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The V.23 standard was an early modem standard approved by the ITU in 1988. It specifies audio frequency-shift keying (AFSK) to encode and transfer data at a rate of 1200 bits per second, half-duplex 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.
In some European countries, (and perhaps elsewhere), V.23 Mode 2 AFSK modulation, (without the backward channel) is used to transmit Caller ID information over POTS lines in the public telephone network.
The 75 baud backward channel was originally envisioned for use in error correction schemes, but V.23 was also widely used in Videotex applications where the backward channel was used to send keyboard data in an asymmetrical full duplex scheme in devices such as the Minitel.
The BELL 202 communications standard defines a similar modulation scheme.