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Dial-Up Modem Standards


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







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