3 Transmission Media
3.1
Introduction
3.2
Guided Media
3.2.1
Twisted pair cables
3.2.1.1
Unshielded Twisted
Pair (UTP)
3.2.1.2
Shielded Twisted Pair
(STP)
3.2.2
3.2.3
Coaxial Cable
3.2.2.1
Thicknet (Thickwire
Ethernet)
3.2.2.2
Thinnet(Thinwie
Ethernet)
3.2.2.3
Thinnet Cable
Connectors
Fibre Optics
3.2.3.1
Single Mode Fibre
3.2.3.2
Multimode Fibre
3.3
Unguided Media
3.3.1
Satellite Communication
3.3.2
Terrestrial Microwave
communication
3.3.3
Radio Communication
3.3.4
Infrared communication
3.4
Summary
3.5
Exercise
Objectives
At the end of this chapter, students should be able to:
§
§
§
§
§
§
§
Learn different types of transmission media
Identify bounded and unbounded transmission media
Know the difference between shielded twisted pairs
and unshielded twisted pair
Compare and contrast coaxial, STP, UTP and fibre
optics
Outline the characteristics of the fibre optics and its
advantages
Know the role played by satellites in data
communication
Learn about radio, terrestrial microwave and infrared
communication
Transmission Media
3.1 Introduction
Due to the variety of transmission media and network wiring methods, selecting the most
appropriate media can be difficult task. However, considering following factors help in
choosing the correct media type:
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§
§
§
Transmission Rate
Distance Covered
Cost and Ease of Installation
Resistance to Environmental Conditions
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Transmission media can be categorised into two:
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§
Guided Transmission Media
Unguided Transmission Media
3.2 Guided Media
Guided Transmission Media uses a "cabling" system that guides the data signals along a
specific path. Guided Media is also known as Bounded Media, since the data signals are
bounded by the “cabling” system. Cabling here meant in a generic sense and is not meant to
be interpreted as copper wire cabling only. Here cable can be any physical or conductive
media like wires, coaxial cables or fibre optics.
Example of Guided Transmission Media
Twisted Pair Cable
Figure 3-1 Twisted Pair Cable
Figure 3-1 shows a twisted pair
cable, which is one of the guided
transmission media, is used for
computer networking. There are
other guided transmission media
like Coaxial cable and fibre optics.
Twisted
Pair Cable comes in
different configuration and quality.
Guided Transmission Media can be one of the following types
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§
§
Twisted Pair Cable
o Unshielded Twisted Pairs (UTP)
o Shielded Twisted Pairs (STP)
Coaxial Cable
Fibre Optic
49
Networking Principles
3.2.1 Twisted pair cables
3
Figure 3-2 Twisted Pair Cables
Twisted pair cables got their name because pairs of wires are twisted around one another.
Twisted pair consists of colour-coded pairs of insulated copper wires. Each wire has
diameter of 0.4 to 0.8 mm. Two insulated copper wires are twisted together in order to
reduce crosstalk and noise susceptibility. Data is transmitted in the form of electrical signal
through these copper conductors. High quality twisted pair cables have about 1 to 3 twists
per inch. The number of twists per meter or foot is known as the twist ratio. For best results,
the twist rate should vary significantly between pairs in a cable. Twisted-pair cable is used
over several different topologies, although it is most often implemented in star or starthybrid topologies.
Twisted pair can be specified as category 1 to 6 and is abbreviated as CAT 1-6.
There are two types of Twisted Pair cables
§
§
50
Unshielded Twisted Pairs (UTP)
Shielded Twisted Pairs (STP)
Transmission Media
3.2.1.1 Unshielded Twisted Pair (UTP)
Copper
Wire
Conductor
Twisted
Pairs
As the name implies, "unshielded twisted pair"
(UTP) cabling is twisted pair cabling that contains no
shielding. The UTP cables in computer networking
have resistance rating of 100 ohm. UTP cabling most
commonly includes 4 pairs of wires enclosed in a
common sheath. UTP is a very flexible, low cost
media, and can be used for either voice or data
communications. It is easy to install and maintain
compared to fibre optic cable. UTP is widely used for
cabling LAN in today’s computer network.
