Data flow diagrams (DFDs) provide a graphical representation of how data moves through a system. DFDs use four main symbols: processes, data stores, external entities, and data flows. They allow system analysts and users to depict and understand the flow of data in a system. DFDs come in two main types: context diagrams provide an overview of the system and its interactions, while level 0 DFDs show more detail about the system's major sub-processes, data stores, and flows at a high level. Together, DFDs enable customers and users to specify requirements by modeling the system's data flows.

1. Systems Requirements Analysis
Data flow diagram (DFD)
Is a picture of the movement of data between external entities and the processes
and data stores within a system. Data flow diagram provide a graphical
representation of the system that aims to be accessible to computer specialist and
non-specialist users alike. The models enable customers and users to work together
effectively during the analysis and specification of requirements.
A data flow diagram (DFD) is a graphical tool that allows system analysts (and
system users) to depict the flow of data in an information system.
The DFD is one of the methods that system analysts use to collect information
necessary to determine information system requirements.
The DFD consists of only four main symbols:
• Process : performs some action on data, such as creates,
modifies, stores, delete, etc. Can be manual or supported by
computer.
• Data store : information that is kept and accessed. May be in paper file
folder or a database.
• External entity : is the origin or destination of data. Entities are external to
the system.
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2. Systems Requirements Analysis
• Data flow : the flow of data into or out of a process, data store or entity.
Rules for Drawing DFD’s
 A minimum of one data flow in and one data flow out of a process.
 A data store must be connected to a process (either in, out, or both).
 An external entity must be connected to a process (either in, out, or both).
 A single data flow must only flow one way.
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3. Systems Requirements Analysis
Rules for Using DFD:
Although all data-flow diagrams are composed of the same types of symbols, and
the validation rules are the same for all DFDs, there are two main types of data-
flow diagram:
 Context Diagram: are diagrams that present an overview of the system and
its interaction with the rest of the “world”. And is Shows the system
boundaries, external entities that interact with the system, and major
information flows between entities and the system.
Example1: Order system that a company uses to enter orders and apply payments
against a customer’s balance.
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4. Systems Requirements Analysis
Figure 4.1. Context Diagram of system payments
 Level 0 data-flow diagrams: Level 0 DFDs present a more detailed view of
the system than context diagrams, by showing the main sub-processes and
stores of data that make up the system as a whole. And is Shows the
system’s major processes, data flows, and data stores at a high level of
abstraction.

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5. Systems Requirements Analysis
Figure 4.2. Level 0 data-flow diagrams of system payments
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