Software Requirements
Engineering
Elaboration Process
Lecture-15
Recap
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Object Oriented Analysis
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Behavioral Modeling
Today’s lecture
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Structure analysis
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Flow oriented modeling
Flow-Oriented Modeling
What is a Data Flow Diagram?
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Known as DFDs
A way to model a real world situation
They are the interface between the real
world activities and an understanding of
how this can be converted into a computer
system.
Why do we use DFDs?
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It is a way of taking the physical view and
converting it into a logical view.
The physical view - all documents
involved
The logical view - the data they contain
Their main purpose is to communicate
with the user, the analyst’s
understanding of the scope of the
required system
Where do they fit in?
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Life Cycle Phases
Planning
Feasibility Study
Analysis
Design
Code and Unit test
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Analysis (What do we do?)
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Fact finding
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investigate business process and
the current system
modelling the current and
required systems
deliverables 
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requirements specification
logical models of the required
system
Data Flow Diagrams (DFD)
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DFDs describe the flow of data or information into and
out of a system
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what does the system do to the data?
A DFD is a graphic representation of the flow of data or
information through a system
Decomposition and Abstraction
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Decomposition - Divide and subdivide
into manageable size problems
Abstraction - Concentrate on the
important issues and ignore the
irrelevant
4 Main Elements
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external entity - people or organisations that send
data into the system or receive data from the system
process - models what happens to the data i.e.
transforms incoming data into outgoing data
data store - represents permanent data that is used
by the system
data flow - models the actual flow of the data
between the other elements
Outside Entity
Is anything outside the system that is of
interest to the system. Can be a person,
a company or another system.
Customer
a
Outside entity shows the Name and a
lowercase alpha character is used to
uniquely identify it.
Customer
a
If an outside entity is repeated
for the purpose of neat layout a
line is added across the top.
Data Flow
Is shown by a line with an arrowhead,
indicating the direction of the flow of
data. Each data flow should be named to
indicate what data is being passed.
Nouns or adjectives only no verbs are
permitted.
The Levels
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Context - Overview - contains only one
process
Level 1 - Utilises all four elements
Level 2 - A breakdown of a level 1 process
Level 3 - A breakdown of a level 2 process
There is no rule as to how many levels of
DFD that can be used.
Rules
Sequence not important - getting the Process correct is
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Context or Level 0 - Identifies the system/
boundary/External Links
Level 1 - Overview of function
Level 2 - Breakdown to Understand
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Hard to know where to stop
Rule of Thumb
If there are more than 8 data flows break it
Process of Identifying major Processes
The Procedure for Constructing DFD’s
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Draw a document flow diagram of the current
situation
Draw a systems boundary around the agencies that
are part of the system
Draw a Context Diagram
Identify processes in the system
Complete the level 1 Current Physical DFD
Notation
• Data Flow
• Process
• External Entity
• Data Store
Level 0 - Context Diagram
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models system as one process which represents scope of
the system
identifies external entities and related inputs and outputs
Level 1 - overview diagram
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gives overview of full system
identifies major processes and data flows between
them
identifies data stores that are used by the major
processes
boundary of level 1 is the context diagram
Level 2 - detailed diagram
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level 1 process is expanded into more detail
each process in level 1 is decomposed to show its
constituent processes
boundary of level 2 is the level 1 process
Other Notation
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Duplicates marked by diagonal line in corner
System Boundary
Elementary Processes - star in corner
Process that is levelled - dots on top
Rules for DFDs
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Numbering
Labelling
Balancing
5
Numbering
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On level 1 processes are numbered 1,2,3…
On level 2 processes are numbered x.1, x.2, x.3…
where x is the number of the parent level 1 process
Number is used to uniquely identify process not to
represent any order of processing
Data store numbers usually D1, D2, D3...
Labelling
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Process label - short description of what the process
does, e.g. Price order
Data flow label - noun representing the data flowing
through it e.g. Customer payment
Data store label - describes the type of data stored
Make labels as meaningful as possible
Balancing and data stores
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Balancing
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any data flows entering or leaving a parent level must by
equivalent to those on the child level
Data stores
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data stores that are local to a process need not be
included until the process is expanded
Identifying operations
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The SafeHome security function enables the homeowner to configure the security
system when it is installed, monitors all sensors connected to the security system, and
interacts with the homeowner through the Internet, a PC, or a control panel.
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During installation, the SafeHome PC is used to program and configure the system. Each
sensor is assigned a number and type, a master password is programmed for arming and
disarming the system, and telephone number(s) are input for dialing when a sensor event
occurs.
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When a sensor event is recognized, the software invokes an audible alarm attached to
the system. After a delay time that is specified by the homeowner during system
configuration activities, the software dials a telephone number of a monitoring service,
provides information about the location, reporting the nature of the event that has been
detected. The telephone number will be redialed every 20 seconds until a telephone
connection is obtained.
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The homeowner receives security information via a control panel, the PC, or a browser,
collectively called an interface. The interface displays prompting messages and system
status information on the control panel, the PC, or the browser window. Homeowner
interaction takes the following form…
Data Flow Diagram
Context-level DFD for SafeHome security function