IT3205: Fundamentals of

Software Engineering
(Compulsory)

BIT – 2nd Year

Semester 3
IT3205 - Software Design

IT3205: Fundamentals of Software

Engineering

Software Design

Duration: 8 hours

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Learning Objectives
• Describe the important software design issues
and concepts.
• Compare different approaches to software
design.
• Identify suitable design approaches for a
problem.

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Detailed Syllabus
4.1 Design concepts
4.1.1 Abstraction
4.1.2 Architecture
4.1.3 Patterns
4.1.4 Modularity
4.1.4.1 Cohesion
4.1.4.2 Coupling
4.1.5 Information hiding
4.1.6 Functional independence
4.1.7 Refinement

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Detailed Syllabus
4.2 Architectural design
4.2.1 Repository model
4.2.2 Client-server model
4.2.3 Layered model
4.2.4 Modular decomposition

4.3 Procedural design using structured methods

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Detailed Syllabus
4.4 User Interface design
4.4.1. Human-computer interaction
4.4.2. Information presentation
4.4.3. Interface evaluation

4.5 Design notations

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What is Software Design ?

• Design is the process of translating the
requirements in to a meaningful engineering
representation that can be implemented.
• In the context of software engineering, design
focuses on transforming the requirements into
an implementable version of the software
system.
• Design stage has the greatest influence on
software quality.
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Software Design – Why it is important?

• A good design
– is the key for a successful software system.
– allows easy maintenance of a system.
– allows to achieve non-functional requirements
such as reliability, performance, reusability and
portability.
– facilitates the development and management
processes of a software project.

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Design activities
• Identification of the software architecture
• Architectural design
– Identification of the sub systems and components
• Data design
– Organizing the data so as to facilitate effective utilization
• Procedural design
– How inputs are transformed into outputs
• Interface design
– How to make the system user friendly
• Algorithm design
• Design specification

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4.1 DESIGN CONCEPTS

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Design Concepts
• Abstraction
– Permits one to concentrate on a problem at some level of generalization
without regard to irrelevant low level details
• Stepwise refinement and partitioning
– Increasing level of detail by allocating functionality to modules
• Modularity
– Self contained and loosely coupled software components
• Information Hiding & Encapsulation
– Modules should be specified and designed so that code / data contained in a
module is directly inaccessible to other modules. Protecting information from
direct access by other modules and providing access to this information
through well defined interfaces is called Encapsulation.
• Polymorphism
– Modules should be flexible and promote reusability

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Abstraction
• This is an intellectual tool (a psychological notion)
which permits one to concentrate on a problem at
some level of generalization without regard to
irrelevant low level details
• Abstraction allows us to proceed with the
development work without been held up in low-level
implementation details (yet to be discovered)
• Two forms of abstraction
– Procedural abstraction
– Data abstraction

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Abstraction
Example:
Develop software that will perform 2-D drafting (CAD)
Abstraction 1
Software will include a computer graphics interface which will enable
the draftsperson to see a drawing and to communicate with it via a
mouse. All line and curve drawing, geometric computations will be
performed by the CAD software. Drawing will be stored in a drawings
file.

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Abstraction
Abstraction 2
CAD software tasks:
user interaction task;
2-D drawing task; Procedural Abstraction
graphics display task;
drawing file management task;
End.

Data Abstraction = defining a data object at different levels.

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Software Architecture
• Overall structure of the software components
and the ways in which that structure provides
conceptual integrity for a system.

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Design Patterns
• When reusing software components, the developer
is constrained by the design decisions that have been
made by the implementers of these components.
• If the design decisions of the reusable components
conflicts whit the current requirements the
reusability will be impossible or the developed
system will become inefficient.
• One way to solve the problem is to reuse more
abstract designs that do not include implementation
details

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Modularity
• Software is divided into separately named,
addressable components called modules.
• The complexity of a program depends on
modularity.
Let C(x) = a measure of complexity and P1 and P2 be problems,
E(x) = a measure of effort to solve
If C(P1) > C(P2) then
E(P1) > E(P2)
Also, C(P1+P2) > C(P1) + C(P2)
Therefore, E(P1+P2) > E(P1) + E(P2)

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Modularity
• Modularity facilitates
– the development process
– the maintenance process
– the project management process
– reusability

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How many modules?

