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Analog Transmission Systems

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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:

300-3,000 kHz

AM radio

3 to-30 MHz

Shortwave and CB radio

30-300 MHz

VHF television and FM radio

300-3,000 MHz

UHF television and cellular telephones, and microwave systems

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.

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