Transmission mode (Data Flow) In Data Communication

Communication between two devices can be simplex, half-duplex, full-diplex
Simplex:

 
In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive. Keyboards and traditional monitors are examples of simplex devices. The keyboard can only introduce input; the monitor can only accept output. The simplex mode can use the entire capacity of the channel to send data in one direction.

Half-duplex:

 
In half-duplex mode, each station can both transmit and receive, but not at the same time. When one device is sending, the other can only receive, and vice vers.
The half-duplex mode is used in cases where there is no need for communication in both directions at the same time; the entire capacity of the channel can be utilized for each direction

Full- Duplex: 

 
In full-duplex mode (also called duplex), both stations can transmit and receive simultaneously.
The full-duplex mode is used when communication in both directions is required all the time. The capacity of the channel, however, must be divided between the two directions.