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Echo (computing)
In computer telecommunications, echo is the display or return of sent data at or to the sending end of a transmission. Echo can be either local echo, where the sending device itself displays the sent data, or remote echo, where the receiving device returns the sent data that it receives to the sender (which is of course simply ''no'' local echo from the point of view of the sending device itself). That latter, when used as a form of error detection to determine that data received at the remote end of a communications line are the same as data sent, is also known as echoplex, echo check, or loop check. When two modems are communicating in echoplex mode, for example, the remote modem echoes whatever it receives from the local modem.
== Terminological confusion: echo is not duplex ==
Whilst local echo and remote echo are sometimes referred to as "half-duplex" and "full-duplex", those latter appellations are strictly incorrect, and are misleading (see duplex for their correct meanings). Whilst local echo is ''often used with'' half-duplex transmission, so that bandwidth is not wasted upon remotely echoing data from the remote end of the communications channel, it is not ''the same as'' half-duplex transmission. A full-duplex communications channel can still employ local echo, and a half-duplex channel can still (albeit wastefully) employ remote echo, or no echo at all.
Indeed, for example, that is the case for echoplex error checking, which ''requires'' full-duplex communication, so that received data can be echoed back as they are being received.
Similarly, for another example, in the case of the TELNET communications protocol a local echo protocol operates on top of a full-duplex underlying protocol. The TCP connection over which the TELNET protocol is layered provides a full-duplex connection, with no echo, across which data may be sent in either direction simultaneously. Whereas the ''Network Virtual Terminal'' that the TELNET protocol itself incorporates is a half-duplex device with (by default) local echo.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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