Multiplexing is a method of transmitting more than one quantity over the same transmission channel. In practice, we are often required to transmit many quantities from the transmitter to the receiver. if a separate channel is used for each quantity, the cost of installation, maintenance and periodic replacement becomes economically prohibitive. the use of multiplexing reduces the overall cost because a single transmission channel is shared by many quantities.
There are two main types of multiplex systems which are:
- Time Division Multiplexing(TDM)
- Frequency Division Multiplexing (FDM)
- TIME DIVISION MULTIPLEXING (TDM)
in this system, the information from different measuring points is transmitted serially one after the other over the same communication channel. When all the measurements have been sampled, the cycle is repeated. The time required to complete the transmission of all the measurement may be sufficiently short so that one measurement cannot change excessively between samplings.
- FREQUENCY DIVISION MULTIPLEXING(FDM)
In this system, several information can be simultaneously transmitted using different carrier frequencies and employing modulation methods (AM, FM OR PM) discussed earlier. By proper selection of the magnitude of different carrier frequencies, several data can simultaneously be transmitted over a communication link. these can be recovered separately at the receiving end by using demodulators and filters.
