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Unit 6: Functional Design

discipline began. DFDs are very useful in understanding a system and can be effectively used Notes
during analysis.

A DFD shows the flow of data through a system. It views a system as a function that transforms
the inputs into desired outputs. Any complex system will not perform this transformation in
a “single step,” and data will typically undergo a series of transformations before it becomes
the output. The DFD aims to capture the transformations that take place within a system to
the input data so that eventually the output data is produced. The agent that performs the
transformation of data from one state to another is called a process (or a bubble). Thus, a DFD
shows the movement of data through the different transformations or processes in the system.
The processes are shown by named circles and data flows are represented by named arrows
entering or leaving the bubbles. A rectangle represents a source or sinks and is a net originator
or consumer of data. A source or a sink is typically outside the main system of study.
In this DFD there is one basic input data flow, the weekly timesheet, which originates from the
source worker. The basic output is the paycheck, the sink for which is also the worker. In this
system, first the employee’s record is retrieved, using the employee ID, which is contained in
the timesheet. From the employee record, the rate of payment and overtime are obtained. These
rates and the regular and overtime hours (from the timesheet) are used to compute the pay. After
the total pay is determined, taxes are deducted. To compute the tax deduction, information from
the tax-rate file is used. The amount of tax deducted is recorded in the employee and company
records. Finally, the paycheck is issued for the net pay. The amount paid is also recorded in
company records.
Some conventions used in drawing this DFD should be explained. All external files such as
employee record, company record, and tax rates are shown as a labelled straight line. The
need for multiple data flows by a process is represented by a “*” between the data flows. This
symbol represents the AND relationship. For example, if there is a “*” between the two input
data flows A and B for a process, it means that A AND B are needed for the process. In the
DFD, for the process “weekly pay” the data flow “hours” and “pay rate” both are needed, as
shown in the DFD. Similarly, the OR relationship is represented by a “+” between the data
flows. (See Figure 6.10)

Figure 6.10: Data Flow Diagrams

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