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Module Coupling
• A measure of the strength of the interconnections
between system components.
• Loose coupling means component changes are likely
to affect other components.
• Shared variables or control information exchange
lead to tight coupling.
• Loose coupling can be achieved by component
communication via parameters or message passing.

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Levels of Coupling
• Data Coupling
– Data is passed from one module to another using arguments
• Stamp Coupling
– More data than necessary is passed via arguments.
Eg. Pass the whole record instead of just the field being changed.
• Control Coupling
– A flag is passed from one module to another affecting the functionality
of the second module
• External Coupling
– Coupling with the environment
(eg. Data files, other programs etc.).

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Levels of Coupling
• Common Coupling
– Occurs when modules access the same global data

MODULE A MODULE B

MODULE C MODULE D

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Levels of Coupling
• Content Coupling
– One module directly affects the working of another. Calling module
can modify the called module or refer to an internally defined data
element.

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Levels of Coupling
• Object Coupling
– Object oriented systems are loosely coupled. No shared state and
objects communicate using message passing. Object coupling occurs
when a class inherits attributes and methods of another class.
Changes to super class propagate to all sub classes.

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Coupling
• Coupling should be minimized.
• Loosely coupled modules facilitate:
– Maintenance
– Development
– Reusability

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Module Cohesion
• Interaction within a module. A measure of
how well a component fits together.
• High cohesion – A component should
implement a single logical entity or function
• Cohesion is a desirable design component
attribute as when a change has to be made, it
is localized in a single component.

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Levels of Cohesion
• Object Cohesion
– Occurs when a single entity is represented by the object and all
operations on the object, and no others are included within it. This is
the strongest type of cohesion and should be aimed by the designer.
• Functional Cohesion
– Occurs when all the elements of the module combine to complete one
specific function. This also strong cohesion and should be
recommended.
• Sequential Cohesion
– Occurs when the activities (more than one purpose to the function)
combine such that the output of one activity is the input to the next.
Not as good as functional cohesion but still acceptable.

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Levels of Cohesion
• Communicational Cohesion
– Occurs when a module performs a number of activities on the same
input or output data.

For example customer maintenance functions in a business

information system such as add customer, delete customer, update
customer details and print customer details exhibits communicational
cohesion because they all operate on a customer file.

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Levels of Cohesion
• Procedural Cohesion
– Occurs when a modules’ internal activities bear little relationship to
one another but control flows one to another in sequence.

class MakeCake {
void addIngredients() { ... }
void mix() { ... }
void bake() { ... } }

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Levels of Cohesion
• Temporal Cohesion
– Occurs when functionality is grouped simply because it occurs at the
same time. For example house keeping tasks at the start and end of an
application.

class InitFuns {
void initDisk() { ... }
void initPrinter() { ... }
void initMonitor() { ... } }

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Levels of Cohesion
• Logical Cohesion
– Occurs when functionality is grouped by type. For example all creates
together, all updates together etc. This should be avoided at all cost.

class AreaFuns {
double circleArea() { ... }
double rectangleArea() { ... }
double triangleArea() { ... }}

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Levels of Cohesion
• Coincidental Cohesion
– Occurs when functionality is grouped randomly. Not even to be
considered as an option in design.

class MyFuns {
void initPrinter() { ... }
double calcInterest() { ... }
Date getDate() { ... }}

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Information Hiding
• The principle of Information Hiding suggests that modules be
characterized by design decisions that (each) hides from all others. In
other words modules should be specified and designed so that
information (procedure and data) contained within a module is directly
inaccessible to other modules.
• However the modules should communicates using well defined interfaces.
Protecting information from direct access by other modules and providing
access to this information through well defined interfaces is called
Encapsulation.
• Because most data and procedure are hidden from other parts of the
software, inadvertent errors introduced during modification are less likely
to propagate to other locations within the software.

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Encapsulation
• Encapsulation is a technique for minimizing
interdependencies among separately written
modules by defining strict external interfaces.
• The external interface acts as a contract between a
module and its clients.
• If clients only depend on the interface, modules can
be re-implemented without affecting the client.
• Thus the effects of changes can be confined.

