1. Data Flow Diagram (DFD) Overview
Example
Why used in SSADM
DFD conventions and components
Hierarchical structure
What for and when

2. Example of a Level 1 DFD Register New Customer * Record Deposits and
Account No
Bank D2
Accounts
Register New
Customer c
Amount of
Salary Paid
Customer details
Amount/
Balance
Employer *
Customer details
Ex Bank 2
Amount/
Balance b
Account
Record d
Manager
Deposits and
Clerk
Customer
Withdrawals
Notification of
Closure D1
Details
Deposit/
Withdrawal
Customer details Ex
Customer
Balance
Notification of
Closure
Notification of 3
Closure a
Customer a
Close
Customer
Accounts *

3. Representation and Communication
DFDs help to represent and communicate
information and data flows
processes that change or handle data
stored data
system boundaries
sources and recipients of information and data
At many levels of detail

4. Benefits of DFDs Provide a pictorial, non-technical representation
Are easy to understand
Quick to produce and easy to amend
Use limited number of symbols with specific meanings
concise system descriptions
Use a simple, top-down expansion
easy progression between levels of detail

5. DFD Components and Conventions
External Entity a
Customer
Represents sources and/or recipients of information or data outside the system described by the DFD
e.g. external persons or organisations, system users, physical systems in the same organisation, or other information systems
If duplicated on same diagram a
Customer

6. DFD Components and Conventions (continued)
Process
Processes transform or handle data
may be clerical, automated or a mixture
can be decomposed to several levels of detail
Name should be a simple imperative statement
Identifier numbers do NOT indicate sequence
Control logic is NOT shown on a DFD
N.B. a DFD is NOT a flowchart 1
Manager
Register New
Customer

7. DFD Components and Conventions (continued)
Data Store
Represents data retained by the system (long and short term)
manual data stores referenced by M1, M2, M3 etc
transient data stores (short term) referenced by T1, T2, T3 etc
computer data stores referenced by D1, D2, D3 etc
transient manual data stores by T1(M), T2(M) T3(M) etc
e.g computer files, record books, box of cards, filing cabinets
or where repeated
D1 Customer Details
D1 Customer Details

8. DFD Components and Conventions (continued)
Data Flows
Show the input and/or output of information or data to and from Processes
Arrows show the direction of the flow
Description normally given to the data flow
Identified by source & destination and data flow description
e.g. data flow a-1 goes from External Entity “a” to Process 1
Show the flow of information or data NOT storage media
Usually flow to or from Processes
an exception might be e.g. letters between external entities.
Amount of deposit

9. DFD Components and Conventions (continued)
Data flows show the direction of information
Update
Customer D1
Customers
Details
Customer for
Amendment
WRONG
RIGHT
Read
Name of
Required

10. Hierarchical Structure of DFDs
Start with Level 1 (or top-level) DFD
Each process on Level 1 can be expanded into a level 2 DFD
Level 2 processes can be further expanded until sufficiently detailed
Any process may be a “window” into another diagram, holding greater detail
Processes which don’t decompose further are marked /* in corner
Lowest level processes may be described further by Elementary Process Descriptions
Elementary Process Descriptions use narrative, Structured English, mathematical formulae, decision tables or trees as appropriate

11. Hierarchical Structure
Level 1 1
a - 1 a
1.1
1.2
a - 1.1
1.3
1.4
a - 1.3 a
Level 2

12. Example of a Level 2 DFD 2 Record Deposits and Withdrawals d Clerk d
Balance of account
Amount of
Deposit
Amount Requested
2..1
2..2
Check and
Record
Withdrawal
Record
Deposit
Amount Requested * *
Amount of
Deposit
Balance of Account
Bank
Accounts D2
Bank
Accounts
2..3
Amount of
Deposit D2
Accept Pay
Deposit *
Amount of
Salary Paid c
Employer

13. Example of a Level 2 DFD
Name and identifier of higher level Process shown at top of lower level diagram
Frame represents the boundary of the Process
Data flows across the frame must relate to data flows at the higher level
Data stores used by only one process are usually shown as internal to that process at the lower level
Processes with no further decomposition are marked /*