Multiplexer & Demultiplexer With Circuit Diagrams | Types & Applications


Table of Contents

What is the Multiplexer?


Fig 1: Multiplexer

Multiplexer is a combinational logic circuit that accepts N input signals and gives one output signal.

Still confused about Multiplexer?

In general, Multiplexer can accept N number of input signals but gives single output signal.

The smart way to remember

Multiplexer – “Many into One”

Note: Multiplexer also called as Mux.

Mainly Multiplexer circuits are used to select & route any one of the available input signals to the output port. A multiplexer is required for the large-scale circuits to be automatically selected.

Why choose Multiplexer than Mechanical Switches?

For Large-scale circuits, mechanical switches will not produce satisfactory results. Due to this, Multiplexers are used for large-scale circuits for high-speed switching.

Do you know this?

Our Hero (Multiplexer) can handle two variants of data

  1. Analog: For this, Relays & Transistor switches are used to build a multiplexer
  2. Digital: For this, Logic gates are used to build a Multiplexer.

4-to-1 Multiplexer with Truth Table & Logic Diagram

The 4×1 multiplexer consists of 4-input bits, 2- control bits & ofcourse 1- output bit. The four input bits are namely I0, I1, I2 and I3. Whereas only one input bit is transmitted to the output. The output ‘Z’ purely depends on the value of control input AB. However, the output is completely based on the control bit AB. The below figure of 4×1 multiplexer circuit is designed using AND gates.



Fig 2: 4×1 Multiplexer

Consider an Example from the above figure:

Let us assume,

Control bits AB =00,

Then the top two AND gate i.e. I0 & I1 are allowed,

Whereas the remaining two AND gates are blocked.

Thus, data input I0 is transmitted to the o/p ‘Z”

Control bits AB =11,

Then the bottom two AND gate i.e. I2 & I3 are allowed,

Whereas the remaining two AND gates are blocked.

Thus, data input I3 is transmitted to the o/p ‘Z”

Types of Multiplexers & Multiplexing Techniques


Fig 3: Types of Multiplexers

What is Multiplexing?

Multiplexing is defined as transmitting N number of signals through a single medium.

Generally, Multiplexers are classified into

  1. Analog
    • Wavelength Division Multiplexing
    • Frequency Division Multiplexing
  2. Digital
    • Time Division Multiplexing
      1. Synchronous Time Division Multiplexing
      2. A Synchronous Time Division Multiplexing

Wavelength Division Multiplexing (WDM)


Fig 4: Wavelength Division Multiplexing Block Diagram

Wavelength Division is Analog Multiplexing technique. WDM’s are used to increase the capacity of a single optical fiber. These Multiplexing techniques are used in the optical fiber. WDM transmit data separately using multiple frequencies at different wavelengths over multiple channels.

Before going further,

Just revamp “Refraction through a prism”


Fig 5: Glass Prism

We have seen this experiment in our childhood,

White light can be separated into multiple individual coloured beams when it passes through the prism as shown in the above figure. Similarly, these multiple individual coloured beams can be combined into one single light beam by the prism.

Whereas WDM uses the same concept.

How Wavelength Division Multiplexing functions?


Fig 6: Wavelength Division Multiplexing functions

The Wavelength Division Multiplexing assigns unique or similar frequencies of light having the certain bandwidth to various optical signals. These multiplexed wavelengths are transmitted through the single fiber. Similarly, at demultiplexer side, these signals are selected using a selector of the desired bandwidth. Refer to Fig 3.


In General, Wavelength Division Multiplexing system combines different coloured lights by WDM device & transfer on a single optical fiber, and each colour is defined as a channel. At the receiving end, with the help of WDM Demultiplexing device, each individual colour is separated into their respective channel.

However, in the early 90’s boom of the internet urged the service provider to increase the capacity of their network, in a most efficient way.

Here comes our Hero “WDM” device invented.

The main thing to remember

  • WDM increased the single fiber capacity by 40 times with 40 channel Wavelength Division Multiplexing.
  • If you want to upgrade the network, no need to dig up underground trenches to install more fiber cables. Just upgrade the end devices.

Advantages & Disadvantages of Wavelength Division Multiplexing

Advantages or Benefits of Wavelength Division Multiplexing (WDM)

  • High security
  • Full duplex transmission is possible.
  • Simple to implement.
  • Optical components are more reliable.
  • WDM provides higher bandwidth.
  • Easier to reconfigure.

Disadvantages or Drawbacks of Wavelength Division Multiplexing (WDM)

  • Cost of the system increases with the addition of optical components.
  • Lightwave carrying WDM are limited to a 2-point circuit.
  • Signals cannot be very close.
  • (WDM in PON:) Inefficiency in BW utilization, difficulty in wavelength tuning, difficulty in a cascaded topology.
  • Scalability is a concern as OLT (Optical Line Termination) has to have a transmitter array with one transmitter for each ONU (Optical Network Unit). Adding a new ONU could be a problem unless transmitters were provisioned in advance. Each ONU must have a wavelength specific laser.

Frequency Division Multiplexing(FDM)


Fig 7: Frequency Division Multiplexing (FDM) Block Diagram

Frequency Division Multiplexing (FDM) is a method in which various signals are merged for transmission on a single channel. This technique is commonly known as Analog multiplexing. Where each individual signal is allocated in a different frequency within the primary (Main) channel. For this Channel synchronization is essential. Frequency Division Multiplexing technique depends on the orthogonality of sinusoids.

The Principle of Frequency Division Multiplexing depends on the sharing the bandwidth that is available in the transmission channel. That means various signals are being transmitted

That means various signals are being transmitted at the same time as each individual signal occupies the different frequency slot inside the same bandwidth.

Signals that are to be transmitted, modulates the different carrier. This modulation includes SSB, AM, PM or FM.

Those modulated signals are combined to form a complex signal that is transmitted via single channel.

Advantages & Disadvantages of Frequency Division Multiplexing

Advantages or Benefits of Frequency Division Multiplexing (FDM)

  • Bunch of signals can be transmitted at the same time simultaneously.
  • Synchronization is not required between transmitter & receiver for FDM.
  • Demodulation process for FDM is easy.

Disadvantages or Drawbacks of Frequency Division Multiplexing (FDM)

  • Need very large bandwidth.
  • Intermodulation distortion occurs.
  • Modulators & filters required in large number.
  • Frequency Division Multiplexing suffers crosstalk.
  • Due to Wideband fading, all FDM channels get affected.
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