Essential Guide to Telecommunications, The, Third Edition

Modems—Analog Telephone Lines for Transmitting Data from Digital Devices

Modems convert digital signals received from computers into analog signals and transmit them over analog telephone lines. The process of converting digital signals to analog and modifying them for transmission is called modulation. This makes the signals compatible with analog telephone lines. At the receiving end, the modem demodulates the signal or converts it from analog to digital and transmits it to the data terminal equipment (DTE) (e.g., computer or T-1 multiplexer).

When modems were first used, the Bell system, at that time AT&T, set modem standards. (See Table 7.4 at the end of this chapter.) Starting in the 1980s, other modem manufacturers began making higher speed ...