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Those Mysterious CCITT numbers

The CCITT (French acronym for International Consultative Committee for Telegraphy and Telephony, which is based in Geneva, a French-speaking area) uses "bis" to indicate the second in a family of related standards, and "ter" to indicate the third.

The assignment of number is NOT in ascending numeric order. The V series recommendations, for example, are broken up into several categories:

V.1-V.7 General
V.10-V.34 Interfaces and voice-band modems
V.35-V.37 Wideband modems
V.40-V.42 Error control
V.50-V.57 Transmission quality and maintenance
V.100-V.230 Interworking with other networks

Within various groups, the numbers have been more or less assigned in ascending order chronologically. There is no direct correlation between the "speed" at which a modem operates and its numeric assignment. It is best to simply consider CCITT number of recommendations to be random, and leave it at that.


v.17, a standard for computer-to-computer fax transmission.

v.22 runs at 1200 bps (bits per second), compatible with the Bell 212A standard used in the US and Canada.

v.22bis runs at 2400 bps, with automatic fallback to 1200 baud and compatibility with Bell 212A and v.22 modems. All 2400 baud modems for personal computers support this standard.

v.23 runs at 1200 bps with a 75 bps back channel. Generally not used in the U.S. or Canada.

v.29 is the universal standard for 9600 bps fax machines.

v.32 runs at 9600 bps, with automatic fallback to 4800 when line quality is poor. Note: the Hayes V-Series 9600 bps modem is not v.32 compatible.

v.32bis runs at 14400 bps, with automatic fallback to 12000, 9600, 7200 and 4800. If line quality improves, speed can actually increase during the course of a session.

v.32turbo is a non-CCITT "standard" for 12 kbps transfer, used by Zoom.

v.32terbo or v.32ter is a non-CCITT "standard" for 19.2 kbps transfer, not universally accepted by modem manufacturers.

v.FAST is a non-CCITT standard for 28.8 kbps data transfer.

v.34 is the protocol for 28.8 kbps modems.

v.42 is the CCITT standard for modem communication covering negotiation for LAPM error control and support for MNP Levels 1-4 error correction.

v.42bis is an extension of v.42 that adds data compression to the v.42 correction protocols.

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Dec. 3, 1995 LC