V.23 is an ITU-T recommendation for half-duplex communication between two analogue dial-up modems using FSK modulation at up to 600 or 1200 bauds to carry digital data at up to 600 or 1200 bit/s respectively. An optional 75 bauds reverse channel carries 75 bit/s.
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V.23 (http://en.wikipedia.org/wiki/V.23)
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