Multiplexing: definition and applications

Multiplexing is a set of techniques that allows many signals to share a single communication link at the same time. At the sender a multiplexer (MUX) combines n input lines into one link, and at the receiver a demultiplexer (DEMUX) separates them back into n output lines.

Why it is needed

• A link such as fibre, coaxial cable or a microwave channel has far more bandwidth than one user needs. Without multiplexing, most of it is wasted.
• Laying a separate cable for each connection is very costly; sharing one link cuts the cost of media, installation and maintenance.
• Better utilisation of bandwidth and equipment.
• Fewer physical links give a simpler network design.

Main techniques

• FDM (frequency division): each signal uses its own frequency band.
• TDM (time division): each signal uses the link in its own time slot.
• WDM (wavelength division): each signal uses its own light wavelength in fibre.
• CDM (code division): each signal uses its own code.

Applications

• Telephone trunks carry thousands of voice calls using FDM or TDM (T1/E1, SONET/SDH).
• Radio and TV broadcasting (AM, FM, TV channels share the air by FDM).
• Cellular networks (FDMA, TDMA in GSM, CDMA).
• Optical fibre backbones and internet service providers (WDM/DWDM).
• DSL, which separates voice and data by frequency using a splitter, and cable TV with many channels on one coaxial cable.