The direction in which data can be transmitted between any two linked devices is of three types:

  1. Simplex,
  2. Half-duplex, and
  3. Full- duplex, or duplex.

In simplex mode the data transmits in one direction only, from one system to another system. The sender device that sends data can only send data and cannot receive it. On the other hand the receiver device can only receive the data and cannot send it. Television is an example of simplex mode transmission as the broadcast sends signals to our TV but never receives signals back from our TV. This is a unidirectional transmission.

In half-duplex mode the communication between sender and receiver occurs in both directions, but only one at a time.  The sender and receiver can both send and receive the information, but only one is allowed to send at any given time. For example, in walkie-talkies, the speakers at both ends can speak, but they have to speak one by one.  They cannot speak simultaneously.

In full duplex transmission mode, the communication between sender and receiver can occur simultaneously.  The sender and receiver can both transmit and receive at the same time. For example, in a telephone conversation, two people communicate, and both are free to speak and listen at the same time.