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
- Frequency Division Multiplexing
- 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
- Synchronous Time division Multiplexing
- 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.