What is a Multiplexer?
A multiplexer is a device that combines and transmits multiple signals over a single transmission medium （copper wires, fiber optics or air）. External circuits are the most difficult and costly part in telecommunications network infrastructure. Long distance or international telephone lines are even more costly. Telecommunications engineers use multiplexers to build many communication routes with these scarce media so that a greater amount of information and data can be transmitted.Basic multiplexing technologies can be divided into several types: frequency division multiplexing （FDM）, time division multiplexing （TDM）, code division multiple access （CDMA）, and wavelength division multiplexing （WDM）.
There are three main types of multiplexers: 1. Signal conversion: Voice or data are converted to signals adequate for multiplexing. 2. Multiplexing and demultiplexing: Signals from multiple channels are aggregated in an efficient and orderly manner and sent to a transmission medium. Or conversely, the composite signal is separated by a device called demultiplexer. 3. Repeating: A weakened or distorted signal is picked up by a device called a repeater that reshapes, amplifies or regenerates the signal for re-transmission. FDM analog transmission was the dominant technique in the early days in Taiwan. PCM TDM communication systems were introduced to Taiwan in the 1970s. Fiber optics technology was introduced in the 1980s. Today, 80-wavelength, 40G WDM-TDM systems are available. CDMA is typically used in microwave mobile and wireless communications systems. Different variations of DSL （xDSL） technology use frequency division multiplexing to allow digital data transmission over existing telephone copper wires.