ITU V-Series

These are the ITU-T V-Series 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.

General standards

Applies to V.1–V.9

V.1 is an ITU-T recommendation, entitled Equivalence between binary notation symbols and the significant conditions of a two-condition code.
V.2 is an ITU-T recommendation, approved in November 1988, titled Power levels for data transmission over telephone lines.
V.4 is an ITU-T recommendation, approved in November 1988, titled General structure of signals of International Alphabet No. 5 code for character oriented data transmission over public telephone networks.
V.5 was an ITU-T recommendation, approved in November 1988, titled Standardization of data signalling rates for synchronous data transmission in the general switched telephone network. It has been withdrawn since.
V.6 was an ITU-T recommendation, approved in November 1988, titled Standardization of data signalling rates for synchronous data transmission on leased telephone-type circuits. It has been withdrawn since.
V.7 is an ITU-T recommendation, approved in November 1988, titled Definitions of terms concerning data communication over the telephone network.
V.8 is an ITU-T recommendation, first approved in September 1994, titled Procedures for starting sessions of data transmission over the public switched telephone network. It has been superseded three times. The current version was approved in November 2000.
V.8bis is an ITU-T recommendation, first approved in August 1996, titled Procedures for the identification and selection of common modes of operation between data circuit-terminating equipments (DCEs) and between data terminal equipments (DTEs) over the public switched telephone network and on leased point-to-point telephone-type circuits. It has been superseded twice. The current version was approved in November 2000.

Interfaces and voiceband modems

Applies to V.10–V.34

V.10 is a 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 RS423.
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, it is compatible with EIA RS 422. The 15-pin ISO 4903 connector is recommended for data network interface.
V.17 is an ITU-T fax protocol that uses 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 bauds 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.
V.22bis is an ITU-T recommendation extending V.22 with a faster rate using QAM modulation 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 bauds to carry digital data at up to 600 or 1200 bit/s respectively. An optional 75 bauds reverse channel carries 75 bit/s.
V.24 is referenced as RS-232.
V.32 is an ITU-T recommendation for a modem, 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 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.34 is an ITU-T recommendation for a modem, allowing up to 28.8 kbit/s bidirectional data transfer. 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. It is the successor to the unofficial V.FC (V.Fast Class) standard created by Hayes and Rockwell, which was the first widely available 28.8 kbit/s protocol. Most V.34 modems support V.FC, although not all modern modems support both.
V.34bis, also known as V.34plus is an 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.

Wideband modems

Applies to V.35–V.39

V.35 is an ITU-T standard located on layer 1 on the OSI model. Max speed is 2 Mbit/s.

Error control and data compression

Applies to V.40–V.49

V.42 is an error correction protocol. Its function is to allow the receiver to immediately request re-transmission of any lost data packets. However, it does not guarantee how quickly the error-free data will be delivered to the receiving end. V.42 is generally included in dialup modems.
V.42bis is an adaptive data compression standard. It uses an adaptive algorithm, based on the Lempel Ziv dynamic dictionary approach, and may go to "transparent mode," in which data is transmitted uncompressed. The specific algorithm is "BTLZ" (British Telecom Lempel Ziv), which was developed by Alan Clark (then with BT, later CTO of Hayes).
V.44 is an adaptive data compression standard incorporated into the v.92 dial-up modem standard. V.44 offers somewhat better compression performance for certain types of data than the V.42bis standard, on average allowing 5% greater throughput. V.44 is based on LZJH (Lempel-Ziv-Jeff-Heath) compression technology developed by Hughes Electronics for its DirecPC satellite Internet connection technology.

Simultaneous transmission of data and other signals

Applies to V.60–V.99

V.90 is an ITU-T recommendation for a modem, allowing 56 kbit/s download and 33.6 kbit/s upload. It was developed between March 1998 and February 1999. It is also known as V.Last as it was anticipated to be the last standard for modems operating near the channel capacity of POTS lines to be developed. A follow-on standard, V.92, was developed later in 1999 to replace V.90.
V.92 is an ITU-T recommendation, titled Enhancements to Recommendation V.90, that establishes a modem standard allowing near 56 kbit/s download and 48 kbit/s upload rates. V.92 was first presented in August 1999. It is intended to succeed the V.90 standards. With V.92 PCM is used for both the upstream and downstream connections; previously 56K modems only used PCM for downstream data.

Interworking with other networks

Applies to V.100–V.199

Interface layer specifications for data communication

Applies to V.200–V.249

V.230 is an ITU-T recommendation, approved in November 1988, titled General data communications interface layer 1 specification.

Control Procedures

Applies to V.250–V.299

Digital circuit modems

Applies to V.300–V.399

V.300 is an ITU-T recommendation, first approved in July 1999, titled A 128 (144) kbit/s data circuit-terminating equipment standardized for use on digital point-to-point leased circuits.

External links

Official V-Series Documentation
V.35 Technical Info - Includes pinouts and history
v.44 FAQ
v.44 vs. v.42bis Compression Comparison
Technical information on the LZJH algorithm in use in the v.44 standard
Basic handshakes & modulations - V.22, V.22bis & V.32 handshakes

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