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Frequency Division Multiplexing (FDM) is a networking technique in which multiple data signals are combined for simultaneous transmission via a shared communication medium. FDM uses a carrier signal at a discrete frequency for each data stream and then combines many modulated signals.
When FDM is used to allow multiple users to share a single physical communications medium (i.e. not broadcast through the air), the technology is called frequency-division multiple access (FDMA).
Frequency Division Multiplexing has many applications. Stereo FM transmissions use carrier signals referred to as subcarriers (a previously modulated signal modulated into another signal of higher frequency and bandwidth), which differentiates signals to left and right channels, one for each speaker and sometimes a third, forth and fifth signals for three, four or five speaker sound systems. Television channel signals are divided up into different subcarrier frequencies for video, audio and color. And DSL (digital subscriber line) transmissions use different subcarrier frequencies for voice, upstream and downstream data transmissions (frequency duplex,” i.e. simultaneous transmissions in both directions); these too are all multiplexed into the same communications medium.
Twentieth century telephone companies used FDM for long-distance connections to multiplex thousands of voice signals through co-axial cable systems. This was done in numbers of stages by utilizing channel banks (a device for multiplexing or demultiplexing groups of channels, i.e. physical transmission paths). Shorter distance connections used communications mediums (cables) with less bandwidth, some with only 12, and later 24, voice channels multiplexed using four wires – a pair for each direction; these were common in households and small businesses. These signals went through twisted pair telephone lines, i.e. copper wires with insulation twisted around each other to prevent signals from interfering with each other, known as crosstalk or more technically, electromagnetic induction. However, near the 21st century use of FDM became rare and instead it was replace with time-division multiplexing (TDM).