Data Flow
There are 3 types of communication between two devices: simplex, half-duplex and full-duplex.
Simplex
In simplex mode the communication is unidirectional. One device on a link can transmit and the other can only receive.
Traditional monitors and keyboards are examples of simplex devices. The monitor can only accept output otherwise the keyboard can only introduce input. The simplex mode use the entire capacity to send data in only one direction.
Half-Duplex
In half-duplex mode, both devices can transmit and receive, but not at the same time. When one device can only receive, the other is sending, and vice versa.
The half-duplex mode is like a one-lane road with traffic allowed in both directions. In this transmission the capacity of a channel is taken over by whichever of the two devices is transmitting at the time.For example citizens band radios and walkie-talkies are both half-duplex systems. The half-duplex mode is used when there is no need for communication in both directions at the same time and the entire capacity can be utilized for each direction.
Full-Duplex
In full-duplex mode, both stations can transmit and receive at the same time.Signals going in one direction share the capacity of the link with signals going in the other direction. The link must contain two paths, one for sending and the other for receiving; or the capacity of the channel is divided between signals traveling in both directions. One example of full-duplex communication is the telephone network.
This mode is used when in both directions communication is required all the time and in this way the capacity of the channel must be divided between the two directions.
