Technical aspects of data communication (3rd ed.)
Technical aspects of data communication (3rd ed.)
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A modem is a device used to transmit data between computers, workstations, and other peripheral devices interconnected by means of conventional telephone communication lines supporting analog transmission. Modems transform (modulate) data from a digital device to analog form suitable for transmission over such lines. Since, in general, data flows in both directions, modems are also able to receive an analog signal from some remote device and restore (demodulate) it back to its original digital form, as shown in Fig. 1. The word "modem" stems from the modulation-demodulation process performed. In addition to performing the basic transformation between digital and analog signals, modems also can perform a variety of control functions that coordinate data flow over the analog communication link. Terms usually used synonymously for modems include data set or data phone. A DSU/CSU (data service unit/channel service unit) performs the same function as a modem, but is used on communication lines supporting digital transmission conforming to AT&T publication 62310.