Multiplexing

Multiplexing

Multiplexing is a technique for sending more than one information signal at a time down a single communication path(e.g. medium, circuit or channel). Multiplexing is sometimes loosely referred to as MANY into ONE.

The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side.

A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX).

Multiplexing can be achieved in a number of ways. The following three will be covered in this class:

1. Space Division Multiplexing
2. Frequency Division Multiplexing
3. Time Division Multiplexing
4. Code division multiplexing.

 

Frequency Division Multiplexing

 In FDM, the frequency bandwidth of the line is divided into a number of partitions, each of which is used as a separate logical channel. Radio and TV broadcasting represent the oldest examples of FDM. To avoid neighboring channels from interfering with one another, the extreme ends of the channel frequencies are left unused to provide a gap. For example, a line that has a bandwidth of 30 kHz can be divided into 3 times 10 kHz channels, each of which consists of 8 kHz of bandwidth for data and two gaps of 1 kHz on either side. FDM requires special multiplexing/demultiplexing hardware (MUX) at either end of the line.

Space Division Multiplexing

When we want to transmit multiple messages, the goal is maximum reuse of the given resources: time and frequency. . It involves grouping many separate wires into a common cable enclosure. A cable that has, for example, 50 twisted pairs inside it can support 50 channels. There is therefore a one-to-one correspondence between physical and logical channels.

SDM has the unique advantage of not requiring any multiplexing equipment. It is usually combined with other multiplexing techniques to better utilize the individual physical channels.

 

 For example If there are six people in the office and they all want to talk at the same time, there obviously will be some interference between the conversations taking place. To reduce the interference they may divide themselves into three groups of two, such that the conversation is between each pair of people. If the pairs continue talking whilst sitting next to each other, the interference would still be present.

The best way for each pair to converse with minimal interference would be to sit a few feet away from the other pairs (within the same room) and converse. They would still be sharing the same medium for their conversations but the physical space in the room would be divided for each conversation. This is an example of SPACE DIVISION MULTIPLEXING.