Duplex: A communication term used to describe how two way communications are handled. If simultaneous communication can occur in both directions, the link is described as "full duplex". If the line is only used in one direction at a time it is said to be "half duplex". Links that can operate in only one direction are "simplex". Lines that could in principle support full duplex are often used as half duplex in practice. Half duplex protocols -- unlike full duplex and simplex -- often include a line release or line turn around element indicating that the transmitter is finished and has gone into receive mode.

ASCII terminals have separate receive and transmit lines. They are generally capable of full duplex in the sense descussed in the first paragraph. For ASCII terminals and modems "half duplex" is often used somewhat imprecisely to indicate that characters typed on the keyboard are displayed directly by the terminal whereas "full duplex" means that the character is sent to the host which -- at its discretion -- sends the character back to the terminal for display. Strictly speaking the operation is echo, not duplex and is so identified by some manufacturers, but the "duplex" terminology is used by others.

Ethernet can support true full duplex if there are two data channels present -- as with unshielded twisted pair. Ethernet repeaters (including hubs) operate in true half duplex. Switches can operate in full duplex. For full duplex devices, the mode can be manually configured or set by autodetection. Autoconfiguration can fail in some circumstances potentially leading to intermittent unexplained network slowdowns when a normally full duplex link gets into half duplex mode and becomes a bottleneck. It is possible for one end of a loop to attempt to operate in full duplex while the other operates half-duplex. This does not work very well if it works at all (which it sometimes does albeit VERY slowly) and is a possible cause of very slow network links.

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