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 bit/s at 300 baud and 212A transmits at 1200 bit/s at 600 baud.
- V.21: Capable of only 300 bit/s, it is an international standard used mainly outside of the U.S.
- V.22: Capable of 1200 bit/s at 600 baud. Used mainly outside the U.S.
- V.22bis: Used in the U.S. and out, it is capable of 2400 bit/s at 600 baud.
- V.23: Used mainly in Europe, it allows the modem to send and receive data at the same time at 75 and 1200 bit/s.
- V.29: A one-way (half-duplex) standard that is used mostly for fax machines. Capable of 9600 bit/s.
- V.32: A full-duplex standard capable of 9600 bit/s 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 bit/s. 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 bit/s.
- V.34: Capable of 28,000 bit/s or fallback to 24,000 and 19,200 bit/s. This standard is backwards compatible with V.32 and V.32bis.
- V.34bis: Capable of 33,600 bit/s or fallback to 31,200 bit/s.
- V.42: A data compression protocol for V.34 which made it 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 bit/s.
- 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 bit/s.
- V.92: Transmits at the same speed as V.90 but offers a reduced handshake time and an on-hold feature.
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