Figure 3-3 Unshielded Twisted Pair (UTP)
3.2.1.2 Shielded Twisted Pair (STP)
Figure 3-4 Shielded Twisted Pair
Shielded Twisted Pair (STP) cabling includes metal shielding over each individual pair of
copper wires. This type of shielding protects cable from external Electromagnetic
Interferences (EMI). Therefore these cables can be used in noisy area.
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3
Networking Principles
Cable types and their uses
Type
Category 1 (CAT 1)
Category 2 (CAT 2)
3
Description
Telephone Wire
Use
It is used for voice only, not for
computer data.
UTP cable that contains four wire It is used for carrying data at rate of 4
pairs.
Mbps for Local Talk
UTP cable containing 4 pair of wires.
Category 3 (CAT 3)
Category 4 (CAT 4)
It has possible max bandwidth of 16 It is used for 10 Mbps Ethernet and 4
Mbps Token Ring.
Mbps
UTP cable containing 4 pair of wires.
Provide more protection to crosstalk It is used to 10 Mbps Ethernet and 16
than CAT 3
Mbps Token Ring.
Category 5 (CAT 5)
UTP cable containing 4 pair of wires. It is used for 100
CAT 5E has better protection for Ethernet(Fast Ethernet)
crosstalk.
Category 6 (CAT 6)
UTP cable containing 4 pair of wires.
Can carry data at rate of 1Gbps.
It is used for Gigabit Ethernet that
can carry data at rate of
1000Mbps
Table 3-1 Cable types and their Uses
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Mbps
Transmission Media
Connector types
Twisted Pair Cable uses RJ45 connector at the end of cable to connect with computer.
3
Figure 3-5 RJ 45 connector and Jack
3.2.2 Coaxial Cable
Figure 3-6 Construction of Coaxial Cable
Coaxial cable is basically a copper cable, mostly used by Cable TV Companies to distribute
TV signals from antenna to users home. Coaxial cable was once the primary medium for
Ethernet and Local Area Networks (LANs), but due to development of STP and UTP, the
use of coaxial cable has almost disappeared over few years. It is commonly called as coax
and carries a signal at much higher frequencies than twisted pair.
Coaxial cable has a single copper conductor at its centre as a medium for transferring an
electrical signal. A plastic layer provides insulation between the centre conductor and a
copper mesh shield (See Figure 3-6). The copper mesh helps to block any outside
interference from fluorescent lights, motors, and generators.
53
Networking Principles
Types of Coaxial Cable
Depending on the use of coaxial cable it can be categorized into two types.
3.2.2.1 Thicknet (Thickwire Ethernet)
3
It is the original Ethernet medium which is approximately 1-cm thick and contains a solid
copper core. Thicknet uses Vampire taps and Transceiver Cable as shown in Figure 3-7 to
connect to a computer. IEEE designates Thicknet as 10BASE-5 Ethernet. Here the "10"
represents its speed (10Mbps), the “base” represent Baseband Transmission and "5"
represent maximum segment length of Thicknet Cable which is 500 meters. It is mostly
found in the older network.
Figure 3-7 Thicknet Cable with Vampire Taps and Transceiver cable to connect to a computer
54
Transmission Media
3.2.2.2 Thinnet (Thinwire Ethernet)
These types of medium were popular in 1980s. Thinnet uses RG-58A/U coaxial cable.
Thinnet has diameter of approximately 0.64cm, which makes it more flexible and easier to
install than Thicknet. IEEE has designated thinnet as 10BASE-2.Similary like thicknet, here
10 means speed of 10Mpbs using Baseband Signal with maximum segment length of 200
meters. It is not popular choice for present due to arrival of twisted pair cables.
