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Wednesday, September 25, 2013

What is meant by multiplexing?

- Multiplexing or muxing is a very important process in computer networks and the telecommunications. 
Using this process, a number of digital data streams or analog message signals are combined as one signal and then transported over the common medium. 
Multiplexing is used wherever it is required to share a resource that is very expensive. 
- The most common example of multiplexing is of using one wire for several telephone calls. 
- The origin of the multiplexing dates back to 1870s when telegraphy was started.
- Now it is used to a great extent in the field of communications. 
- The telephone carrier multiplexing was developed by George Owen Squire in the field of telephony. 
- The communication channel over which the multiplexed signal might be transmitted might be a physical transmission medium. 
- The high level communication channel’s capacity is divided by multiplexing process in to a number of low level logical channels where for each message or data stream one channel is used. 
- Demultiplexing is the reverse process of multiplexing. 
- This is used for the extraction of the original signals on the reception side.
- A multiplexer or MUX is a device that is used for carrying out the multiplexing process and the demultiplexer or DEMUX is the device that performs demultiplexing. 
- IMUX or inverse multiplexing is another process whose aim is just the opposite of the multiplexing.
- It breaks down a single data stream in to various streams while transferring them at the same time over various communication channels.
- Later, the original stream is recreated.

Types of Multiplexing

Many different types of multiplexing technologies are available today. Each has its own significance:

Ø SDM or space-division multiplexing: 
This technique implies on using different point – to – point wires for individual communication channels. For example, an audio cable of analogue stereo, multi – pair telephone cable, switched star network, mesh network. However typically the wired SDM is not usually considered as multiplexing. In SDM a phased array antenna is formed by multiple antennas. For example MIMO (multiple – input and multiple – output), SIMO (simple – input and multiple – output), MISO (multiple – input and single – output) etc.

Ø  FDM or frequency-division multiplexing: 
This is considered to be an analog process, here the signals are sent in to different frequency ranges over a shared medium. For example, TV and radio broadcasting from satellite stations through the earth’s atmosphere. One cable is given in each house but over this cable many signals can be sent to other subscribers also. For accessing the desired signal, the users require to tune to that particular frequency. WDM or wavelength division multiplexing is a variant of FDM.

Ø TDM or time-division multiplexing: 
Unlike FDM, TDM is a digital technology but very rarely it might be used as an analog technology also. The process involves putting bytes in a sequence for each input stream one by one. this sequencing is done in such a way that the receiver can appropriately  receive them. If this is done quickly, the fact that another logical communication path was served in that circuit time won’t be detected by the receiver.

Ø  CDM or code-division multiplexing: 
In this multiplexing technique, the same frequency spectrum is shared by the several channels at the same time. Also the bandwidth of the spectrum is quite high when compared to the symbol rate or the bit rate. It is implemented in either of the two forms namely direct sequence spread spectrum and frequency hopping.

Some other types of multiplexing techniques which are less prominent are:
Polarization-division multiplexing: Used in optical and radio communications.
Orbital angular momentum multiplexing

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