The ITU-T V-Series Recommendations on Data communication over the telephone network specify the protocols that govern approved modem communication standards and interfaces.
Note: the bis and ter suffixes are ITU-T standard designators of successive iterations of a standard (bis and ter are derived from the Latin for "twice" and "thrice").
Interfaces and voiceband modems
Applies to V.10–V.34
- V.10 is an ITU-T recommendation, first agreed in 1976, for unbalanced electrical circuits for data communication at up to 100 kbit/s. It can interwork with V.28, provided it is not exposed to signals greater than 12 volts. Used with the 37-pin ISO 4902 connector, it is compatible with EIA RS-423.
- V.11 is an ITU-T recommendation, first agreed in 1976, for balanced electrical circuits for data communication at up to 10 Mbit/s. Used with the 37-pin ISO 4902 connector ("DC-37"), it is compatible with EIA-422. The 15-pin ISO 4903 connector ("DA-15") is recommended for data network interface.
- V.17 is 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 transfer using TCM modulation. Other additional defined data transfer rates are 24.0 kbit/s and 19.2 kbit/s as well as all the permitted V.32 and V.32bis rates. Additionally, V.34 modems employ shell mapping as shaping code to reduce the transmit power.
- V.34 (10/96) is an updated ITU-T recommendation for a modem, building on the V.34 standard but allowing up to 33.6 kbit/s bidirectional data transfer. Other additional defined data transfer rates are 31.2 kbit/s, as well as all the permitted V.34 rates. Modems implementing this standard were often marketed under the moniker V.34+.
- V.34 (02/98) commonly rendered as V.34bis, is a further update to V.34 which corrected some errata in the original 1996 document.
Ad hoc standards
In order to gain first-mover advantage, many modem companies introduced models based on upcoming V-series standards before they reached final ratification. In other cases, companies introduced non-standard systems but gave them ITU-like names.
- V.32terbo, or V.32ter for short, was a 19.2 kbit/s standard introduced by AT&T Paradyne. It was based on V.32bis and did little other than increase the data rate. V.32ter is compatible with V.32bis at speeds of 14.4 kbit/s and lower, but it is not compatible with V.34 at 19.2.
- V.FC, short for V.Fast Class and sometimes referred to as V.FAST, was developed by Hayes and Rockwell to introduce a 28.8 kbit/s standard while the V.34 efforts dragged on. V.FC was not compatible with V.34, although most V.34 modems could support V.FC, notably, those using Rockwell chipsets.
- K56flex was developed by Rockwell and Lucent while the V.90 standards were underway. Compatibility problems between the two companies' implementations were a concern, and the standard quickly disappeared.
- X2 was USRobotics' answer to K56flex, and likewise disappeared rapidly with the introduction of V.90.