ITU-T V.23

The V.23 standard was an early <a href="/facts/Modem/9H0Xwtq0">modem</a> standard first approved by <a href="/facts/ITU-T/9nJwkPYx">ITU-T</a> precursor CCITT in 1964.

It specifies <a href="/facts/Frequency-shift_keying/Bp1jJDPX">audio frequency-shift keying</a> (AFSK) to encode and transfer data at a rate of 1200 <a href="/facts/Bits_per_second/Nv9VoT7T">bits per second</a>, <a href="/facts/Duplex_(telecommunications)/LCULeVxr">full-duplex</a> at 1200 baud (Mode 2), (or at a "fallback rate" of 600 baud, mode 1) for the forward data-transmission channel, and an optional 75 baud backward channel.

<ul><li>V.23 Mode 1 AFSK uses a 1300 Hz tone (FZ) for mark (typically a binary 1) and 1700 Hz (FA) for space (typically a binary 0), and a 1500 Hz center frequency (F0).</li>
<li>V.23 Mode 2 AFSK uses a 1300 Hz tone (FZ) for mark (typically a binary 1) and 2100 Hz (FA) for space (typically a binary 0), and a 1700 Hz center frequency (F0).</li>
<li>V.23 backward channel AFSK uses a 390 Hz tone (FZ) for mark (typically a binary 1) and a 450 Hz (FA) for space (typically a binary 0).</li></ul>
In some <a href="/facts/Europe/DTAtrKfh">European</a> countries, (and perhaps elsewhere), V.23 Mode 2 AFSK modulation, (without the backward channel) is used to transmit <a href="/facts/Caller_ID/hh9DqEEj">Caller ID</a> information over <a href="/facts/Plain_old_telephone_service/7uEc2wFY">POTS lines</a> in the <a href="/facts/Public_Switched_Telephone_Network/eVrCTDU7">public telephone network</a>.
The 75 baud backward channel was originally envisioned for use in error correction schemes, but V.23 was also widely used in <a href="/facts/Videotex/psxycPcf">Videotex</a> applications where the backward channel was used to send keyboard data in an asymmetrical full duplex scheme in devices such as the French <a href="/facts/Minitel/bTmoGeyh">Minitel</a> service, or the British <a href="/facts/Prestel/ov6p8pzW">Prestel</a> service.

ITU-T V.23 open-in-new

ITU-T V.23