Page 53 - DCAP305_PRINCIPLES_OF_SOFTWARE_ENGINEERING

Unit 3: Software Requirements

in a layered manner. At the first layer, the DFD is made at block level and in lower layers, the Notes
details are shown. Thus, level “0” DFD makes a fundamental system. (See Figure 3.2)

Figure 3.2: Depiction of Process

I 1
Output
I
2

The DFD can represent the system at any level of abstraction. DFD of “0” level views entire
software element as a single bubble with indication of only input and output data. Thus, “0”
level DFD is also called as Context diagram. These symbols can be seen in Figure 3.3.

Figure 3.3: Symbols of a Data Flow Diagram

Symbol Name Description
Data Flow Represents the connectivity between
various processes
Process Performs some processing of input data

External Defines source or destination of system
Entity data. The entity which receives or
supplies information.

Data Store Repository of data

Rules for making DFD
The following factors should be considered while making DFDs:
1. Keep a note of all the processes and external entities. Give unique names to them. Identify
the manner in which they interact with each other.
2. Do numbering of processes.

3. Avoid complex DFDs (if possible).
4. The DFD should be internally consistent.
5. Every process should have minimum of one input and one output.

The data store should contain all the data elements that flow as input and output.
Data Flow Model
The data flow model is based on the program’s control flow graph do not confuse that with the
program’s data flow graph. We annotate each link with symbols (for example, d, k, u, c, p) or
sequences of symbols (for example, dd, du, ddd) that denote the sequence of data operations on
that link with respect to the variable of interest. Such annotations are called link weights. The
control flow graph structure is same for every variable: it is the weights that change.

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