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. e.g. 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.
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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.
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CONTACT US
Dingshengwei Electronics Co., Ltd
Company Address: Buliding A,the first industry park of
Guanlong,Xili Town,Nanshan,Shenzhen,Guangdong,China
Tel : +86 0755 26909863
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