MULTIPLEXING-MODULE 2
• Multiplexing (sometimes contracted to muxing) is the set of techniques that allow the
simultaneous transmission of multiple analogue or digital signals across a single data link.
• The aim multiplexing is to share a rare or expensive resources. For example, in
telecommunications, several telephone calls may be carried using one wire no dedicated
connection is required for each pair.
• By promoting sharing of rare or expensive resources, multiplexing can also reduce the
wattage of bandwidth resource, results efficient utilization of bandwidth of a medium.
• The multiplexed signal is transmitted over a communication channel such as a cable. The
multiplexing divides the capacity of the communication channel into several logical
channels, one for each message signal or data stream to be transferred.
• A reverse process, known as demultiplexing, extracts the original channels on the receiver
end.
• A device that performs the multiplexing is called a multiplexer (MUX), and a device that
performs the reverse process is called a demultiplexer (DEMUX or DMX).
• In a multiplexed system, n lines share the bandwidth of one link. The lines on the left direct
their transmission streams to a multiplexer, which combines them into a single stream
(many-to-one).
• At the receiving end, that stream is fed into a demultiplexer, which separates the stream
back into its component transmissions (one-to-many) and directs them to their
corresponding lines.
• The “link” refers to the physical path.
• The “channel” refers to the logical portion of a link that carries a transmission between a
given pair of lines. One link can have many (n) channels.
• There are five different types of multiplexing techniques, each designed to handle various
types of data and communication needs. These techniques include:
• Frequency Division Multiplexing (FDM)
• Time-Division Multiplexing (TDM)
• Wavelength Division Multiplexing (WDM)
• Code-division multiplexing (CDM)
• The first two are techniques designed for analogue signals and the last two are for digital
signals.
FREQUENCY-DIVISION MULTIPLEXING (FDM)
• Frequency-division multiplexing (FDM) is an analogue technique that can be applied when
the bandwidth of a link is greater than the combined bandwidths of the signals to be
transmitted.
• In FDM, the bandwidth of a single physical medium is divided into a number of smaller,
independent frequency channels.
• There is a suitable frequency gap between the 2 adjacent signals to avoid over-lapping.
Since the signals are transmitted in the allotted frequencies so this decreases the
probability of collision.
• The frequency spectrum is divided into several logical channels, in which every user feels
that they possess a particular bandwidth.
• In FDM systems, the multiplexer accepts input from the user(s), converts the data streams
to analogue signals using either fixed or dynamically assigned frequencies, and transmits
the combined analogue signals over a medium .
• The demultiplexer then accepts the combined analogue signals, separates out one or more
of the individual analogue signals, and delivers these to the appropriate user(s).
• To keep one signal from interfering with another signal, a set of unused frequencies called
a guard band is usually inserted between the two signals to provide a form of insulation.
Advantages of FDM
• FDM does not need synchronization between its transmitter and receiver for proper
operation
• FDM provides more latency than TDM.
• Using FDM, a large number of the signal can be transmitted simultaneously
• The demodulation process in FDM is very easy and simple.
• Using FDM system multimedia data can be transferred with very high efficiency and low
noise and distortion.
• FDM system has high reliability and is less expensive.
• In FDM, due to slow narrowband fading, only one channel gets affected.
Disadvantages of FDM
• FDM suffers problem of cross-talk.
• FDM is used only when a few low speed channels are desired.
• FDM suffers from intermodulation distortion.
• A large number of modulators and filters are required.
• FDM system needs a carrier wave or carrier signal
• FDM requires more hardware than TDM.
• The full bandwidth of a channel cannot be used in FDM.
• The communication channel must have a very large bandwidth.
Applications of FDM
• FDM is commonly used in TV networks.
• FDM is used for FM & AM radio broadcasting.
• First generation cellular telephone also used FDM.
• FDM system is used for multimedia data such as video, audio image transmission.
• FDM system is used in the telephone system. FDM helps to transmit multiple phone calls
over a single transmission line of a single link.
WAVELENGTH -DIVISION MULTIPLEXING
• It is an analogue technique.
• Wavelength division multiplexing (WDM) is a technique of multiplexing multiple optical
carrier signals through a single optical fiber channel by varying the wavelengths of laser
lights.
• WDM allows communication in both the directions in the fiber cable.
• WDM is designed to use the high data rate capability of fibre optical cable.
• WDM can be achieved using prism.
• The technique assigns a uniquely coloured laser to each input source and combines the
multiple optical signals of the input sources and amplified as a group and transported over
a single fibre.
• A WDM system uses a multiplexer at the transmitter to join the several signals together
and a demultiplexer at the receiver to split them apart.
• The 3 different types are wavelength patterns normal (WDM), coarse (CWDM) and dense
(DWDM).
• Normal WDM uses the two normal wavelengths 1310 and 1550 on one fibre.
• Coarse WDM provides up to 16 channels across multiple transmission windows of silica
fibres.
• Dense WDM (DWDM) uses the C-Band (1530 nm-1565 nm) transmission window but with
denser channel spacing.
Advantages of WDM:
o WDM is a quite simple technique.
o Fall duplex transmission is possible.
o Easier to reconfigure
o Optical components are similar and more reliable.
o It provides higher bandwidth.
o It allows secured transmission of optical signal
o Long distance communication with low signal loss
o High security
Disadvantages of WDM:
o The presence of optical components increases the overall cost of the system.
o Cost of system increases with addition of optical components
o Signals cannot be very close. Proper wavelength spacing must be required otherwise
it will lead to signal interference.
o Light wave carrying WDM are limited to 2-point circuit.
TIME DIVISION MULTIPLEXING
• It is a digital technique.
• Time division multiplexing (TDM) is a technique used to transmit multiple signals
simultaneously over a single communication channel.
• The total time available in the channel is distributed among different users.
• In TDM, the channel is divided into several time slots of fixed length per subchannel, and
each signal is transmitted during its allocated time slot.
• TDM is commonly used in telecommunications, broadcasting, and computer networking to
increase data transmission efficiency.
• There are 2 types: Synchronous and Asynchronous
Advantages:
o The hardware required for TDM is very less.
o In the TDM system, the full bandwidth of a channel can be used.
o TDM has dynamic coordination.
o TDM provides more throughput than FDM.
o TDM system does not need any carrier wave or carrier signal
o There is no intermodulation distortion.
o Time division multiplexing systems are more flexible than frequency division
multiplexing.
o Time division multiplexing circuitry is not complex.
Disadvantages:
o Synchronization is required in time division multiplexing.
o Complex to implement.
o Due to slow narrowband fading, all the TDM channels may get wiped out.
o . In Time Division Multiplexing system, address information and buffer is needed.
o Another noticeable disadvantage of TDM is that it provides less latency than FDM
and thus extra guard time are necessary.
o It is not much suitable for analogue signals.
Applications of TDM:
o Used in digital audio mixing.
o Used in PCM transmission system.
o In Half Duplex Communication system, TDM is used.
o TDM is used in Synchronous Digital Hierarchy or SDH system.
o TDM is used in GSM or Global System for Mobile communication technology.
o TDM is used in Satellite Access
o TDM is used in Cellular Radio.
