The V Series Recommendations from the ITU-TS are summarized in the table below. They include the most commonly used modem standards and other telephone network standards. Prior to the ITU-T standards, the American Telephone and Telegraph Company and the Bell System offered its own standards (Bell 103 and Bell 212A) at very low transfer rates. Another set of standards, the Microcom Networking Protocol, or MNP Class 1 through Class 10 (there is no Class 8), has gained some currency, but the development of an international set of standards means these will most likely prevail and continue to be extended. (Some modems offer both MNP and ITU-T standards.)

In general, when modems handshake, they agree on the highest standard transfer rate that both can achieve.

Beginning with V.22bis, ITU-T transfer rates increase in 2400 bps multiples. (bis refers to a "second version." terbo refers to a "third version.")







Standard Meaning
V.22 Provides 1200 bits per second at 600 baud (state changes per second)
V.22bis The first true world standard, it allows 2400 bits per second at 600 baud
V.32 Provides 4800 and 9600 bits per second at 2400 baud
V.32bis Provides 14,400 bits per second or fallback to 12,000, 9600, 7200, and 4800 bits per second
V.32terbo Provides 19,200 bits per second or fallback to 12,000, 9600, 7200, and 4800 bits per second; can operate at higher data rates with compression; was not a CCITT/ITU standard
V.34 Provides 28,800 bits per second or fallback to 24,000 and 19,200 bits per second and backwards compatility with V.32 and V.32bis
V.34bis Provides up to 33,600 bits per second or fallback to 31,200 or V.34 transfer rates
V.35 The trunk interface between a network access device and a packet network at data rates greater than 19.2 Kbps. V.35 may use the bandwidths of several telephone circuits as a group. There are V.35 Gender Changers and Adapters.
V.42 Same transfer rate as V.32, V.32bis, and other standards but with better error correction and therefore more reliable
V.90 Provides up to 56,000 bits per second downstream (but in practice somewhat less). Derived from the x2 technology of 3Com (US Robotics) and Rockwell's K56flex technology.

An industry standard, Integrated Services Digital Network uses digitally-encoded methods on phone lines to provide transfer rates up to 128,000 bits per second. Another technology, Digital Subscriber Line, provides even faster transfer rates.

This was last updated in September 2005

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