Data Flow Diagram :

Data Flow Diagram is a simple graphical formulation that can be used to represent a system in terms of input data. A DFD model used a limited number of symbols to represent the function performed by the system. DFD is a popular system because it’s simple to understand.

DFD is two types –

1. Physical DFD : If two processes are directly connected by a data flow arrow , then this DFD is called Physcial DFD. In Physical DFD there are two processes are must be operated in same speed.

The figure below shows the structure of Physical DFD - 

Logical DFD : If two processes are connected through a data store, then it’s called Logical DFD. In this DFD the speed of operation of processes are independent. The data produced by a process stored in the data store then another process collects that from the data store, So the speed of operation may not be same.

The figure below shows the structure of the Logical DFD -