Structured Systems Analysis and Design Method. SSADM with ConceptDraw DIAGRAM
Structured Systems Analysis and Design Method (SSADM) is the method which is used at projecting and analysis of information systems. This method was developed for government of Great Britain in the beginning of 80th of the past century. It was accepted as the national standard of Great Britain for information systems development in 1993. Thus SSADM usage is expedient for those who work with governmental organizations as this method is a standard for these organizations.
SSADM is based on the data flow diagrams. At the early stages of projecting at description of models (functional, informational and event-trigger) the top-down method is used.
At the description of data flows out of the system and into the system DFD, which denote boundaries of the system, are used.
At the description of data models LDS (Logical Data Structure) diagrams are used. LDS describes which data the system operates with. It is created for existing system and is added at the development of the new one.
For modelling events, happened in the system, ELN (Entity Life History) diagrams are used. These diagrams support states indications and the possibility of description not only consecutive but parallel or reiterative events and also description of the choice of events course.
ELN describes how data change in the system in the course of time at different variants of events.
Thus, the most important SSADM elements are flows modelling with the help of DFD, data logic modelling with the help of LDS and description of entities behavior.
Analysis, projecting and documenting of information system accordingly to SSADM involves 6 main stages.
Each stage is divided into several steps, which define tasks that should be fulfilled at the given stage. The system is studied for getting system requirements. Detailed project is created on the logic level and then transformed into physical project.
SSADM stages:
- Analysis of the existing system or estimation of practicability. It involves the analysis of the existing system and creation of DFD for visualization of known problems and system description. If the system is developed from the beginning than the projecting starts from definition of new system requirements.
- Requirements definition. On the basis of available data about the system new functions, which the system must execute are defined. Also boundaries of the future system and data which will be processed by the system are defined. Infological model of requirements is constructed.
- Definition of technical requirements and device equipment cost. Definition of the expected profit with the introduction of new functions.
- Development of logical data model. Specification of the list of functional requirements. After development of the logical project the adding of new requirements on SSADM is forbidden. Only correction of the existing requirements, their specification and concrete definition are allowed.
- Projecting of logical requirements. Specification of requirements.
- Physical projecting. Physical information model and specification to program elements are developed and optimized. Specifications to program elements are specified and documentation is drawn up.
The advantages of SSADM are the precise definition and support of so-called “non functional requirements”. Such requirements define the level of the quality with which the system must execute its functions. For example, access limitations, the mean time of lifelength to denial, time of the response, safety requirements, etc.
In comparison with other methods of information system projecting, particularly with E. Yordon and A. Young methods, SSADM is characterized with presence of clear consecution of steps at projecting.
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Pic.1. Structured Systems Analysis and Design Method solution
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