unguided media, wireless media in computer network

1. UNGUIDED MEDIA
Unguided medium transport electromagnetic
waves without using a physical conductor. This
type of communication is often referred to as
wireless communication. Signals are normally
broadcast through free space and thus are
available to anyone who has a device capable of
receiving them.
The below figure shows the part of the
electromagnetic spectrum, ranging from 3 kHz to
900 THz, used for wireless communication.

3. Unguided signals can travel from the source to the
destination in several ways: Ground
propagation, Sky propagation and Line-of-sight
propagation as shown in below figure.


4. Propagation Modes
 Ground Propagation: In this, radio waves travel through the lowest
portion of the atmosphere, hugging the Earth. These low-frequency
signals emanate in all directions from the transmitting antenna and
follow the curvature of the planet.
 Sky Propagation: In this, higher-frequency radio waves radiate upward
into the ionosphere where they are reflected back to Earth. This type of
transmission allows for greater distances with lower output power.
 Line-of-sight Propagation: in this type, very high-frequency signals are
transmitted in straight lines directly from antenna to antenna.
We can divide wireless transmission into three broad groups:
 Radio waves
 Micro waves
 Infrared waves

5. Radio Waves
 Electromagnetic waves ranging in frequencies between 3 KHz
and 1 GHz are normally called radio waves.
 Radio waves are omnidirectional. When an antenna transmits
radio waves, they are propagated in all directions. This means
that the sending and receiving antennas do not have to be
aligned. A sending antenna send waves that can be received by
any receiving antenna. The omnidirectional property has
disadvantage, too. The radio waves transmitted by one
antenna are susceptible to interference by another antenna
that may send signal suing the same frequency or band.

6.  Radio waves, particularly with those of low
and medium frequencies, can penetrate
walls. This characteristic can be both an
advantage and a disadvantage. It is an
advantage because, an AM radio can receive
signals inside a building. It is a disadvantage
because we cannot isolate a communication
to just inside or outside a building.

7. Omnidirectional Antenna for Radio Waves
 Radio waves use omnidirectional antennas that
send out signals in all directions.

8. Applications of Radio Waves
 The omnidirectional characteristics of radio
waves make them useful for multicasting in
which there is one sender but many receivers.
 AM and FM radio, television, maritime radio,
cordless phones, and paging are examples of
multicasting.

9. Advantages and disadvantages
 Radio waves are easy to generate and penetrate
buildings also can travel long distances.
 Radio waves cover a large area and can penetrate the
buildings. By this, an AM radio can receive signals inside
a building.
 This can also be disadvantageous because we cannot
isolate a communication just inside or outside a
building. Cause of this, governments strictly legislate
the use of radio transmitters.

10. Micro Waves
 Electromagnetic waves having frequencies
between 1 and 300 GHz are called micro
waves. Micro waves are unidirectional. When
an antenna transmits microwaves, they can
be narrowly focused. This means that the
sending and receiving antennas need to be
aligned. The unidirectional property has an
obvious advantage. A pair of antennas can be
aligned without interfering with another pair
of aligned antennas.

11. The following describes some characteristics of
microwaves propagation:
 Microwave propagation is line-of-sight. Since the
towers with the mounted antennas need to be in
direct sight of each other, towers that are far
apart need to be very tall.
 Very high-frequency microwaves cannot
penetrate walls. This characteristic can be a
disadvantage if receivers are inside the buildings.

12.  The microwave band is relatively wide,
almost 299 GHz. Therefore, wider sub-bands
can be assigned and a high date rate is
possible.
 Use of certain portions of the band requires
permission from authorities.

13. Unidirectional Antenna for Micro Waves
 Microwaves need unidirectional antennas that
send out signals in one direction.Two types of
antennas are used for microwave
communications: Parabolic Dish and Horn.

14.  A parabolic antenna works as a funnel, catching a
wide range of waves and directing them to a
common point. In this way, more of the signal is
recovered than would be possible with a single-point
receiver.
 A horn antenna looks like a gigantic scoop. Outgoing
transmissions are broadcast up a stem and deflected
outward in a series of narrow parallel beams by the
curved head. Received transmissions are collected
by the scooped shape of the horn, in a manner
similar to the parabolic dish, and are deflected down
into the stem.

15. Applications of MicroWaves
 Microwaves, due to their unidirectional properties, are very
useful when unicast(one-to-one) communication is needed
between the sender and the receiver. They are used in cellular
phones, satellite networks and wireless LANs.
 There are 2 types of MicrowaveTransmission :
 Terrestrial Microwave
 Satellite Microwave
Advantages of MicrowaveTransmission
 Used for long distance telephone communication
 Carries 1000's of voice channels at the same time
Disadvantages of MicrowaveTransmission
 It is very costly

16. Terrestrial Microwave
 For increasing the distance served by
terrestrial microwave, repeaters can be
installed with each antenna .The signal
received by an antenna can be converted into
transmittable form and relayed to next
antenna as shown in below figure. It is an
example of telephone systems all over the
world

17. There are two types of antennas used for
terrestrial microwave communication :
 1. Parabolic Dish Antenna
 In this every line parallel to the line of symmetry
reflects off the curve at angles in a way that they
intersect at a common point called focus. This
antenna is based on geometry of parabola.

