Frequency Division Multiplexing (FDM)

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).

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).

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