an ITU-T recommendation for a fax modem using TCM modulation at 12 and 14.4 kbit/s. V.21 is an ITU-T recommendation for full-duplex communication between two analogue dial-up modems using audio frequency-shift keying modulation at 300 baud to carry digital data at 300 bit/s. It is a variant of the original Bell 103 modulation format. V.22 is an ITU-T recommendation for full-duplex communication between two analogue dial-up modems using PSK modulation at 600 baud to carry data at 1200 or 600 bit/s. It is a variant of the Bell 212A modulation format. V.22bis is an ITU-T recommendation extending V.22 with a faster rate using QAM at 600 baud to carry digital data at 2400 or 1200 bit/s. The 1200 bit/s mode is compatible with V.22. 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 baud to carry digital data at up to 600 or 1200 bit/s respectively. An optional 75 baud reverse channel carries 75 bit/s. V.24 is referenced as RS-232 which also includes V.28. V.250 (also known as V.25ter) is extended data modems ITU-T recommendation . V.27ter is an ITU-T recommendation for a half-duplex modem, allowing 2400 and 4800 bit/s (PSK modulation). V.28 is an ITU-T recommendation defining the electrical characteristics for unbalanced double-current interchange circuits. V.29 is an ITU-T recommendation for a modem, allowing 4.8 kbit/s, 7.2 kbit/s and 9.6 kbit/s transfer modes (PSK and QAM modulations). V.32 (11/88) is an ITU-T recommendation for a modem operating as full-duplex on a 4-wire circuit, or half-duplex on a two-wire circuit, allowing bidirectional data transfer at either 9.6 kbit/s or 4.8 kbit/s at a symbol rate of 2,400 baud instead of the 600 baud of the V.22 standards. V.32bis (02/91) is an ITU-T recommendation for a modem, allowing up to 14.4 kbit/s bidirectional data transfer. Other additional defined data transfer rates are 12.0 kbit/s, 9.6 kbit/s, 7.2 kbit/s, and 4.8 kbit/s. The standard was extended by several modem manufacturers to allow bidirectional data transfer rates of 19.2 kbit/s, but never ratified as a V.32ter standard. These non-ITU-T standard modems were often referred to as 'V.32terbo' modems. V.33 is an ITU-T recommendation for a modem operating as full-duplex on a 4-wire point-to-point leased line allowing bidirectional data transfer at either 14.4 kbit/s. V.34 (09/94) is an ITU-T recommendation (superseded) for a modem, allowing up to 28.8 kbit/s bidirectional data