18. 2. HornAntenna
 It is a like gigantic scoop. The outgoing transmissions are
broadcast up a stem and deflected outward in a series of
narrow parallel beams by curved head.

19. Characteristics of microwaves
 It is inexpensive for short distance expensive as it
requires a higher tower for a longer distance.
 Due to environmental conditions and antenna
size attenuation (loss of signal) occurs.
 There is a capacity in very high-frequency
microwaves that they cannot penetrate walls.
This characteristic can be a disadvantage of
microwaves if the receiver is inside the buildings.

20. Advantages
 Microwave transmission is cheaper than using cables.
 It does not require any land for the installation of cables
that is free from land acquisition.
 Microwave transmission provides easy communication.
Disadvantages
 Bandwidth is limited in microwave transmission.
 A signal can be moved out of phase and any
environmental change such as rain, wind can distort the
signal so these signals are susceptible to weather
conditions.
 Cause of eavesdropping insecure communication occurs
in which any user can catch the signal in the air by using
its antenna.

21. Satellite Microwave
 This is a microwave relay station which is placed
in outer space. The satellites are launched either
by rockets or space shuttles carry them.
 These are positioned 36000 Km above the
equator with an orbit speed that exactly matches
the rotation speed of the earth. As the satellite is
positioned in a geo-synchronous orbit, it is
stationery relative to earth and always stays over
the same point on the ground. This is usually
done to allow ground stations to aim antenna at
a fixed point in the sky.

23. Features of Satellite Microwave
 Bandwidth capacity depends on the frequency used.
 Satellite microwave deployment for orbiting satellite is difficult.
Advantages of Satellite Microwave
 Transmitting station can receive back its own transmission and check
whether the satellite has transmitted information correctly.
 A single microwave relay station which is visible from any point.
Disadvantages of Satellite Microwave
 Satellite manufacturing cost is very high
 Cost of launching satellite is very expensive
 Transmission highly depends on whether conditions, it can go down in
bad weather

24. Infrared Waves
 Infrared waves, with frequencies from 300 GHz to 400 THz, can
be used for short-range communication. Infrared waves, having
high frequencies, cannot penetrate walls. This advantageous
characteristic prevents interference between one system and
another, a short-range communication system in on room cannot
be affected by another system in the next room.
 When we use infrared remote control, we do not interfere with
the use of the remote by our neighbours. However, this same
characteristic makes infrared signals useless for long-range
communication. In addition, we cannot use infrared waves
outside a building because the sun's rays contain infrared waves
that can interfere with the communication.

25. Applications of InfraredWaves
 The infrared band, almost 400 THz, has an excellent
potential for data transmission. Such a wide
bandwidth can be used to transmit digital data with
a very high data rate.
 The Infrared Data Association(IrDA), an association
for sponsoring the use of infrared waves, has
established standards for using these signals for
communication between devices such as keyboards,
mouse, PCs and printers.
 Infrared signals can be used for short-range
communication in a closed area using line-of-sight
propagation.

26. Difference between guided and
unguided media
1. The guided media uses a physical path or conductor to
transmit the signals whereas, the unguided
media broadcast the signal through the air.
2. The guided media is also called wired
communication or bounded transmission media.
However, the unguided media is also called wireless
communication or unbounded transmission media.
3. The guided media provide direction to the signal whereas,
the unguided media does not direct the signal.
4. Categories of guided media are twisted pair cable, coaxial
cable and optical fibre. On the other hands, the categories
of unguided media are radio wave,
microwave, and infrared signal.

27. Quiz
1. Which of the following primarily uses guided
media?
a) Cellular telephone system
b) Local telephone system
c) Satellite communications
d) Radio broadcasting

28. 2.Which of the following is not a guided
medium?
a) twisted-pair cable
b) Coaxial cable
c) Fiber-optic cable
d) atmosphere

29. 3.What is the major factor that makes coaxial
cable less susceptible to noise than twisted-
pair cable?
a) Inner conductor
b) Diameter of cable
c) Outer conductor
d) Insulating material

30. 4.The inner core of an optical fiber is _____ in
composition.
a) Glass or plastic
b) Copper
c) Bimetallic
d) liquid

31. 5. Radio communication frequencies range
from ___
a) 3KHz to 300KHz
b) 300KHz to 3GHz
c) 3KHz to 300GHz
d) 3KHz to 300THz

32. 6.The radio communication spectrum is divided
into bands based on ___.
a) Amplitude
b) Frequency
c) Cost an hardware
d) Transmission medium

33. 7.VLF propagation occurs in ____
a) The troposphere
b) The ionosphere
c) Space
d) All of the above