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A Modem is Modulation De-Modulation device. It converts the discrete stream of digital on-off electrical pulses generated by computing equipment into a type of continuously variable analog wave patterns, for example, used for transmission of human voice. Digital pulses cannot effectively travel long distance over the transmission network that was designed years ago for voice communications. Thus a modem is needed to modulate or convert the digital pulses into analog wave patterns for transmitting data on telephone lines. Similarly another modem is needed at the receiving end to demodulate or reconvert, or recover, the digital data from the transmitted signal.

When the I/O equipment is not likely to be moved, modem is hard-wired to the equipment. Some such direct-connect modems have built in micro processors and other specialized communication chips. These intelligent modems can automatically perform dialing, answering, and disconnecting functions. But not all modems are hard wired. Where communication network is designed for all-digital transmission, modem is not required.

A data transmission channel carries data from one location to the other and is classified as narrowband, voice band or wideband category. The wider the band, more data it can transmit in a given period of time. Telegraph line are narrowband channels with transmission rate of 5 to 30 characters per second (cps, whereas standard telephone lines are voice band with rate of over 1000cps. Broadband channels are used for higher speeds (over 100,000 cps). Coaxial cables, microwave circuits, and communications satellites are used to provide these broadband channels, Fiber optic cables and laser technology provides broadband channels, cheaper and in large numbers.

In many cases, a terminal operator at a remote station uses a regular dial-up telephone switching network, calls up a number at the CPU location and enters data. When data volume is sufficient, it is economical to acquire dedicated or leased line which can be used both for voice and date purposes by the users like Professional Web Design.