SAD-Data Flow Diagram

DFD is also known as “bubble chart”. Its main purpose is to clarify the system requirements and identify major transformations that will become programs in system design. It is the starting point of the design phase. A DFD consist of a series of bubbles joined by lines. The bubbles represent data transformations and the line represents data flow in the system.

DFD symbols

There are mainly four symbols for DFD.

1.A Square defines a source or destination of system data.
2.An arrow identifies data flow, i.e.: data in motion. It is a pipeline through which information flows.
3.A circle represents a process that transforms incoming data flows into outgoing data.
4.An open rectangle is a data store, ie: data at rest or a temporary repository of data.

A DFD describes what data flow rather than how they are processed, so it does not depend on hardware, software, data structure or file organization.

Constructing a DFD.

Rules for constructing DFDs are

1.Processes should be named and numbered for easy reference.
2.The direction of flow is from top to bottom and from left to right. Data flows from source to the destination.
3.Process should be numbered, if they are exploded into lower level details. 
4. The names of data stores, sources and destination should be in capital letters. Process and data flow names first letter should be in capital letter.
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