V.25 | Article about V.25 by The Free Dictionary
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V.25 (communications, standard)
V.25An ITU standard (1968) for automatic calling and/or answering equipment on dial-up lines. It uses parallel circuits and is similar in function to RS-366 and Bell 801 autodialers used in the U.S. The answer tone defined in V.25 is the first thing heard when calling a modem. It serves a dual function of identifying the answering equipment as being a modem and also disabling the echo suppression and echo cancellation equipment in the network so that a full-duplex modem will operate properly.