Data Flow Diagrams (DFDs) are categorized as either logical or physical. A logical DFD focuses on the business and how the business operates. It describes the business events that take place and the data required and produced by each event. On the other hand, a physical DFD shows how the system will be implemented.
What the model depicts
How the business operates
How the system will be implemented
What the process represent
Programs, program modules, and manual procedures
What the data store represent
Collection of data, regardless of how the data is stored
Physical files and databases, manual files
Type of data source
Show data stores representing permanent data collections
Master files, transaction files. Any process that operates at two different times must be connected by a data source. System controls
Show business controls
Show controls for validating input data, for obtaining a record, for ensuring successful completion of a process and for system security.
CONTEXT DATA FLOW DIAGRAM
A Context Data Flow Diagram depicts the entire process being modeled as a
single task. The diagram shows important interactions between the system and the external agents. To draw a context diagram, you start by placing a single process symbol in the center of the page. The symbol represents the entire information system, and you identify it as process 0 (the numeral 0, not the letter O). Then you place the system entities around the perimeter of the page and use data flows to connect the entities to the central process. Data stores are not shown in the context diagram because they contained within the system and remain hidden until more detailed diagrams are created.