Almost all modern modems can inter-operate with fax machines. Digital faxes, introduced in the

The next major advance in modems was the Hayes Smartmodem, introduced in 1981. The Smartmodem was an otherwise standard 103A 300-bit/s modem, but it was attached to a small microcontroller that let the computer send it commands. The command set included instructions for picking up and hanging up the phone, dialing numbers, and answering calls. This eliminated the need for any manual operation, a handset, or a dialler. Terminal programs that maintained lists of phone numbers and sent the dialing commands became common. The basic Hayes command set remains the basis for computer control of most modern modems.
The Smartmodem and its clones also aided the spread of bulletin board systems (BBSs) because it was the first low-cost modem that could answer calls. Modems had previously been typically either the call-only, acoustically coupled models used on the client side, or the much more expensive, answer-only models used on the server side. These were fine for large computer installations, but useless for the hobbiest who wanted to run a BBS but then periodically use the same telephone line to call other systems. The first hobby BBS system, CBBS, started as an experiment in ways to better use the Smartmodem.
Almost all modern modems can inter-operate with fax machines. Digital faxes, introduced in the 1980s, are simply an image format sent over a high-speed (commonly 14.4 kbit/s) modem. Software running on the host computer can convert any image into fax format, which can then be sent using the modem. Such software was at one time an add-on, but has since become largely universal.
1200 and 2400 bps[edit source | editbeta]
The 300 bit/s modems used audio frequency-shift keying to send data. In this system the stream of 1s and 0s in computer data is translated into sounds which can be easily sent on the phone lines. In the Bell 103 system, the originating modem sends 0s by playing a 1,070 Hz tone, and 1s at 1,270 Hz, with the answering modem transmitting its 0s on 2,025 Hz and 1s on 2,225 Hz. These frequencies were chosen carefully, they are in the range that suffer minimum distortion on the phone system and not harmonics of each other.
In the 1,200 bit/s and faster systems, phase-shift keying was used. In this system the two tones for any one side of the connection are sent at similar frequencies as in the 300 bit/s systems, but slightly out of phase. Voiceband modems generally remained at 300 and 1,200 bit/s (V.21 and V.22) into the mid-1980s. A V.22bis 2,400-bit/s system similar in concept to the 1,200-bit/s Bell 212 signaling was introduced in the U.S., and a slightly different one in Europe. The limited available frequency range meant the symbol rate of 1,200 bit/s modems was still only 600 baud (symbols per second). The bit rate increases were achieved by defining 4 or 8 distinct symbols, which allowed the encoding of 2 or 3 bits per symbol instead of only 1. The use of smaller shifts had the drawback of making each symbols more vulnerable to interference, but improvements in phone line quality at the same time helped compensate for this. By the late 1980s, most modems could support all of these standards and 2,400-bit/s operation was becoming common.