12.2 Modems, Bauds, and Bits per Second

A voice-grade telephone line has a useful frequency range of 300-3000 Hz but modems typically use tones restricted to the range 300-2400 Hz, primarily to avoid a 2600 Hz signaling tone that causes call disconnect. Various modulation schemes are used to convert the representation of information from the RS-232C discrete voltage levels to amplitude, phase, or frequency shift keyed analog signals, and vice versa for demodulation; each analog symbol may represent one or more bits. The number of symbols per second sent over a communication line called BAUD (after J.M.E. Baudot, 1845-1903, a French inventor who studied telegraph codes.)

Simple modulation techniques carry one bit per symbol. For example, in a "type 103" 300 baud modem each bit is translated to one of two tone frequencies using FSK (Frequency Shift Keying). Two sets of frequencies are used to provide full-duplex operation; each set is used for either transmit or receive, depending on whether the modem originated or answered the call. Details are given in Figure 12-2.

Complex modulation schemes such as CCITT V.22bis carry 4 bits per symbol, using a combination of amplitude and phase shift keying, to achieve a data transfer rate of 2400 bits/sec. This rate is usually called "2400 baud" in reference to the 2400 levels/sec on the RS-232C line; the symbol rate on the telephone line is 600 baud.