Standard telephone lines can transmit only analog signals. Computers, however, store and transmit digitally. Modems can transmit digital computer 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. Odem’s can be used to connect computer 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) data-up lines and make a connection only when one is required. Odem’s enable networks to exchange extremely limited. By themselves, modems don’t enable remote networks to connect to each other and directly exchange data.

Until recently, modem manufactures 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.


Modems are classified a/c to the transmission method they use for sending & receiving data. The two basic types of modems are as follows:

Asynchronous modems.

Synchronous modems.

The following sections describe asynchronous & synchronous transmission.


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