Modems provide serial communications across ordinary telephone lines. This chapter discusses modem standards, general modem setup, and specific configuration recommendations when attaching a modem to the server family of asynchronous adapters.
A modem is a device that can 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 upon reception. It is from these actions that the name MODEM is derived (MOdulate, DEModulate).
Modern modems generally communicate at speeds ranging from 2400 baud to 14,400 baud. Some modems use proprietary means to communicate at even faster rates (for example 19,200 baud). The disadvantage to these proprietary, faster modems is that in order to achieve faster speeds, identical modems (or more precisely, modems implementing the same proprietary protocol) must be used at both ends. Older modems communicate at speeds ranging from 300 baud to 1200 baud.
Often, the term baud is used to refer to a modem's speed instead of bps. Baud is actually a measurement of the modulation rate. In older modems, there was only 1 bit encoded in each signal change, so a modem's baud rate was equal to the modem's speed. Modems that operate at higher speeds, however, still generally operate at 2400 (or even 1200) 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, 2400 baud x 6 bits per signal change = 14,400 bps). Most modern modems can communicate at a variety of speeds (for example: 14400, 9600, 7800, 4800, and 2400 baud). In general, the terms bps and baud are interchangeable.
Figure showing serial communications using modems:
The following sections contain detailed information about modems:
