V.21

<protocol, standard>

An ITU-T modem protocol for 300 bps two-wire full-duplex communications using Frequency Shift Keying modulation.

All modern modems support both V.21 and the close variant, Bell 103.

Last updated: 1996-07-22

V.22

<protocol, standard>

An ITU-T modem protocol which allowed data rates of 1200 bps. V.22bis doubled this.

Last updated: 2004-07-26

V.22bis

<protocol, standard>

V.22 twice. An ITU-T modem protocol which allowed a data rate of 2400 bits per second, twice that of V.22.

V.23

<protocol, standard>

An ITU-T modem protocol which allowed half-duplex (unidirectional) data transmission at 1200 bits per second with a 75 bps back channel and fall back to 600/75 bps. This was the main standard used for British Telecom's Prestel service.

Last updated: 2004-07-26

V.24

<standard>

The ITU-T standard defining interchange circuits between DTE and DCE. V.24 is the ITU-T equivalent of EIA standard EIA-232C, though V.24 only specifies the meaning of the signals, not the connector or the voltages used.

V.24 recommends 12 modem carrier frequencies that will not interfere with Dual Tone Multi-Frequency or other telephone control tones. These are: 

  GROUP A   =    920 Hz, 1000 Hz, 1080 Hz, 1160 Hz
  GROUP B   =    1320 Hz, 1400 Hz, 1480 Hz, 1560 Hz
  Group C   =    1720 Hz, 1800 Hz, 1880 Hz, 1960 Hz

Last updated: 2004-08-02

V.25

<communications, standard>

A standard allowing an auto-answer modem to determine the correct modulation standard to use.

Last updated: 2001-04-06

V.25 bis

<communications, standard>

A cryptic command language for modems.

Last updated: 2004-08-02

V.27 ter

<protocol, standard>

An ITU-T modem protocol which allowed 4800 bps communications with fall back to 2400 bps. V27.ter was used by Fax machines.

Last updated: 2004-07-26

V.28

<communications, standard>

An ITU-T physical interface standard for serial data communications equipment, formally titled "Electrical Characteristics for Unbalanced Double-Current Interchange Circuits". The combination of V.28 and V.24 is equivalent to the EIA's RS-232 and uses the same 25-pin connectors.

Last updated: 2001-04-06

V.29

<communications, standard>

An ITU-T modem protocol which allows 9600 bps half duplex or four wire communications with fall back to 7200 bps and 4800 bps. V.29 is used by fax machines.

Last updated: 2004-07-30

Nearby terms:

uz ♦ V ♦ V.10 ♦ V.11 ♦ V.17 ♦ V.21 ♦ V.22 ♦ V.22bis ♦ V.23 ♦ V.24 ♦ V.25 ♦ V.25 bis

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