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What are modulation protocols?

Modulation protocols determine how modems convert digital data into analog signals that can be sent over a phone line. The first modulation protocols were Bell 103 and V.21, which allowed transmission at 300 bps. It is impossible to transmit data faster than 600 bps by converting a singal bit into a singal signal. By combining digital data into more complex signals, it is possible to transmit at higher speeds. How this is done is determined by the transmission protocol. These standards are today controlled by the International Telecommunications Union Telecommunication Standardization Sector (ITU-T, formerly the CCITT). All of their various protocols are referred to by "V.##". Below is a table of V. standards for transfer rates over a general switched telephone network. Realize that there is a V.1 all the way up, with slight revisions for various types of circuits and phone lines.

Modulation Transfer rate (in bits/sec)
V.21 300
V.22 1200
V.22bis 2400
V.27 4800
V.32 9600
V.32bis 14400
V.32ter 19200
V.34 21400, 24600, 28800
V.34+ 28800, 33600
V.90 56000 (down), 33600 (up)
V.92 56000 (down), 48000 (up)


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