What Is Multiplexing, Types Of Multiplexing, Time Division Multiplexing, Frequency Division Multiplexing, Multiplexing With Advantages And Disadvantages

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Whenever the Bandwidth of a medium linking two devices is greater than Bandwidth of the device, the link can be shared.
Multiplexing is the technique that allows the simultaneous transmission of multiple signals across a single data link or channel.
The process of multiplexing increases the bandwidth requirement for the transmission

Bandwidth is one of the most precious resources we have in data communication.
In a Multiplexed system N lines shares the bandwidth of one link.
Block Diagram of Multiplexing:

Multiplexing is the method of dividing a physical channel into many logical channels so that the independent signals may be simultaneously transmitted  to network.
It is also known as muxing.
A device that performs the multiplexing is called a multiplexer (MUX).
A device that performs the demultiplexing is called a demultiplexer (DEMUX).

Aim of multiplexing
To share an expensive resource that is band width.
To maximize the utilization of channel

Categories of Multiplexing
1. Analog Multiplexing:
FDM (Frequency Division Multiplexing)
WDM (Wave length Division Multiplexing)

2. Digital Multiplexing:
TDM ( Time Division Multiplexing)


Frequency-division multiplexing (FDM)
is an analog technique that can be applied when the band width of a link (in Hz) is greater than the combined bandwidth signals to be transmitted.
In FDM signal generated by each sending device modulates different carrier frequencies.
These modulated signals are then combined in to a single composite signal that can be transmitted by the link.

Carrier frequencies are separated by sufficient bandwidth ranges of the channels through which the various signals travel.
Channels must be separated by strips of unused bandwidth (guard bands) to prevent signals from overlapping.
FDM is an analog multiplexing technique that combines signal.
Example: Radio, television broad casting, etc.



Application of FDM
FDM is used for A.M. Radio & Radio broadcasting.
550 KHz bandwidth is assign to each station.
FDM is used for T.V broadcasting.
In this frequency range is 80 – 108 MHz.
In this each channel is of 700 Mhz.

Advantages OF FDM
Popular with Radio, TV, Cable TV
All the receivers, cellular telephones, need not to be at the same location
It is not sensitive to propagation delays.
It allows maximum transmission link usage.

Disadvantages OF FDM
In FDM there is need of filters, which are very expensive and complicated to construct and design.
Analog signal only having limited frequency range.
Sometimes, it is necessary to use more complex linear amplifiers in FDM systems.


Time Division Multiplexer
Time Division multiplexing (TDM) is a digital process.
It allows several connections to share the high bandwidth of a link.
Instead of sharing a portion of the bandwidth as in FDM, here in TDM time is shared.
Each connection occupies a portion of time in the link.
Example: telephone system

In this figure of TDM a portion of signals 1,2,3 and 4 occupy the link sequentially.
The analog signals are digitized at the end office by a device called a Codec (Coder-Decoder), producing a series of 8-bit numbers.

Time Slots and Frames
The data flow of each connection is divided in to units and the link combines one unit of each connections to make a frame.
The size of the unit can be one bit or several bits.
For n input connections a frame is organized in to minimum of n time slots.
Each slot is carrying one unit from each connection as shown in figure 1.1
Telephone companies implements TDM.

Types of TDM
Synchronous TDM
Asynchronous TDM  or Statistical  TDM

Synchronous Time Division Multiplexing

In Synchronous Time Division Multiplexing, multiplexer allocates exactly the same time slot to each device at all times.
For Example: Time slot A, is assigned to device A alone and cannot be used by any other device.
Each time its allocated time slot comes up, a device has the opportunity to send a portion of its data.
If a device is has to transmit or does not have data to send, its time slot remains empty.


Synchronous Time Division Multiplexing
T-1 multiplexing (the classic)
ISDN (Integrated Services Digital Networks) multiplexing
SONET (Synchronous Optical NETwork)

Advantages of synchronous TDM
Digital signals
Relatively simple
Commonly used with ISDN (Integrated Services digital network)

Disadvantages of synchronous TDM
Wastage of bandwidth

synchronous TDM:
 Asynchronous means something different in multiplexing in the area of data communication.
It means flexible or not fixed.
Suppose number of inputs =5, then number of slots in each frame = 3
Asynchronous Multiplexer transmits only the data from active work stations.
If the workstation is not active, no space is wasted on the multiplexed streams.


A Statistical or Asynchronous multiplexers occupy the incoming data streams and creates frames containing only the data to be transmitted.
Hence, the Asynchronous Time Division Multiplexing or Statistical TDM is designed to avoid this type of wastage of bandwidth.

Diffrence between FDM & TDM