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System Design

System Design

System design can be explained and presented in narrative form. But the benefits of diagrammatic view
cannot be understated. This helps to give a snapshot of what the entire system looks like. Various
diagrammatic tools can be used while designing the system.
As an example consider the following DFD which indicates a simple process of recording transactions and
posting into general ledger
User/Accountant uses chart of accounts to access the relevant accounts in order to prepare different
vouchers according to requirements. The purpose behind this entire activity is to record various
transactions. The next step is posting of all these transactions in the system. This process updates the
general ledger.

19.1 Entity Relationship Diagram (ERD)
Another diagrammatical tool used in system design is ERD. ERD as shown below indicates simple
relationships. These relationships can be read as follows.
One department has one supervisor
A department may have more than one employees
An employee may be in more than one departments
An employee may not be working on any project but a project must have at least one employee working
on it
This is another form of ERD used to show the relations between various fields in files used to record
specific data.
Room ID
User ID
The above figure shows a hotel booking system. Various records have been kept for each entity.
However each entity shares a relationship with for logical purpose. For instance, the field for room ID
has been kept in reservation for access to further data. User information has been kept separate,
however link has been made to reservation, session and logs by making user ID common to all three
tables. Such kind of relationship helps in keeping
Reservation ID
Room ID
User ID
Start time
End time
Session ID
User ID
Session String
Time Stamp
Log ID
User ID
Time Stamp


19.2 Design of the information flow
It is a major step in the conceptual design. Following aspects should be covered
Flow of data & information and transformation points
The frequency and timing of flows
The extent of formality in these flows – input forms, report formats.

19.3 Design of data base
It involves determining scope and structure:
Scope – Whether the database is local or global. If interdependence of organizational units is
high, the data base has to be global in order to prevent sub-optimization of sub units. As it
becomes global, the cost of maintenance enhances.
Structure – refers to the ways data is stored in partitions and sequences. Various design
methodologies can be used for devising a suitable structure in accordance with the needs of the
organization and the new system.

19.4 Design of the User Interface
This phase involves determining the ways the information system will interact with the users. Some
elements are
Source Documents to capture raw data
Hard-copy output reports
Screen layouts
Inquiry screens
Interrogation languages for the data base
Graphics and colour displays
Voice output to guide users or answer queries
Screen layouts for manipulation by a light pen or mouse
Icons for pictorial representations
The design process begins with stratifying system users and then identifying their needs. e.g.
New users dealing with system infrequently,
Experts dealing regularly

19.5 Physical Design
The logical design is converted to physical design in this phase. The physical design involves breaking
up the logical design into units, which in turn can be decomposed further into implementation units
such as programs and modules.

Design of the Hardware/ Software Platform
New system requires new software and hardware not currently available in the organization.
For example
User workstations might have to be purchased to support an office automation system.
A minicomputer might have to be purchased to provide extra processing resources to the new

19.6 Program Development
The development phase involves converting design specifications into executable programs. Once the
analysis and design is complete, the software is either developed according to the needs or most suitable is
purchased. Similarly the specifications of the hardware are seen and acquisition is made according to the
situation. Primary procedural programming activities include
The creation and testing of source code
The refinement and finalization of test plans
Writing and reviewing program modules or components
Integration of Completed components with other components to ensure the components properly
interact. The process continues as component groups are progressively integrated and as interfaces
between component groups and other systems are tested.

19.7 Procedures Development
In this phase, following documents are prepared.
Technical manual – This is meant for the Data Base Management and highlights the system
infrastructure, inputs-outputs of the system and flows of system processes. Documents include
DFD’s (Data Flow Diagrams)
ERD’s (Entity Relationship Diagram)
Use cases, test cases

User manual
It defines the operations of the system in layman’s terms i.e.
Getting started with the software
Operating the software
These manuals are generally function related.

19.8 Testing
The purpose of this phase is to identify as far as possible any errors and deficiencies in the system prior to
its final release into production use. For instance errors in
User interface
Procedure manuals
Job design
Organizational structure design
In reality all system features cannot be checked at the outset. For instance, users might realize that the
system has inadequate procedures manual only after the system has been properly implemented.

Change Over
This phase comprises of those activities undertaken to replace the new system in operation from the
existing system. Following ways of change over can be undertaken
Abrupt change over – stop the existing system abruptly to shift over to new one
Phased change over – Both are run but output of both the systems is used since functions
performed are different.
Parallel change over – Both systems are run simultaneously for a period of time and output of
either of the systems is used. Functions performed by both are same.

19.9 Operations & Maintenance
The new system is run as a production system and is periodically modified to adjust for better functioning.
Following can be various forms of errors.
Removal of coding/logic errors – Logic errors discovered in the system are corrected.
Modifications / system rewrite – Changes in the system environment may necessitate system
Perfective maintenance – Changes might be made to improve processing efficiency.

19.10 Evaluating Waterfall
Arguments for water fall
Waterfall model places emphasis on documentation (such as requirements documents and design
documents) as well as source code.
Other methodologies which save time in software development can de-emphasize documentation. In
such methodologies project knowledge is stored mentally by team members. Should team members
leave, this knowledge is lost, and substantial loss of project knowledge may be difficult for a project to
recover from. Extreme Programming is an example which will be discussed later.
Waterfall model is preferred for its simple and arguably more disciplined approach. The model itself
progresses linearly through discrete, easily understandable and explainable "phases" and is thus easy to
It also provides easily mark able "milestones" in the development process. It is perhaps for this reason
that the waterfall model is used as a beginning example of a development model in many software
engineering texts and courses.

Arguments against water fall
It is argued that it is impossible to get one phase of a software product's lifecycle "perfected" before
moving on to the next phases and learning from them.
For example clients may not be aware of exactly what requirements they want before they see a working
prototype and can comment upon it - they may change their requirements constantly, and program
designers. This is an example of iterative model (to be discussed later)
Waterfall model advocates more reliance on fixed, static requirements. Designers may not be fully
aware of future implementation difficulties when writing a design for an unimplemented software
product. That is, it may become clear in the implementation phase that a particular area of program
functionality is extraordinarily difficult to implement.
Another problem is that the waterfall model assumes that the only role for users is in specifying
requirements, and that all requirements can be specified in advance. Unfortunately, requirements grow
and change throughout the process and beyond, calling for considerable feedback and iterative
consultation. Thus many other SDLC models have been developed. The choice of phases differs in
various standards and organizations.

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