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Bandwidth Utilization: Multiplexing and Spreading

* Multiplexing is the simultaneous transmission of multiple signals across a single data link.
* Frequency-division multiplexing (FDM) and wave-division multiplexing (WDM) are techniques for analog signals, while time-division multiplexing (TDM) is for digital signals.
* In FDM, each signal modulates a different carrier frequency. The modulated carriers are combined to form a new signal that is then sent across the link.
* In FDM, multiplexers modulate and combine signals while demultiplexers decompose and demodulate.
* In FDM, guard bands keep the modulated signals from overlapping and interfering with one another.
* Telephone companies use FDM to combine voice channels into successively larger groups for more efficient transmission.
* Wave-division multiplexing is similar in concept to FDM. The signals being multiplexed, however, are light waves.
* In TDM, digital signals from n devices are interleaved with one another, forming a frame of data (bits, bytes, or any other data unit).
* Framing bits allow the TDM multiplexer to synchronize properly.
* Digital signal (DS) is a hierarchy of TDM signals.
* T lines (T-1 to T-4) are the implementation of DS services. A T-1 line consists of 24 voice channels.
* T lines are used in North America. The European standard defines a variation called E lines.
* Inverse multiplexing splits a data stream from one high-speed line onto multiple lower-speed lines.