UNIT -03 Transmission Media and Connecting Devices

1. UNIT -03
Transmission Media and
Connecting Devices

2. A transmission medium can be broadly defined as
anything that can carry information from a source to
a destination. The transmission medium is usually
free space, metallic cable, or fiber-optic cable.
Transmission media are actually located below the
physical layer and are directly controlled by the
physical layer.

4. Factors to be considered while selecting a Transmission
Medium
Transmission Rate
Cost and Ease of Installation
Resistance to Environmental Conditions
Distances

5. Twisted-Pair Cable
 A twisted pair consists of two conductors (normally copper),
each with its own plastic insulation, twisted together,
 The type of cable currently used in local area networks has
four pairs of wires. Until recently, category 5 or category 5E
cable has been used, but category 6 is now used for most
new installations. The main difference between the various
categories is in the data rate supported - category 6 cable
will support gigabit Ethernet. Its frequency range is 0 to 3.5
kHz.

6. It comes in two forms:-
 Unshielded Twisted Pair Cable
 It is the most common type of telecommunication when
compared with Shielded Twisted Pair Cable which consists of
two conductors usually copper, each with its own colour
plastic insulator. Identification is the reason behind coloured
plastic insulation.
 UTP cables consist of 2 or 4 pairs of twisted cable. Cable with
2 pair use RJ-11 connector and 4 pair cable use RJ-
45 connector.

7. Shielded Twisted – Pair (STP) Cable:
- STP cable has a metal foil or braided – mesh
covering that encases each pair of insulated
conductors. The metal casing prevents the
penetration of electromagnetic noise. It also can
eliminate a phenomenon called crosstalk, which is
the undesired effect of one circuit (or channel) on
another circuit (or channel). It occurs when one line
picks up some of the signals traveling down another
line.

8. Applications of Twisted pair cables
Twisted-pair cables are used in telephone
lines to provide voice and data channels.
Local-area networks also use twisted-pair
cables. 10baseT

9. Advantages of Twisted pair cables
High availability—More than 1 billion telephone
subscriber lines are based on twisted-pair
Low cost of installation on premises—The cost of
installing twisted-pair on premises is very low.
Low cost for local moves, adds, and changes in places—
assuming that the wiring is already in place an individual
can simply pull out the twisted-pair terminating on a
modular plug and replace it in another jack in the
enterprise.

10. Disadvantages of Twisted pair cables

Limited frequency spectrum—The total usable frequency
spectrum of twisted-pair copper cable is about 1MHz.
Limited data rates—The longer a signal has to travel over
twisted-pair, the lower the data rate.
Short distances required between repeaters—More
components need to be maintained, which leads to higher
long-term operational costs.
High error rate—Twisted-pair is highly susceptibility to signal
interference


11. Coaxial Cable
 A Coaxial cable is a cable used in the transmission of video,
communications, and audio. This cable has high bandwidths
and greater transmission capacity.
 It consists of four primary components, as follows:
 A core copper wire, which serves as the primary channel
 A dielectric plastic insulator, which surrounds the copper
 A braided copper/aluminum sheath beneath the insulator.
This is used to protect from external electromagnetic
interference.
 The last layer, which is made of Teflon or plastic coating, is
used to protect the inner layers from physical damage, such
as fire and water.

13. Applications
Coaxial cable was widely used in analog
telephone networks where a single coaxial
network could carry 10,000 voice signals.
Later it was used in digital telephone networks
where a single coaxial cable could carry digital
data up to 600 Mbps.
Cable TV networks also use coaxial cables.
Coaxial cable is also used in traditional Ethernet
LANs because of its high bandwidth, and
consequently high data rate,

14. The three most common cable sizes are RG-6,
RG-11 and RG-59:
RG stands for "radio guide." The numbers of the
various versions of RG cable refer to the diameter
(59 meaning .059, and 6 meaning .06, etc.). They
are also called RF cables, which stands for "radio
frequency".
Most non-industrial coax is now known as RG-6,
but installers may use a thicker cable, like RG-11.

18. Advantages
Broadband system -Coax has a sufficient frequency range to support
multiple channels, which allows for much greater throughput.
Greater channel capacity -Each of the multiple channels offers substantial
capacity.
Greater bandwidth -Compared to twisted-pair, coax provides greater
bandwidth system wide. Because it has greater bandwidth per channel, it
supports a mixed range of services.
Lower error rates -Because the inner conductor is in a Faraday shield,
noise immunity is improved, and coax has lower error rates .
Greater spacing between amplifiers -Coax's cable shielding reduces noise
and crosstalk, which means amplifiers can be spaced farther apart than
with twisted-pair.


19. Disadvantages
 Problems with the deployment architecture -The bus topology in which coax is
deployed is susceptible to congestion, noise, and security risks.
 Bidirectional upgrade required -the cable systems were designed for broadcasting. To
offer the subscriber any form of two-way services, networks have to be upgraded to
bidirectional systems.
 High installation costs -Installation costs in the local environment are high.
 Susceptible to damage from lightning strikes -Coax may be damaged by lightning
strikes.

