Multiplexing

Multiplexing is a technique that combines data from n number of channels and transmit that data over a single communication channel for efficient Bandwidth utilization of that transmission medium. eg a water pipe carries water to several separate houses at once.

Why we need Multiplexing?

Most of the individual data-communicating devices typically require modest data rate. But, communication media usually have much higher bandwidth. So, two communicating stations do not utilize the full capacity of a data link. When the bandwidth of a medium is greater than individual signals to be transmitted through the channel, a medium can be shared by more than one channel of signals. That is known as multiplexing.

Types of Multiplexing

1.  Frequency Division Multiplexing
2. Time Division Multiplexing

Frequency Division Multiplexing

1. FDM is an analog multiplexing technique. Basic approach is to divide the available bandwidth of a single physical medium into a number of smaller, independent frequency channels.

2. Thus, many relatively narrow Bandwidth channels can be transmitted over the single wide bandwidth transmission system without interfering with each other.

For example Commercial AM broadcast band occupies frequency spectrum from 535KHZ-1605KHZ.

Each broadcast station carries an information signal (voice and music) that occupies a Bandwidth between 0 Hz to 5KHz.

Each station amplitude modulates a different carrier frequency and produces a 10 KHz signal. Carrier frequencies of adjacent stations are separated by 10KHz, the total commercial AM broadcast band is divided into 107 10 KHz frequency slots stacked next to each other in frequency domain.

            

Figure shows, how three voice signals from three sources are modulated by different carrier and multiplexed together and transmitted over single transmission medium. Voice signal range is 0-5KHz. suppose carrier f1 is 100 KHz, Carrier f2 is 105KHz and carrier f3 is 110 KHz. So output signal of Modulator 1 is 100-105 KHZ, Modulator 2 is 105-110 KHz and modulator 3 is 110-115 kHz using SSB-SC Modulation. After multiplexing these signals output band is 100-115KHz band that is transmitted over communication medium.

Other applications of FDM like commercial FM and television broadcasting, cable television etc.

Time Division Multiplexing

TDM is a technique used to transmitting several message signals from different sources over a single communication channel by dividing time frame into slots, one slot for each message signal.

            As shown in Diagram there are four channels, channel 1 red, channel 2 blue, channel 3 yellow and channel 4 green. Multiplexing means combining information from all channels and send it on common carrier like coaxial cable, optical fiber cable etc. Here we are using time division multiplexing, where each channel uses entire bandwidth for particular allotted time. Information is forwarded in form of frames and frame is further divided into time slots. Each channel uses its own time slot to forward information and get full access of bandwidth for that particular time.

As shown in above diagram, channel 1, channel 2, channel 3 and channel 4 uses their allotted time slot and form a frame and that frame is transmitted over transmission medium. MUX and DEMUX acts as a digital switch. Take data from channel 1 and immediately switch to channel 2 and so on. At the receiver end, DE multiplexer, receives that frames and sends information to respective channel by getting information from particular slots.

Types of Time division Multiplexing

1.   Synchronous Time division Multiplexing
2.   Asynchronous Time Division Multiplexing 

Synchronous Time division Multiplexing

1. In synchronous TDM, each device is allotted a time slot in frame to transmit their data. If any device has no information to send, that slot is kept empty in that particular frame and transmits that frame. This causes wastage of bandwidth.

 2. There is abundance of time slots within each frame, which contain no information (i. e. at any time instant, several of the channels may be idle).

For example in PCM-TDM system, the voice conversation over telephone, information is transferred only in one direction at a time and causes several pauses that causes wastage of Bandwidth.

As shown in above figure, there are four sources 1,2 and 3. During each sample time, data is collected from all sources in a frame.  In frame 4, only source A and B has data to send, rest slot is transmitted empty as it has nothing to send. This causes wastage of Bandwidth in TDM.

Asynchronous TDM

1. Asynchronous TDM is alternative to synchronous TDM to avoid wastage of Bandwidth.

2. It dynamically allocates time slots on demand basis.

3. Asynchronous MUX has a finite number of low speed data input lines with one high speed multiplexed data output line and each line has its own digital encoder and buffer.

4 MUX scans the input buffers and collect data until frame is filled and then transmitted.

5. At receiver end, DE multiplexer removes the data from frame and distributes them to their appropriate output buffers.