3
Figure 3-8 Thinnet Cable Connection
3.2.2.3 Thinnet Cable Connectors
BNC cable connector (Figure 3-9) is either soldered or crimped to the end of a cable.
Figure 3-9 BNC Cable Connector
Note: BNC stands for Bayonet Neill Concelman or British Navel connector
55
Networking Principles
BNC T connector (Figure 3-10) is used to join the Network Interface Card (NIC) in the
computer to the network cable.
3
Figure 3-10 BNC T Connector
BNC barrel connector (Figure 3-11) is used to join two lengths of thinnet cable to make one
longer length.
Figure 3-11 BNC Barrel Connector
56
Transmission Media
The BNC terminator (Figure 3-12) closes each end of the bus cable to absorb stray signals
and prevent signal bouncing.
3
Figure 3-12 BNC Terminator
Comparison of Thicknet and Thinnet Cables
Thicknet
Thinnet
Thicknet was the original Ethernet wiring.
Thinnet replaces early thicknet.
It is also called as thickwire.
It is also called as thinwire.
Thickwire, is 0.4 inches in diameter.
Thinnet has diameter of approximately
0.64cm.
Uses Vampire Taps and Transceiver cable as
connectors.
Uses different BNC connectors to connect
cable.
Since it is thick, it is heavy and difficult to
bend and install.
It is light, easy to install and bend.
Table 3-2 Thicknet and Thinnet Comparison
57
Networking Principles
3.2.3 Fibre Optics
3
Figure 3-13 Fibre Optics Construction
Fibre optic cable consists of a centre glass core surrounded by several layers of protective
materials (as shown in Figure 3-13). It transfer data in the form of light rather than electrical
signals, thus there is no any electrical or electromagnetic inference to such cable. This
makes it ideal for certain environments that contain a large amount of electrical interference.
It has also made it the standard for connecting networks between buildings, due to its
immunity to the effects of moisture and lightning.
Fibre optic cable has the ability to transmit signals over much longer distances than coaxial
and twisted pair. It also has the capability to carry information at vastly greater speed
compared to other transmission media. This capacity broadens communication possibilities to
include services such as video conferencing and interactive services, but cost is high as
well as difficulty in installation.
Fibre optics can be classified into two categories:
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§
Single Mode Fibre
Multimode Fibre
3.2.3.1 Single Mode Fibre
Single Mode Fibre can transfer data for longer distance without help of a repeater and has
high bandwidth with small diameter of 10 micron. In Single Mode Fibre, the laser light
travels over single path with little reflection. Since light does not disperse, it can travel
longer distance without requiring a repeater as shown in Figure 3-14.
Figure 3-14 Signal travel over Single Mode Fibre
58
Transmission Media
3.2.3.2 Multimode Fibre
Multimode Fibre contains a core with a larger diameter than a single-mode fibre (between
50 to 115 microns). Due to large diameter, many pulses of light which is generated by laser
can travel at different angles as shown in Figure 3-15.
3
Figure 3-15 Signal travel over Multimode Fibre
Media Comparison Chart
Media Type
Twisted Pair Cable
Coaxial Cable
Optical Fibre Cable
Advantages
Inexpensive
Well understood
Easy to add computers
Disadvantages
Sensitive to noise
Short distance
Limited bandwidth
Security-easily tapped
High Bandwidth
Long distances
Nose immunity
Physical dimensions
Security-easily tapped
Very high bandwidth
Noise immunity
Long distances
High Security
Small size
Connections
T splitters
Expensive
High installation cost
Table 3-3 Media Comparison Chart
59
Networking Principles
3.3 Unguided Media
Unguided Media consists of a means (e.g. air, space) for the data signals to travel, however
there is nothing to guide them along a specific path, like in wires. In such cases, the data
signals are not bounded to a cabling media. Therefore such transmission media area often
called as Unbound Media. Unbounded media is electromagnetic waves in form of radio,
microwave, infrared or others. Wireless communication (unbound media) is used where
cables are difficult to use or install.
