Tools and TechniquesOf

Structured Systems Analysis and Design

Structured Systems Analysis and Design

Structured Systems Analysis and Design Method (SSADM) is a systems approach to

the analysis and design of information systems.

History

1980: Central Computer and Telecommunications Agency (CCTA) evaluate analysis and design methods.

1983: SSADM made mandatory for all new information system developments

1984: Version 2 of SSADM released

1986: Version 3 of SSADM released.

1990: Version 4 launched

1995: SSADM V4+ announced, V4.3 launched

Objectives

• Improvements in communication.

• Ensure the maintainability of the system to be analyzed.

• Useful tools for representing the system.

• Must be able to effectively partition a complex problem.

Benefits of SSADM

• Usability • Timelines• Respond to changes in the business

environment• Effective use of skills• Better quality• Improvement of productivity• Cuts costs

Disadvantages of SSADM

• SSADM puts special emphasis on the analysis of the system and its documentation. This causes the danger of over-analysing, which can be very time and cost consuming. Due to various types of description methods, checks of consistence cannot be carried out. Especially with large systems, the outline diagram can become very unclear, because all relevant data flows have to be included.

SSADM Tools

• Data Flow Diagram (DFD).• Data Dictionary. • Structured English. • Decision Trees. • Decision Tables.

Data Flow Diagram:

• Shows processes and flow of data in and out of these processes.

• Contains 5 graphic symbols.• Can be used to show logical and physical .• Still used today to document business and/or

other processes.• Data flow refers to the movement of data

through a system from entry to destination

Data Flow Diagram: Signs

• Symbol:

• Meaning: Process

• Description: A series of one or more steps that converts inputs to outputs. Each process is followed by a mini-spec

Data Flow Diagram: Signs

• Symbol:

• Meaning: Data Flow

• Description: Shows a data path (flow of data)

Data Flow Diagram: Signs

• Symbol:

• Meaning: External Agent.

• Description: A source or sink of data. Lies outside the system

Data Flow Diagram: Signs

• Symbol:

• Meaning: Data Store.

• Description: Data at rest, usually a file or database table

Data Flow Diagram: Signs

• Symbol:

• Meaning: Real-time link

• Description: A communication link. This symbol added later. When ???

Rules for drawing the DFD:--

• Process should be named and numbered . Name should represent the process.

• The direction of flow is from top to bottom and from left to right.

• Sources cannot leak data directly to a data store.

• The name of data stores, sources and destinations are written in capital letter. Process and data flow names have the first letter capitalized.

cont…..

Rules for drawing the DFD:--

• A data store cannot pass the data directly to the destination.

• Each sub-system must be a process on the next higher level diagram.

• A data flow out of a data store is read only.• Data cannot flow from one data store to another.

Levels Of DFD

The level-0 partitions the entire system. It has only one process (the system), and from it, the data flows to/from the external agents.

The level 1 diagram- decomposes the system into 3 processes: Schedule Courses, Enrol Student, and Product Class List.

The level 2 diagram decomposes the Schedule Course process into 3 sub-process.

Note that the data flow Schedule Data from level 0 is broken into 2 sub data flows in the level 1: Course and Available Faculty. Also note that the Offered Course file is still preserved.

DATA DICTIONARY

Meaning

Data dictionary contains definition of “DATA”, used in system

It contains accurate definition of all

DFDs, data elements & data structure

Features Of Data Dictionary:

Structured repository of dataReference Work of dataSupport Documentation Improve CommunicationDescribe Interrelationship

Data Dictionary Contains Mainly Three Items:

Data element

Data structure

Data flow and data store

Structured English

Structured English is the use of the English language with the syntax of structured programming

structured English aims at getting the benefits of both the programming logic and natural language. Program logic helps to attain precision while natural language helps in getting the convenience of spoken languages.

cont…..

Structured English consists of the following elements:

Operation statements written as English phrases executed from the top down

Conditional blocks indicated by keywords such as IF, THEN, and ELSE

Repetition blocks indicated by keywords such as DO, WHILE, and UNTIL

Guidelines when writing Structured English

• Statements should be clear and unambiguous.

• Use one line per logical element.• All logic should be expressed in

operational, conditional, and repetition blocks.

• Logical blocks should be indented to show relationship.

• Keywords should be capitalized.

Example

• If customer purchase > Rs 1000

Then give 10% discount

Else do not provide any discount

End- if

Decision tree

• A decision tree is a decision support tool that uses a tree-like graph or model of decisions and their possible consequences

A decision Tree consists of 3 types of nodes:-

1. Decision nodes - commonly represented by squares2. Chance nodes - represented by circles3. End nodes - represented by triangles

Advantages

• Are simple to understand and interpret.

• Have value even with little hard data.

• Can be combined with other decision techniques.

Decision table

Decision tables are a precise yet compact way to model complicated logic. Decision tables, like if-then-else and switch-case statements, associate conditions with actions to perform.

Decision table Structure

• The four quadrants• Conditions• Condition alternatives• Actions• Action entries

ExamplePrinter trouble shooter

Rules

Conditions

Printer does not print Y Y Y Y N N N N

A red light is flashing Y Y N N Y Y N N

Printer is unrecognised Y N Y N Y N Y N

Actions

Check the power cable     X          

Check the printer-computer cable

X   X          

Ensure printer software is installed

X   X   X   X  

Check/replace ink X X     X X    

Check for paper jam   X   X        

SSADM Techniques

The 3 most important techniques that are used in SSADM are:

Logical Data Modelling.

Data Flow Modelling.

Entity Behaviour Modelling.

Logical Data Modelling

This is the process of identifying, modelling and documenting the data requirements of the system being designed. The data are separated into entities (things about which a business needs to record information) and relationships (the associations between the entities).

Data Flow Modelling

• This is the process of identifying, modelling and documenting how data moves around an information system. Data Flow Modelling examines processes (activities that transform data from one form to another), data stores (the holding areas for data), external entities (what sends data into a system or receives data from a system), and data flows (routes by which data can flow).

Entity Behaviour Modelling

• This is the process of identifying, modelling and documenting the events that affect each entity and the sequence in which these events occur.

How is SSADM Structured• SSADM consists of 5 main modules,

which are in turn broken down into a complex hierarchy of stages, steps and tasks.

• Feasibility Study.

• Requirements Analysis.

Cont…..

How is SSADM Structured

• Requirements Specification.

• Logical System Specification.

• Physical Design.

Conclusion

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