Modems

Modems provide serial communications across ordinary telephone lines. Modem concepts include standards, general modem setup, and specific configuration tips for popular modems.

A modem is a device that allows you to connect one computer to another across ordinary telephone lines. The current telephone system is incapable of carrying the voltage changes required for a direct digital connection. A modem overcomes this limitation by modulating digital information into audio tones for transmission across the phone line, and by demodulating those tones back into digital information on reception. Modems are commonly used with Basic Network Utilities (BNU) or other implementations of the UNIX-to-UNIX Copy Program (UUCP). A high-speed (14,400 bps or greater) modem can be used with Serial Line Interface Protocol (SLIP) to provide Transmission Control Protocol/Internet Protocol (TCP/IP) connectivity as well.

Often, the term baud is used to refer to modem speed instead of bps. Baud is actually a measurement of the modulation rate. In older modems, only 1 bit was encoded in each signal change, so modem baud rate was equal to modem speed. Modems that operate at higher speeds, however, still generally operate at 2,400 (or even 1,200) baud, and encode two or more bits per signal change. A modem's bps rate is calculated by multiplying the number of data bits per signal with the baud (for example, 2,400 baud x 6 bits per signal change = 14,400 bits per second). Most modern modems can communicate at a variety of speeds (for example, 28,800, 14,400, 9,600, 7,800, 4,800, and 2,400 bps).