System Analysis and Design
System Design
- Mr. Ahmad Al-Ghoul
learning Objectives
 Discuss guidelines and objectives
for systems design
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System Analysis and Design
System Design
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Introduction
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If management decides to develop the system in-house,
then the transition to the systems design phase begins
In a smaller company, you will assigned full responsibility
for the design task
In larger company you will be a member of the design
team
System design is the first phase of the system
development life cycle in which you and the user develop
a concrete understanding of how the system will operate
Design phase
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Decide how to build the system
Create system requirements that describe technical details for
building the system
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Systems Design Guidelines
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Preparing for Systems Design Tasks
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It is essential to have an accurate and understandable
system requirements document
System requirements document is the starting point
for the system design phase
Errors, omissions, ambiguities, and other problems
will affect the quality of the finished product
Before proceeding to the design phase, you must be
certain that you performed a complete and accurate
system analysis and communicated the results in your
system requirements document
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Systems Design Guidelines
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The Relationship between Logical and Physical
Design
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The logical design defines the functions and features
of the system and the relationships among its
components
Logical design done during the system analysis phase
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Logical design includes
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Output that must be produced by the system
Input needed by the system
Process that must be performed by the system, without regard
to how tasks will be accomplished physically
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Systems Design Guidelines
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The Relationship between Logical and Physical Design
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The physical design of an information system is a plan for the actual
implementation of the system
The physical design is built on the system’s logical design and describes a
specific implementation
System design usually begins after completing system analysis phase
Some overlap is possible, you might return to fact-finding if you discover
that you overlooked an important issue, if user have significant new needs,
or if legal or governmental requirements change
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Physical design describes
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Input design
Output design
Interface design
Report design
Actual processes of entering, verifying, and storing data
Physical layout of data files
Sorting procedures
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Systems Design Guidelines [1]
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The systems analyst must understand the logical design of the system
before beginning the physical design of any one component
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Systems Design Guidelines
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Design phase is not a series of clearly
defined steps because the components of
the system are interdependent
You might start with one component, or
you can work on several components at
the same time
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System Design Objectives
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The goal of systems design is to build a system
that is effective, reliable, and maintainable
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A system is effective if it defined requirements and
constraints, accepted by users, and support the
organization’s business objectives
A system is reliable if it adequately handles errors,
input errors, processing errors, hardware failures, and
human mistakes.
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A system is maintainable if it is well designed, flexible,
and developed with future modifications in mind
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An approach to building a reliable system is to plan errors,
detect them as early as possible, allow for their correction, and
prevent them from damaging the system
Modification necessary to correct problems, to adapt to
changing user requirements, to enhance the system, and to
take advantage of changing technology
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System Design Objectives
System design considerations
 User Considerations
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System must be user friendly
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Carefully consider any point where users receive output
from, or provide input to, the system
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Anticipate future needs of the users, the system, and the
organization
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hard-coded solution is straightforward, but its not a good
choice when the organization want to add anther rules
Provide flexibility
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The user interface must be easy to learn and use
Input processes should be well documented, easy to follow,
intuitive, and forgiving of errors
Output should be attractive and easy to understand
there are many issues to take into consideration to make the
system more flexible
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System Design Objectives
System design considerations
 Data Considerations
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Data should be entered into the system where and
when it occurs because delays cause data errors
Data should be verified when entered to catch errors
immediately
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Automated methods of data entry should be used
whenever possible
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The system also should allow corrections at any time
Automated data entry methods reduce input errors and
improve employee productivity
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System Design Objectives
System design considerations
 Data Considerations
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Access for data entry should be controlled and all
entries or changes to critical data values should be
reported
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Dollar fields and many volume fields are considered critical
data fields
Critical data volume include the number of checks processed,
the number of insurance premium payments received
Report that trace the entry of and changes to critical data
values are called audit trails and are essential in every
system
Every instance of entry and change to data should be
logged
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System Design Objectives
System design considerations
 Data Considerations
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Data should be entered into a system only
once
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Data duplication should be avoided
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If there are some data needed by two systems,
you should design program interface between the
systems so data can transfer automatically
In some cases we need to store the same data
with different records, those data must stored
once, so if we need to update the data we will
update it once
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System Design Objectives
System design considerations
Architecture considerations
 Use a modular design
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In a modular design, you create individual processing component,
called modules, which connect to a higher level program or process
In structural design each module represents a specific process or sub
process shown in a DFD and documented in a process description
In object-oriented design, object classes are represented by code
modules
Design modules that perform a single function are easier to understand,
implement, and maintain
Independent modules provide greater flexibility because they can be
developed and tested individually
Modular design is helpful especially when developing large scalesystems
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System Design Objectives
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Design Trade-Offs
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Design goals often conflict with each other
In the system design phase, you must analyze
alternatives and weight trade-offs
Most design trade-off decisions that you will face
come down to the basic conflict of quality versus cost
Avoid decisions that achieve short-term savings but
might mean higher costs later
Each trade-off must be considered individually, and
the final result must be acceptable to users, the
systems staff, and company management
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Sequence Summary
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System design is the first phase of the system
development life cycle in which you and the user develop
a concrete understanding of how the system will operate
System requirements document is the starting point for
the system design phase
The logical design defines the functions and features of
the system and the relationships among its components
The physical design of an information system is a plan
for the actual implementation of the system
The goal of systems design is to build a system that is
effective, reliable, and maintainable
Systems design guidelines and suggestions, including
user considerations, data considerations, and processing
considerations
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System Design
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Sequence Summary
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In this Sequence we have
 Described the tasks needed to prepare for
system design phase
 Explained the Relationship between Logical
and Physical Design
 Explained the system design objectives
 Explained the systems design guidelines and
suggestions, including user considerations,
data considerations, and architecture
considerations
 Described the design trade-offs
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Reference
[1] System Analysis and Design, Sixth
Edition
Authors: Gary B. Shelly, Thomas J.
Cashman and Harry J. Rosenblatt
Publisher: SHELLY CASHMAN SEWIES.
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System Analysis and Design
System Design
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