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Modems
Standard telephone lines can transmit only analog signals. Computers, however, store and transmit data digitally. Modems can transmit digital computer signals over telephone lines by converting them to analog form.

Converting one signal form to another (digital to analog in this case) is called modulation. Recovering the original signal is called demodulation. The word “modem” derives from the terms modulation/demodulation.

Modems can be used to connect computer devices or entire networks that are at distant locations. (Before digital telephone lines existed, modems were about the only way to link distant devices.) Some modems operate constantly over dedicated phone lines. Others use standard public switched-telephone network (PSTN) dial-up lines and make a connection only when one is required.

Modems enable networks to exchange e-mail and to perform limited data transfers, but the connectivity made possible is extremely limited. By themselves, modems don’t enable remote networks to connect to each other and directly exchange data. In other words, a modem is not an internetwork device. Nevertheless, modems can be used in conjunction with an internetwork device, such as a router, to connect remote networks through the PSTN or through an analog service, such as a 56 KB line.


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Modems don’t necessarily need to connect through the PSTN. Short-haul modems frequently are used to connect devices in the same building. A standard serial connection is limited to 50 feet, but short-haul modems can be used to extend the range of a serial connection to any required distance.

Many devices are designed to operate with modems. When you want to connect such devices without using modems, you can use a null-modem cable, which connects the transmitter of one device to the receiver of the other device.

Until recently, modem manufacturers used a parameter called baud rate to gauge modem performance. The baud rate is the oscillation speed of the sound wave transmitted or received by the modem. Although baud rate is still an important parameter, recent advances in compression technology have made it less meaningful. Some modems now provide a data transfer rate (in bits per second—a more meaningful measure of network performance) that exceeds the baud rate. In other words, you can no longer assume the baud rate and the data transfer rate are equal.

Modems are classified according to the transmission method they use for sending and receiving data. The two basic types of modems are as follows:
The following sections describe asynchronous and synchronous transmission.

Further Information


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