20. Fiber-Optic Cable
A fiber-optic cable is made of glass or plastic and
transmits signals in the form of light. Light travels in
a straight line as long as it is moving through a
single uniform substance. If a ray of light traveling
through one substance suddenly enters another
substance (of a different density), the ray changes
direction. Optical fibers use reflection to guide light
through a channel. A glass or plastic core is
surrounded by a cladding of less dense glass or
plastic.

22. Advantages
• Higher bandwidth. Fiber-optic cable can support dramatically higher bandwidths (and hence
data rates) than either twisted-pair or coaxial cable.
• Less signal attenuation. Fiber-optic transmission distance is significantly greater than that
of other guided media. A signal can run for 50 km without requiring regeneration.
• Immunity to electromagnetic interference. Electromagnetic noise cannot affect fiber-optic
cables.
• Resistance to corrosive materials. Glass is more resistant to corrosive materials than
copper.
• Light weight. Fiber-optic cables are much lighter than copper cables.

23.  Disadvantages
 Installation and maintenance. Fiber-optic cable is a relatively new technology. Its
installation and maintenance require expertise that is not yet available
 Unidirectional light propagation. Propagation of light is unidirectional. If we need
bidirectional communication, two fibers are needed.
 Cost. The cable and the interfaces are relatively more expensive than those of
other guided media.

24. Unguided or wireless media:
Radio Waves, Infra-Red, Micro-Wave, Satellite, Light wave
Unguided media 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.

26. Unguided signals can travel from the source to destination in
several ways: ground propagation, sky propagation, and line-of-
sight propagation
 In ground propagation, radio waves travel through the
lowest portion of the Atmosphere. These low-frequency
signals emanate in all directions from the transmitting
antenna and follow the curvature of the planet. Distance
depends on the amount of power in the signal: The greater
the power, the greater the distance.
 In sky propagation, higher-frequency radio waves radiate
upward into the ionosphere (the layer of atmosphere where
particles exist as ions) where they are reflected back to earth.
This type of transmission allows for greater distances with
lower output power.


27. In line-or-sight propagation, very high-frequency
signals are transmitted in straight lines directly from
antenna to antenna. Antennas must be directional,
facing each other and either tall enough or close
enough together not to be affected by the curvature of
the earth.

29. Radio Waves
Electromagnetic waves ranging in frequencies
between 3 kHz and 1 GHz are normally called
radio waves;
Radio waves, for the most part, 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 sends waves that can be
received by any receiving antenna. Disadv- Radio waves
sent by one antenna are susceptible to interference by
another antenna that may send signals using the same
frequency or band.

31. Microwaves
Electromagnetic waves having frequencies
between I and 300 GHz are called microwaves.
Microwaves are unidirectional. When an
transmits microwave waves, they can be
narrowly focused. This means that the sending
and receiving antennas need to be aligned (eg.
dish antenna) The following describes some
characteristics of microwave propagation:

32. 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. Repeaters are
needed for long distance communication.
Very high-frequency microwaves cannot penetrate walls. This
characteristic can be a disadvantage if receivers are inside
buildings.
The microwave band is relatively wide, almost 299 GHz.
Therefore wider sub-bands can be assigned, and a high data
is possible
Use of certain portions of the band requires permission from
authorities.


33. Applications
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.

34. Infrared Waves
Infrared waves, with frequencies from 300 GHz to
400 THz (wavelengths from 1 mm to 770 nm), can
be used for short-range communication. Infrared
waves, having high frequencies, cannot penetrate
walls. This short-range communication system in
one room cannot be affected by another system in
the next room

35. Satellite Communication
 A satellite is a body that moves around another body in a
mathematically predictable path called an Orbit. A communication
satellite is nothing but a microwave repeater station in space that
is helpful in telecommunications, radio, and television along with
internet applications.
 A repeater is a circuit which increases the strength of the signal it
receives and retransmits it. But here this repeater works as
a transponder, which changes the frequency band of the
transmitted signal, from the received one.

36. A repeater is a circuit which increases the strength of
the signal it receives and retransmits it. But here this
repeater works as a transponder, which changes the
frequency band of the transmitted signal, from the
received one.
The frequency with which the signal is sent into the
space is called Uplink frequency, while the frequency
with which it is sent by the transponder is Downlink
frequency.
The following figure illustrates this concept clearly.

38. Satellite Communication − Advantages
 There are many Advantages of satellite communications such
as −
 Flexibility
 Ease in installing new circuits
 Distances are easily covered and cost doesn’t matter
 Broadcasting possibilities
 Each and every corner of earth is covered
 User can control the network

39.  Satellite Communication − Disadvantages
 Satellite communication has the following drawbacks −
 The initial costs such as segment and launch costs are too high.
 Congestion of frequencies
 Interference and propagation
 Satellite Communication − Applications

 Satellite communication finds its applications in the following areas −
 In Radio broadcasting.
 In TV broadcasting such as DTH.
 In Internet applications such as providing Internet connection for data transfer,
GPS applications, Internet surfing, etc.
 For voice communications.
 For research and development sector, in many areas.
 In military applications and navigations.