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Functional Independence
• Achieved by developing modules with single-
minded purpose and an aversion to excessive
interaction with other models

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Refinement
• Process of elaboration where the designer
provides successively more detail for each
design component

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Design with reuse

• Another important design consideration is reusability
of software components. Designing reusable
software components is extremely valuable.
• Some benefits of software reuse are;
– Increased reliability
– Reduced process risks
– Effective use of specialists
– Standards compliance
– Accelerated Development

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4.2 ARCHITECTURAL DESIGN

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Software Architectural Design

• The architectural design process is concerned with
establishing a basic structured framework for a system. It
involves identifying the major components of the system and
the communications between these components.
• Large systems are always decomposed into subsystems that
provide some related set of services. The initial design process
of identifying these sub-systems and establishing a framework
for subsystem control and communication is called
architectural design and the output of this design process is a
description of the software architecture.

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Sub-Systems and Components

• Sub- Systems- A Sub-system is a system in its own right whose
operations are not depend on the services proved by the
other sub-systems. Sub systems are composed of modules
(components) and have defined interfaces which are used for
communication with other sub-systems.
• Components– A component (module) is normally a system
component that provides one or more services to other
modules. It makes use of services provided by other modules.
It is not normally considered as an independent system.
Modules are usually composed from a number of other,
simpler system components.

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Software components - An Example

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Software components - An Example

• GetPDfile –A service to retrieve the printer description file for
a printer type
• PrinterInt –A service that transfers commands to a specified
printer.
• Print –A service to print a document
• GetQueue –Discover the state of a print queue
• Remove –Remove a job from the queue
• Transfer –Transfer a job to another queue
• Register -Register a printer with the printing service
component
• Unregister –unregister a printer

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Architectural Design Process

• System structuring
– The system is structured into a number of principal sub-
systems where a sub-system is an independent software
unit. Communications between sub-systems are identified.
• Control modelling
– A general model of the control relationships between the
parts of the systems is established.
• Modular decomposition
– Each identified sub-system is decomposed into modules.
The architect must decide on the types of module and
their interconnections.
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Architectural Design – An example

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Repository Model
• Sub-systems making up a system must exchange information
so that they can work together effectively. One approach is to
keep all shared data in a central database that can be
accessed by all sub-systems. A system model based on a
shared database is called repository model.

• This model is suited to applications where data is generated

by one sub-system and used by another. Examples of this type
of systems include command and control systems,
management information systems CAD systems and CASE
tools.

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Repository Model - An example

The architecture of an integrated CASE tool.

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Repository Model - Advantages

• It is efficient way to share large amount of data. There os no
need to transmit data explicitly from one sub-system to
another.
• Activities such as backup recovery, access control and
recovery from error are centralized. They are the
responsibility of the repository manager. Tools can focus on
their principal function rather than be concerned with these
issue.
• The model of sharing is visible through the repository schema.
It is straight forward to integrate new tools given that they are
compatible with the agreed data model.

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Repository Model - Disadvantages

• Sub-systems must agree on a repository data model.
Inevitably, this is a compromise between the specific needs of
each tool. Performance may be adversely affected by this
compromise. It may be difficult or impossible to integrate new
sub-systems if their data models do not fit the agreed
schema.
• Evolution may be difficult as a large volume of information is
generated according to an agreed data model. Translating this
to a new model will certainly be expensive.
• Different sub-systems may have different requirements for
security, recovery and backup policies. The repository model
forces the same policy on all sub-systems.

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Client-Server Model
• The client-server architectural model is a distributed system
model which shows how data and processing are distributed
across a range of processors.
• The major components of the model are:
1. A set of stand-alone servers which offer services to other
subsystems. Examples of servers are print servers, web servers and
data base servers.
2. A set of clients that call on the services offered by the servers.
Theses are normally sub-systems in their own right. There may be
several instances of a client program executing concurrently.
3. A network which allows the clients to access these services.

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Client-Server Model – An Example

The above system is multi-user hypertext system to provide a film and

photograph library. In this system, there are several severs which manage and
display the different type of media. Video frames need to be transmitted
quickly and in synchrony but at relatively low resolution. They may be
compressed in a store. Still pictures, however, must be sent at a high
resolution. The catalogue must be able to deal with a variety of queries and
provide links into the hypertext information systems. The client program is
simply an integrated user interface of these services.

