For the physical arrangement of data links and the logical flow of information over them, there are various transmission types depending on the transmission systems and the desired applications, from which the following configuration options can be derived: simplex, half- duplex, echoplex and duplex.
Insimplex operation, data can only be transmitted in one direction.
Echoplex operation refers to the operating mode of asynchronous transmission methods. Data is mirrored back from the receiving station, usually the central computer, to the transmitting station (input terminal), which enables simple checking of error-free transmission. An entered character is therefore not displayed on the screen until it has been mirrored by the host. Echoplex is a special form of duplex operation.
With duplex, data can be transmitted in both directions simultaneously. Duplex operation is only possible with suitable transmission protocols, e.g. with the HDLC protocol, High Level Data Link Control (HDLC).