular it does not cover data on establishing the feasibility of an IS, nor on strategic decisions involved in selecting an information area. Similarly nothing is said about the design of forms, codes, screens and reports nor of the techniques for the analysis of the requirements of the users.

source : http://www.uh.edu/~mrana/try.htm