Multiplexing
In digital communication and computer networks, multiplexing or simply mux is a method by which multiple
analog or digital signals are combined into one signal over a shared medium. The aim is to share a limited resource over a
signal channel.
Multiplexing is achieved by using a
device called multiplexer that combines n input lines to generate a single
output line. Multiplexing follows many-to-one approach, i.e., n input lines and
one output line.
Demultiplexing is used at the receiver
end for decoding the multiplex signal or combined signal. Demultiplexer
separates a signal into its component signals (i.e., one input and n outputs).
Therefore, we can say that demultiplexing follows the one-to-many approach.
Advantages of Multiplexing:
1.
More than one signal can be sent over a single medium.
2.
The bandwidth of a medium can be utilized effectively.
Multiplexing Techniques:
Multiplexing can be classified as:
1.
Frequency
Division Multiplexing (FDM)
2.
Time
Division Multiplexing (FDM)
3.
Wavelength
Division Multiplexing (WDM)
The Time Division
Multiplexing can be further divided into:
1.
Synchronous
TDM
2.
Asynchronous
TDM
The below figure shows the classification of modulation
techniques.
Let’s take an
overlook on all the multiplexing techniques for better understanding purpose.
Frequency Division Multiplexing (FDM):
Frequency division multiplexing is a multiplexing technique in which
multiple separate information signals can be transmitted over a single
communication channel by occupying the different frequency slots within common
channel bandwidth.
In Frequency division multiplexing, simultaneous transmission of the signal takes place over a common
channel in which the channel bandwidth is divided into
various sub-channels. These sub-channels comprised of different frequency slots
for carrying individual signal during transmission.
When we talk about the transmission of various signals to a long
distance through FDM technique, the required bandwidth needed for transmission
must also be large. Now, the question arises how can we provide
multiple signals, various frequency slots under the same channel
bandwidth?
The answer to the above question is modulation. The
different input signal modulates different carriers having different
frequencies. These signals are then mixed to form a hybrid signal for
transmitting it over a single channel.
When we talk about a common communication channel the thing that first
comes to our mind is overlapping of signals. But here the total available
bandwidth is divided into various non-overlapping frequency bands which will
carry different input signal thus preventing them from causing interference. We
can use the full allotted time for transmission, the only need is the division
of frequency. Sometimes this transmission during the same time interval leads
to crosstalk.
FDM Transmitter:
The numerous signals that are to be transmitted along a common channel
modulate different carriers in the modulating section. The output of the
modulator will have multiple signals of different carrier frequency.
The modulated signals are then fed to a linear mixer which is different
from a normal mixer.
Linear mixer simply produces the algebraic sum of the generated
modulated signals. The combined signal at the output of the mixer is then
transmitted along a single channel.
The below figure describes the frequency division multiplexing (FDM) transmitter
concept in an easy way.
Fig. 2: Frequency Division Multiplexing (FDM) transmitter.
FDM Receiver:
Now, the receiver section will have a composite signal that was
transmitted by the linear mixer over a channel.
This composite signal is then fed to different filters mainly BPF each
having a centre frequency corresponding to the carrier frequency.
The BPF passes the channel information without any distortion. BPF
rejects signals of all other frequencies and accepts the signal of the desired
centre frequency.
Further, the signals after being processed by the BPF goes to individual demodulator section where demodulation of the signals takes place to separate modulating signal from that of the carrier signal.
So, after demodulation, we can have separate signals that were actually
transmitted over the same channel.
The below figure describes the frequency division multiplexing (FDM) receiver
concept in an easy way.
Fig. 3: Frequency Division Multiplexing (FDM) receiver.
Wavelength
Division Multiplexing (WDM):
Wavelength
Division multiplexing (WDM) is an analog technique, in which many data streams
of different wavelengths are transmitted in the light spectrum. If the
wavelength increases, the frequency of the signal decreases. A prism which can
turn different wavelengths into a single line, can be used at the output of MUX
and input of DEMUX.
Example − Optical fiber Communications use
the WDM technique, to merge different wavelengths into a single light for the
communication.
Time Division Multiplexing (TDM):
Time-division multiplexing (TDM) is a method of transmitting and receiving
independent signals over a common signal path by means of synchronized switches
at each end of the transmission line so that each signal appears on the line
only a fraction of time in an alternating pattern.
As we know, multiplexing allows the transmission of several signals over
a common channel. However, one may need to differentiate between the various
signal for proper data transmission. So, in time division
multiplexing, the complete signal gets transmitted by occupying different time
slots.
The below figure describes the time division multiplexing (TDM) concept
in an easy way.
Fig. 4: Time division multiplexing (TDM)
Synchronous
TDM:
In
Synchronous TDM, the input is connected to a frame. If there are ‘n’ number of
connections, then the frame is divided into ‘n’ time slots. One slot is
allocated for each input line.
In this technique, the sampling
rate is common for all signals and hence the same clock input is given. The MUX
allocates the same slot to each device at all times.
Asynchronous
TDM:
In
Asynchronous TDM, the sampling rate is different for each of the signals and a
common clock is not required. If the allotted device, for a time slot transmits
nothing and sits idle, then that slot is allotted to another device,
unlike synchronous.
This type of TDM is used in
Asynchronous transfer mode networks.
Advantages of TDM:
1. Complete
Channel Bandwidth can be utilized for each channel.
2. lnter-modulation distortion is not there.
3. Complexity is not high.
4. Crosstalk problem is severe.
Disadvantages of TDM:
1.
Synchronization is required for each channel is required.
2. Slow narrow band fading needs to be handle carefully.
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