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    The CCITT has classified dial-up modems according to modulation standards. Listed here are the standards and their different transmission speeds.

    Dial-up modems utilize traditional copper phone lines to transmit analog signals. Out of all types of modems dial-up, ISDN, DSL, cable dial-up connections offer the slowest transmission speeds.

    The CCITT, an international committee that specifies the way modems and fax machines transmit information to ensure compatibility among modems, has classified dial-up modems according to the following modulation standards:

    • Bell 103M & 212A: Older standards, Bell 103 transmits at 300 bps at 300 baud and 212A transmits at 1200 bps at 600 baud.
    • V.21: Capable of only 300 bps, it is an international standard used mainly outside of the U.S.
    • V.22: Capable of 1200 bps at 600 baud. Used mainly outside the U.S.
    • V.22bis: Used in the U.S. and out, it is capable of 2400 bps at 600 baud.
    • V.23: Used mainly in Europe, it allows the modem to send and receive data at the same time at 75 bps.
    • V.29: A one-way (half-duplex) standard that is used mostly for fax machines. Capable of 9600 bps.
    • V.32: A full-duplex standard capable of 9600 bps at 2400 baud. V.32 modems automatically adjust their transmission speeds based on the quality of the lines.
    • V.32bis: A second version of V.32, it is capable of 14,400 bps. It will also fallback onto V.32 if the phone line is impaired.
    • V.32ter: The third version of V.32, capable of 19,200 bps.
    • V.34: Capable of 28,000 bps or fallback to 24,000 and 19,200. This standard is backwards compatible with V.32 and V.32bis.
    • V.34bis: Capable of 33,600 bps or fallback to 31,200.
    • V.42: Same transfer rate as V.34 but is more reliable because of error correction.
    • V.42bis: A data compression protocol that can enable modems to achieve a data transfer rate of 34,000 bps.
    • V.44: Allows for compression of Web pages at the ISP end and decompression by the V.44-compliant modem, so transmitting the same information requires fewer data packets.
    • V.90: The fastest transmissions standard available for analog transmission, it is capable of 56,000 bps.
    • V.92: Transmits at the same speed as V.90 but offers a reduced handshake time and an on-hold feature.

    Note: CCITT was renamed to ITU-T in 1993, however modem standards are still called CCITT standards. The full list of ITU-T (V Series Standards) can be found here.

    Related Webopedia Categories: Data, Communications

    This article was originally published on September 18, 2009