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Code division multiplexing (CDM) is a networking technique in which multiple data signals are combined for simultaneous transmission over a common frequency band.
When CDM is used to allow multiple users to share a single communications channel, the technology is called code division multiple access (CDMA).
CDMA uses spread spectrum, a technology that was developed in World War II to prevent enemies from intercepting and jamming transmissions. In spread spectrum, a data signal is sent over a range of frequencies in an assigned frequency spectrum.
A pseudo-random spreading code is used to multiplex the base signal. Multiplexing with a spreading code increases the bandwidth required for the signal, spreading it out over the available spectrum. The receiving device is aware of the spreading code and uses it to demultiplex the signal.
CDMA provides a certain amount of built-in security, as the transmissions of multiple users are mixed together within the frequency spectrum. The spreading code is required to decode a specific transmission.
Different variations of CDM and CDMA are used in 2G and subsequent generations of cellphone technology.