Updated: 11/12/2023 by Computer Hope
United Nations Emblem used for ITU

Formerly known as CCITT, ITU (International Telecommunication Union) is the United Nations committee founded on May 17, 1865. Its job is to ensure all telecommunications devices (e.g., telephones, fax machines, and modems) can "talk to" each other, no matter what company makes them or in what country they're used.

For example, the Bell 212A standard for 1200-bps communications in North America is observed internationally as ITU-T V.22. For 2400-bps communications, the U.S. manufacturers observe the V.22bis extended standard.

ITU-T V-series modem standards

Below are the ITU-T V-series computer modem and fax standards with their transmission speeds, newly-introduced features, and links to the official ITU PDF (Portable Document Format) and Word versions of the standard.


We've listed each of these as "standards," but in the ITU-T's terminology, they're known as "recommendations."


The bis and ter suffixes found on the ITU-T standards indicate the iteration of the standard. For example, bis is Latin for "twice" (second), and ter is Latin for "thrice" (third).


The V.10 standard was agreed upon in 1976 with data communication rates up to 100 kbit/s.


The V.17 standard provides simplex 14,400 bps over telephone lines using TCM modulation. Most V.17 modems can also do V.29 modulation and have a fallback speed of 9,600 and 7,200 bps.


A modem standard for full-duplex data transmission at 0 to 300 bps (bits per second). V.21 is used in Japan and Europe. In the United States, the Bell 103 standard is used instead.

V.22 and V.22bis

The V.22 is an ITU-T modem standard mostly used in Europe that was introduced in 1980 for full-duplex data transmission at 1,200 bps over a dial-up telephone line. V.22 can handle synchronous or asynchronous data. Data transfer between modems is synchronous, and DTE (data terminal equipment) communication is asynchronous. V.22bis is the same but doubles the data transfer speed to 2400 bps.


The V.23 modem standard was approved in 1964 for asynchronous and synchronous using FSK modulation up to 600 bps and half-duplex data transmission at speeds up to 1200 bps on dial-up lines.


Outlined in ISO 2110, V.24 is a standard that defines the interchange circuits between DTE (data terminating equipment) and DCE (data circuit-terminating equipment) used with RS-232.

V.25 and V.25bis

The V.25 and V.25bis standard for interfaces standards between DTE (data terminal equipment) and DCE (data circuit-terminating equipment) in a modem-to-modem communication setup. One of its essential functions is to allow echo control devices to be disabled and help determine the proper modulation.

V.25 works over analog phone lines and specifies details like signal voltage levels, connector types, and interface functionality to ensure devices can interoperate when connecting.

V.26 and V.26bis

The V.26 standard was approved in 1968 for full-duplex data transmission at speeds up to 2400 bps. The V.26bis standard offered full-duplex data transmission at 1200 to 2400 bps over telephone lines.

V.27, v.27bis, and V.27ter

The V.27, V.27bis, and V.27ter standards offered full-duplex data transmission of up to 4800 bps over telephone lines using PSK modulation.


The V.29 standard is used by fax modems for data transmission speeds up to 9600 bps with PSK and QAM modulation.

V.32 and V.32.bis

The V.32 is a CCITT V series standard for full-duplex dial-up modems transmitting data at a rate of up to 9600 bps. A fallback speed of 4,800 bps is included in the V.32 standard.


Although the V.32 standard supported asynchronous and synchronous data communications, not all modem manufacturers included both. Some did not support synchronous.

V.32bis increased the speed up to 14,000 bps. Fallback speeds of 12,000, 9,600, 7,200, and 4,800 bps were also supported with the standard.


AT&T Paradyne introduced ad hoc standards V.32terbo and V.32ter for increasing the data transfer rates. They are not compatible with V.34 at 19.2 bps.


The V.34 standard for full-duplex data transmission at speeds of up to 28,800 bps on a computer modem. Asynchronous and synchronous data communications are supported with a faster handshaking method based on V.8. Other improvements over V.32bis include line probing, precoding, and smaller feedback.


The V.35 standard is obsolete and no longer used, but was capable of transmitting 48,000 bps.


V.90 logo

The V.90 standard was agreed upon on February 6, 1998, to merge the KFlex56k and X2 standards into a single standard. The V.90 standard is used with the V.42bis data compression standard.

V.90 standard history

  • September 1996 - UsRobotics (3COM) submitted the first V.90 56k proposal to ITU (International Telecommunication Union).
  • November 15, 1996 - Lucent and Rockwell announce the KFlex standard.
  • April 1997 - ITU called for special working party to determine a 56k standard.
  • September 1997 - ITU met but did not reach consensus on several technical aspects.
  • December 1997 - Compromise on spectral shaping and multiple conversions. 25 of 30 favorable votes were cast.
  • February 1997 - Standard determination.
  • September 1998 - Standard ratification expected.
  • February 6, 1998 - V.90 becomes a standard after ITU comes to final agreement.

How should I upgrade to V.90?

Because of the variety of modem chipsets, we recommend you contact your computer or modem manufacturer to obtain complete information on upgrading your modem to V.90. Because this upgrade is a flash of a chip on the modem card, if done incorrectly, it can cause the modem to stop functioning.

I upgraded to V.90 and now my modem does not work

Downloading the incorrect V.90 modem update can cause this problem. Reinstall the modem and its drivers, and then verify the V.90 update is correct. If you're unsure about the V.90 update, contact your computer or modem manufacturer for additional support and information.


The V.91 standard was published in 1999 and can transmit up to 64,000 bps.


The V.92 standard was introduced on February 27, 2001, and offers new features such as modem on hold, which can suspend a user's data call and answer an inbound call. Quick connect, another feature, shortens the modem's connection time by remembering the line conditions to the dial-up server. V.PCM-Upstream, another feature, allows faster upstream communication with upload speeds reaching 48,000 bits per second (V.90 upstream speeds were limited to 31,200 bps). This standard uses V.44 as its compression method.

ITU-T V-series data compression and error control standards

Below are the ITU-T V-series related to modem data compression and error control standards.

V.42 and V.42bis

The V.42 standard helps regulate error detection in high-speed modems and reach data transfer speeds up to 34,000 bps. This standard allows computer modems to work with analog and digital phone lines and could re-transmit data when lost data packets are encountered. The standard used the error correction protocol LAPM (link access procedure for modems) and MNP 4 as a fallback.

V42.bis is a data compression standard variation of the Lempel Ziv algorithm called BTLZ. Because the data needs to be clean, V42.bis requires V.42.

V.44 is a different type of data compression standard based on the LZJH (Lempel-Ziv-Jeff-Heath) algorithm that was part of the V.92 modem standard.

Business terms, Computer acronyms, Fax, Group 3, Group 4, H.323, LAPM, Phone terms, Standard, X.25, X.400, X.500