There are several categories of multiplexing, and they include analog and digital. Analog has Frequency-Division Multiplexing and Wave-Division Multiplexing and digital has Time-Division Multiplexing. The need for different frequency bands ensures noise is filtered out and avoids overlapping.
Frequency-Division Multiplexing was initially used with telephone exchange networks. With Frequency-Division Multiplexing, analog signals are combined. A high frequency signal is the basis for the signal and this signal is called the carrier signal. With Frequency-Division Multiplexing, all signals are transmitted simultaneously with non-overlapping frequency bands. Frequency-Division Multiplexing must have sufficient bandwidth. Signal overlapping is separated by guard bands. Frequency-Division Multiplexing divides single bandwidths into different frequencies. With digital, information is modulated; modifying its amplitude, frequency or phase, and this is called the modulating signal.
With Wave-Division Multiplexing, it is really the same as Frequency-Division Multiplexing however; it involves signals through fiber-optic channels. In this scenario, light is separated by a prism. Colors (light beams) are essential to sending and receiving with Wave-Division Multiplexing.
Time-Division Multiplexing allows many signals to be used in the same channel, but different time slots can be used. With Time-Division Multiplexing, it is a digital process that handles data rate capacity if greater than required by sending and receiving devices. Essentially, Time-Division Multiplexing is a digital multiplexing that combines data by collecting and storing transmissions and allocating and sending it, as appropriate.
Time-Division Multiplexing can be used both synchronous and asynchronous.