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In telecommunications, frequency-division multiplexing (FDM) is a technique by which the total bandwidth available in a communication medium is divided into a series of non-overlapping frequency sub-bands, each of which is used to carry a separate signal. This allows a single transmission medium such as a cable or optical fiber to be shared by many signals. An example of a system using FDM is cable television, in which many television channels are carried simultaneously on a single cable. FDM is also used by telephone systems to transmit multiple telephone calls through high capacity trunklines, communications satellites to transmit multiple channels of data on uplink and downlink radio beams, and broadband DSL modems to transmit large amounts of computer data through twisted pair telephone lines, among many other uses.
An analogous technique called wavelength division multiplexing is used in fiber optic communication, in which multiple channels of data are transmitted over a single optical fiber using different wavelengths (frequencies) of light.
How it works 
At the source end, for each frequency channel, an electronic oscillator generates a carrier signal, a steady oscillating waveform at a single frequency such as a sine wave, that serves to "carry" information. The carrier is much higher in frequency than the data signal. The carrier signal and the incoming data signal (called the baseband signal) are applied to a modulator circuit. The modulator alters some aspect of the carrier signal, such as its amplitude, frequency, or phase, with the data signal, "piggybacking" the data on the carrier. Multiple modulated carriers at different frequencies are sent through the transmission medium, such as a cable or optical fiber.
Each modulated carrier consists of a narrow band of frequencies, centered on the carrier frequency. The information from the data signal is carried in sidebands on either side of the carrier frequency. This band of frequencies is called the passband for the channel. As long as the carrier frequencies of separate channels are spaced far enough apart so that their passbands do not overlap, the separate signals will not interfere with one another. Thus the available bandwidth is divided into "slots" or channels, each of which can carry a data signal.
At the destination end of the cable or fiber, for each channel, an electronic filter extracts the channel's signal from all the other channels. A local oscillator generates a signal at the channel's carrier frequency. The incoming signal and the local oscillator signal are applied to a demodulator circuit. This translates the data signal in the sidebands back to its original baseband frequency. An electronic filter removes the carrier frequency, and the data signal is output for use.
Modern FDM systems often use sophisticated modulation methods that allow several data signals to be transmitted through each frequency channel.
For long distance telephone connections, 20th century telephone companies used L-carrier and similar co-axial cable systems carrying thousands of voice circuits multiplexed in multiple stages by channel banks.
For shorter distances, cheaper balanced pair cables were used for various systems including Bell System K- and N-Carrier. Those cables didn't allow such large bandwidths, so only 12 voice channels (Double Sideband) and later 24 (Single Sideband) were multiplexed into four wires, one pair for each direction with repeaters every several miles, approximately 10 km. See 12-channel carrier system. By the end of the 20th Century, FDM voice circuits had become rare. Modern telephone systems employ digital transmission, in which time-division multiplexing (TDM) is used instead of FDM.
The concept corresponding to frequency-division multiplexing in the optical domain is known as wavelength-division multiplexing.
Group and supergroup 
A once commonplace FDM system, used for example in L-carrier, uses crystal filters which operate at the 8 MHz range to form a Channel Group of 12 channels, 48 kHz bandwidth in the range 8140 to 8188 kHz by selecting carriers in the range 8140 to 8184 kHz selecting upper sideband this group can then be translated to the standard range 60 to 108 kHz by a carrier of 8248 kHz. Such systems are used in DTL (Direct To Line) and DFSG (Directly formed super group).
132 voice channels (2SG + 1G) can be formed using DTL plane the modulation and frequency plan are given in FIG1 and FIG2 use of DTL technique allows the formation of a maximum of 132 voice channels that can be placed direct to line. DTL eliminates group and super group equipment.
DFSG can take similar steps where a direct formation of a number of super groups can be obtained in the 8 kHz the DFSG also eliminates group equipment and can offer:
- Reduction in cost 7% to 13%
- Less equipment to install and maintain
- Increased reliability due to less equipment
Both DTL and DFSG can fit the requirement of low density system (using DTL) and higher density system (using DFSG). The DFSG terminal is similar to DTL terminal except instead of two super groups many super groups are combined. A Mastergroup of 600 channels (10 super-groups) is an example based on DFSG.
FDM can also be used to combine signals before final modulation onto a carrier wave. In this case the carrier signals are referred to as subcarriers: an example is stereo FM transmission, where a 38 kHz subcarrier is used to separate the left-right difference signal from the central left-right sum channel, prior to the frequency modulation of the composite signal. A television channel is divided into subcarrier frequencies for video, color, and audio. DSL uses different frequencies for voice and for upstream and downstream data transmission on the same conductors, which is also an example of frequency duplex.
FDMA is the traditional way of separating radio signals from different transmitters.
In the 1860s and 70s, several inventors attempted FDM under the names of Acoustic telegraphy and Harmonic telegraphy. Practical FDM was only achieved in the electronic age. Meanwhile their efforts led to an elementary understanding of electroacoustic technology, resulting in the invention of the telephone.
See also 
- Duplex (telecommunications)
- Single-sideband modulation
- Orthogonal frequency-division multiplexing (OFDM),
- Time-division multiplexing (TDM)
- Harold P.E. Stern, Samy A. Mahmoud (2006). "Communication Systems: Analysis and Design", Prentice Hall. ISBN 0-13-040268-0.