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Layered Model
• The layered model organizes the system in to layers
by modeling the interfacing of sub systems.
• Each layer provides a set of services.
• This model is also called abstract machine model.
• Each layer defines an abstract machine language is
used to implement the next level of abstract
machine.

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Layered model of a version management system

Configuration management layer

Object management system layer

Database system layer

Operating system layer

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Layered Model
• An example for layered model is OSI reference model
for network protocols.
• This model support the incremental development of
systems because some of the services provided by
the layer is made available to users while the layer is
being developing.

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Modular Decomposition
• After a structural architecture has been
designed, the next stage of architectural
design process is the decomposition of sub-
system in to modules.
• There is not a rigid distinction between system
decomposition and modular decomposition.

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Modular Decomposition
• We consider two modules which may be used
when decomposing a sub-system into
modules.
1. An object oriented model
• The system decomposed into a set of communicating
objects.
2. A data flow model
• The system decomposed into functional modules which
accept input data and transform it, in some way, to
output data. This is also called a pipeline approach.

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4.3 PROCEDURAL DESIGN USING

STRUCTURED METHODS

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Procedural design using structured methods

• For the component-level design for conventional
software components has proposed a set of
constrained logical constructs from which any
program could be formed.
• Each construct had a predictable logical structure.
• Structured programming is a design technique that
constraints logic flow to three constructs that are
sequence, construct and repetition.

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Procedural design using structured methods

• The structured constructs reduces program
complexity and enhances readability, testability and
maintainability.
• These allow reader to recognize procedural elements
of a module rather than reading the design or code
line by line.

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4.4 USER INTERFACE DESIGN

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User Interface Design

Good user interface design is critical to the success of a system.
An interface that is difficult to use will, at best, result in a high
level of user errors. At worst, users will simply refuse to use the
software system irrespective of its functionality.
If information is presented in a confusing or misleading way,
users may misunderstand the meaning of information. They may
initiate a sequence of actions that corrupt data or even cause
catastrophic system failure.
The system should assist the user providing help facilities and
should guide the user in the case of occurrence of an error.

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Graphical User Interfaces

• Although text based interfaces are still widely used, especially
in legacy systems, computer users now expect application
systems to have some form of graphical user interface.
• The advantages of GUI are:
1. They are relatively easy to learn and use. Users with no computing
experience can learn to use the interface after a brief training
session.
2. The users have multiple screens (windows) for system interaction.
Switching from one task to another is possible without losing sight
of information generated during the first task.
3. Fast, full-screen interaction is possible with immediate access to
anywhere on the screen.

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Graphical User Interfaces - characteristics

• Windows
– Multiple windows allow different information to be displayed simultaneously
on the user’s screen
• Icons
– Icons represent different types of information. On some system icons
represents files, on others, icon represents processes.
• Menus
– Commands are selected from a menu rather than typed in a command
language.
• Pointing
– A pointing device such as a mouse is used for selecting choices from a menu
or indicating items of interest in a window.
• Graphics
– Graphical elements can be mixed with text on the same display.

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The user interface design process

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Important Design Considerations for Interfaces

• Response time
– too long and too short response times are unacceptable. Constancy in
response time helps the user to learn a pace for the interaction with
the computer
• Help
– help may be provided as either electronic manuals (add on help) or
integrated help (Microsoft office assistant)
• Error handling
– The error, cause and possible solutions should explain in simple terms.

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Important Design Considerations for Interfaces

• Adaptability
– with growing familiarity users may migrate from one user class to
another
• Familiarity
– The interface should use terms and concepts which are drawn from
the experience of the users
• Consistency
– Activating similar operations in the same way
• Options
– Alternative ways of doing things

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User interface design principles

• User familiarity
– The interface should use terms and concepts which are drawn from the
experience of the people who will make most use of the system.
• Consistency
– The interface should be consistent in that, wherever possible, comparable
operations should be activated in the same way.
• Recoverability
– The interface should include mechanisms to allow users to recover from
errors.
• User guidance
– The interface should provide meaningful feedback when errors occur and
provide context-sensitive user help facilities.
• User diversity
– The interface should provide appropriate interaction facilities for different
type of system user.

