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Analog Transmission Systems
Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.
There are analog transmission systems and digital transmission systems. In an analog transmission system, signals propagate through the medium as continuously varying electromagnetic waves. In a digital system, signals propagate as discrete voltage pulses (that is, a positive voltage represents binary 1, and a negative voltage represents binary 0), which are measured in bits per second.
The medium for an analog transmission may be twisted-pair cable, coaxial cable, optical-fiber cable, the atmosphere, water, or space. A technique called "modulation" is used to combine an input signal (the data) onto a carrier signal. The carrier signal is a specific frequency. When tuning a radio, you select a particular carrier frequency in order to tune in that radio station. There are two primary modulation techniques: amplitude modulation, which varies the amplitude (height) of the carrier signal; frequency modulation, which modulates the frequency of the carrier. Refer to "Modulation Techniques" for more information.
The frequency ranges of several analog transmission systems are listed here:
In data communications, analog signals are used to transmit information over the telephone system or over radio transmission systems (such as satellite links). A modem converts digital data to analog signals. Alternatively, analog signals can be converted to digital information using a codec (coder/decoder). This process is called digitizing. Phones that connect to all-digital communication links use codecs to convert analog voice signals to digital signals. The phone company digitizes voice transmissions between its central offices and long-distance sites. In fact, the only remaining analog portion of the phone system is the twisted-pair wire that runs between homes and the telephone companies' central offices, which are usually less than a mile distance from the subscriber.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.