Engineering Made Easy: communication channels

We have already discussed the communication system and also the basic building blocks of it with the help of block diagram of the electronic communication system.
Although we have talked about communication channels while describing the block diagram of communication system. But now we will discuss the communication channels in depth here.
Here we will see what we mean by communication channels and different types of communication channels.
So let's first understand what is a channel in the communication system-

What is a Channel in Communication

A channel is a medium through which the message having some information travels from the transmitter to the receiver.

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Communication Channel Types-

We will discuss 4 types of communication channels here that are used nowadays-

1.Mobile Radio Channel
2'Telephone Channel
3.Optical Fibres
4.Satellite Communication

Channels can broadly be classified into following two types-

*Point to Point Channels
*Broadcast Channels

Point-to-Point Channels- Channels having one transmitter and one receiver.

Some examples of point to point channels are- Wirelines, microwave links and optical fibers etc.
Broadcast Channels- These channels have one transmitter and multiple receivers.
Satellite communication is an example of Broadcast channel.

Now we will discuss various communication channels one by one-

1.Mobile Radio Channels

Mobile radio channels have made the mobility possible in the telecommunication networks. Mobile radio channels do not have any line of sight for communication. The radio propagation takes place mainly because of two phenomenons known as-

*Scattering and
*Diffraction

The scattering takes place from the surfaces of buildings that are present in the surroundings.
These obstacles are also responsible for the diffraction of the signals.
Because of this scattering and diffraction the energy reaches to the receiving station via different paths which leads to different time delays. This problem is known as the multipath phenomena.

2.Telephone Channels

The telephone channels are used to establish an end to end communication link on a temporary basis. Switching mechanism is used in the telephone networks that is known as a circuit switching.
In Telephone channel communication, the sender speaks into the microphone. The sound vibrations are converted into electrical signals that is to be transmitted through the wired channel.
At the receiving end these electrical signals are converted back into the sound signals.
The telephone channel can pass frequencies between 300 to 3100 Hz that covers all the frequencies that are present in the human speech.

3.Optical Fibers

An optical fiber is a flexible and transparent fibre, made by drawing glass (silica) or plastic to a diameter slightly more than that of a human hair (including outer coating, its diameter is 0.25 mm-0.5 mm).
You can watch the complete video about optical fiber structure and working.

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The light rays propagate inside the optical fiber by the phenomenon of Total Internal Reflection (TIR). Now we will know the basic principle of operation of the optical fiber.

Working Principle of Optical Fiber

*Total Internal Reflection (TIR)

The optical fibre has a core and a cladding layer. The refractive index of the core is more than the refractive index of the cladding. So when the light rays move from denser medium to rarer medium (core to cladding) with an incidence angle greater than the critical angle (90 degrees), the total internal reflection phenomenon takes place and the light rays return back into the same denser medium (core). So the propagation of light rays take place inside the core of the Optical Fibre with successive Total Internal Reflection (TIR).

To understand, how this phenomenon of total internal reflection takes place inside the optical fibre when the light rays propagate through it, see the image given below (Click on the image to enlarge it)-

Total Internal Reflection (TIR)

The image above shows, 3 different cases of refraction of light, when the light ray propagates from denser medium to the rarer medium.

As we know, when the light rays move from denser medium to rarer medium, then it deviates away from the normal, as you can see in the image. This deviation (refraction) can be seen in all the three cases shown in the image.
Therefore the angle of refraction is more than the angle of incidence when the light rays move from denser medium to rarer medium.
Now look at the image, here you can see that, in the first case, when the incidence angle is less than the critical angle (the incidence angle, when the angle of refraction is 90° degrees), the light ray moves away from the normal in the rarer medium and goes into the rarer medium.
The second case shows the case of critical angle. Critical angle is the angle of incidence, when angle of refraction is 90 degrees (when the light ray moves from denser tor rare medium).
Now see the third case, it is the case where total internal reflection takes place. This is the same case that happens inside the optical fibre, when the light rays propagate through it.
In this third case, when the light ray moves from denser to rarer medium, with an angle of incidence more than the critical angle, it returns back into the same denser medium. This is called as total internal reflection.
This phenomena is known as total internal reflection, because in this phenomena, the light ray is reflected totally back into the same medium like reflection phenomena.

*Structure and Working of the Optical Fiber

Now we will see how this phenomenon of total internal reflection takes place inside the optical fiber. To understand the concept clearly look at the image shown below (Click on the image to enlarge it)-

Propagation of light inside optical fiber, Total Internal Reflection (TIR), Total Internal Reflection (TIR) in optical fiber

Propagation of light inside optical fiber
with Total Internal Reflection (TIR)

This image shows the structure and working principle of the optical Fibre. You can see in this image the two layers of the Optical Fiber, known as Core and cladding.
Now observe the image carefully. When the light ray moves inside the core and reaches to the core- cladding interface, then the phenomenon of total internal reflection takes place. This phenomena is seen here because, the light ray moves from the denser medium to the rarer medium (core to cladding).

But for the total internal reflection to take place, it is necessary that the angle of incidence must be more than the critical angle. Only the incident light rays for which the angle of incidence is more than the critical angle, can propagate through the core of the fiber with total internal reflection. The light ray moves through the core of the fiber, with total internal reflection taking place each time when the light ray reaches at the core-cladding interface.
For the light rays that enter into the Optical Fiber with an angle of incidence lesser than the critical angle; the phenomenon of total internal reflection doesn't take place, and these light rays move into the cladding instead of returning back into the same denser medium (core).
So, we can see that for the light rays to travel through the Optical Fiber, it is necessary that the light rays must have the angle of incidence more than the critical angle at the core-cladding interface, for the total internal reflection to take place.

*Advantages of Optical Fibres

Following are the main benefits of Optical Fibers-
*Very large bandwidth (2x10^13 Hz)
*Negligible transmission losses (0.2 db/km)
*Small size and very lightweight
*Highly flexible
*Immune to Electromagnetic Interference (EMI)
*No risk of electric shock

4.Satellite Communication

Satellite communication can be used for Continental or Intercontinental communication. It can cover large areas and also the areas that are not easy to access using other conventional modes of communication like wired communication or optical fibers.
The communication satellites are situated in the Geostationary Orbit, at an altitude of 22300 miles above the Earth.
These satellites are placed directly above the equator on eastward heading.
We call these orbits as geostationary because the satellite that is placed in the geostationary orbit appears stationary from the Earth as it completes one revolution around the earth in exactly 24 hours. And we know that the earth completes one rotation about its axis in 24 hours. Therefore the satellites placed in geostationary orbit appear stationery (fixed) from the Earth.
The Image given below illustrates the satellite communication system-