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Color in interface design

Some guidelines for effective use of color in user interfaces.
1. You should not use more than four or five separate colors in a window
and no more than seven in a system interface. Colors should be used
selectively and consistently.
2. Use color change to show a change in system status. If the display
changes color, this should mean that a significant event has occurred.
3. Use color coding to support the task which users are trying to perform. If
they have to identify anomalous instances, highlight these instances.
4. Be careful about color pairing. Some color combinations are not good for
the eye. (eg. Red and Blue)
5. Use color coding in a useful and consistent way. If one part of a system
displays error messages in red, then red should not be used for anything
else.

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Human computer Interaction

• This can be seen when a user enter data
into a computer system.
• There are number of approaches that
enhance user interaction.

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Human computer Interaction

• Direct interaction
– Involve a pointing device. User interact directly with objects on the
screen.
Eg: To delete a file, user may drag it directly to the trash can using mouse.
• Menu selection
– User has to select an option from a list of possibilities. It is often the
case that another screen object is selected at the same time and
command operates on that object. In this approach to delete a file,
user select the file menu and then select the delete command from
the list.
• Form fill
– User fills a form that includes fields and buttons. Some fields have
associated menus. Buttons initiate some actions on press.

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Human computer Interaction

• Command language
– User give necessary commands and parameters to instruct the
computer to perform a specific activity.
Eg: To delete a file, user issues delete command with the file name as
a parameter.

• Natural language
– User issues commands in natural language. To delete a file, user may
type ”Delete file name xxx”.

The above styles can be mixed in an application.

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Interaction Styles - Advantages and Disadvantages

Interaction Style Main Advantage Main Application
Disadvantage example
Direct manipulation Fast and intuitive May be hard to -Video games and
interaction. implement. Only CAD system
Easy to learn. suitable where
there is a visual
metaphor for tasks
and objects.
Menu selection Avoid user error. Show for -Most general
Required little experienced user. purpose systems
typing. Can be complex if
many menu
options.
Form fill-in Simple data entry. Takes up lot of -Stock control
Easy to learn. screen space. -Personal loan
processing

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Interaction Styles - Advantages and Disadvantages

Interaction Style Main Advantage Main Application
Disadvantage example
Command language Powerful and Hard to learn. -Operating systems
flexible. Poor error -Library information
management. retrieval systems.
Natural language Accessible to casual Requiring more -Time table systems
users. typing. -www information
Easily expanded. Natural language retrieval systems
understanding
systems are
unreliable.

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Information Presentation
• All interactive systems have to provide some way of
presenting information to users.
• The information presentation may simply be a direct
representation of the input information or it may present the
information graphically.
• It is a good system design practice to keep software required
for information presentation separate from the information
itself.
• To some extend, this contradicts object-oriented philosophy
which suggest that operation on data should be defined with
the data itself.

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Information Presentation

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Interface Evaluation
• Interface evaluation is the process of
accessing the usability of an interface and
checking that it meets user requirements.
• Systematic evaluation of a user interface
design can be as expensive process involving
cognitive scientist and graphics designers.
• An evaluation should be conducted against a
usability specification based on usability
attributes as shown below.
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Interface Evaluation
Attribute Description
Learn ability How long does it take a new user to
become productive with the system?
Speed of operation How well does the system response to
match the user’s work practices?
Robustness How tolerant is the system of user error?
Recoverability How good is the system at recovering from
user errors?
Adaptability How closely is the system tied to a single
middle of work?

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4.5 DESIGN NOTATIONS

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Design Notations
• Graphical design notation
– The activity diagram allows a designer to
represent all elements of structured
programming.
– Activity diagram is the descendent of flowchart.

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Design Notations
• Graphical design notation
– Flow chart is simple pictorially

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Tabular Design Notation

• Decision tables provides a notation to translate
actions and conditions into a tabular form.
• Following steps are applied to develop a decision
table.
1. List all actions that can be associated with a specific
procedure.
2. List all conditions during execution of the procedure.
3. Associate specific sets of conditions with specific actions,
eliminating impossible combinations of conditions.

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Tabular Design Notation

• Define rules by indicating what action(s) occurs for a set of conditions.

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Program Design Language

• This is also known as structured English or pscudo code.
• This uses vocabulary of one language (English) and overall
syntax (structured programming language) of another.
• PDL actually is not a programming language due to the use of
narrative text.
• There are tools that can translate PDL into programming
language.

Program design language has been found as the best design

notation.

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