3
Unguided Transmission Media can be one of the following types
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§
Satellite Communication
Terrestrial Microwave
Radio Communication
Infrared
3.3.1 Satellite Communication
Figure 3-16 Satellite Communication
An artificial satellite is a man made object placed into orbit around the Earth for the purpose
of communication and scientific research. The communication satellite consists of a radio
transmitter and receiver which are called transponders and usually placed about 22,300
miles above the Earth’s equator. The function of transponders is to receive the incoming
radio signal from the earth, amplify it and transmit back towards the earth.
Satellite provides communication over longer distance compared to normal radio
communication. A ground station on one side of ocean transmit a signal to the satellite,
satellite receiver receives the signal and transmit back to another station on the other side.
Since satellite communication is so costly, a single satellite is shared by many organizations.
The most common (or useful) satellites are Geosynchronous satellites.
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Transmission Media
Satellite Communication is used for following purposes:
§ Air Navigation
§ Television and Radio Transmission
§ Videoconferencing
§ Global Positioning System (GPS)
Global Positioning System(GPS)
Global Positioning System (GPS) consist of one or more earth-based receivers that accept
and analyse signal sent by satellite in order to determine the receiver’s geographical location.
A GPS receiver can be handheld or mounted on an object like automobile, boat, aeroplane,
farm and construction equipment or computer. It helps scientist, farmer, pilot and rescue
workers operate more productively as well as safely.
3.3.2 Terrestrial Microwave communication
Figure 3-17 Microwave Communication
Terrestrial Microwave communication is used extensively in situations when physical
transmission media is impractical or difficult to install, for example between high building,
across rivers, mountains, jungles and remote areas. Microwaves are transmitted via earth
atmosphere between two stations. It can be used for only short distance up to 50km.
Repeater station is needed if transmission distance is more than 50km. Microwave
transmission is line of sight transmission, the transmit station must be in visible contact with
the receive station. Such type of communication can be easily affected by atmospheric
changes for example weather.
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3
Networking Principles
3.3.3 Radio Communication
3
Figure 3-18 Radio Communication
Apart from public broadcast of radio and television programs, radio communication is also
used for the purpose of wireless communication. It is used for private communication with
devices like portable phones. Wireless communication is also used for sending and receiving
computer data.
Radio communication systems consist of transmitter and receiver called base stations,
between which radio signal are sent and receive. Radio communication uses electromagnetic
waves but operate at radio frequency range. Area covered by such communication system
depends on the power of transmitter.
3.3.4 Infrared communication
Infrared technology allows computing devices to communicate via short-range wireless
signals. With help of infrared, computers can transfer files and other digital data between
each other. Infrared transmission requires line-of-sight transmissions. That means the
sender and receiver must be aligning so that nothing obstructs the path of the infrared wave.
Infrared communication is usually used over short distance. Today most of the consumer
devices like mobile phones, Personal Digital Assistant (PDA), Computer, laptop have built
in infrared port for infrared communication. One infrared device can easily recognize other
infrared device nearby and communicate with easily.
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Transmission Media
3.4 Summary
§
Transmission media can be categorized into Guided or Bounded and Unguided or
Unbounded transmission media.
§
Guided Transmission media uses a cabling system to guide data signal along a
specific path.
§
Guided transmission media are twisted pairs copper wire, coaxial cable or fibre
optics.
§
There are two types of Twisted Pair Cable; Unshielded Twisted Pairs (UTP) and
Shielded Twisted Pairs (STP).
§
Shielded Twisted Pairs (STP) has metal shielding over each individual pair of
copper wires, which protect from Electromagnetic Interference (EMI) compared to
UTP.
§
UTP cable range from CAT1 to CAT6 based on data transfer rates and its uses.
§
Coaxial cable can be used for Cable TV as well as to carry data for computer
networks. Coaxial cables are of Thicknet and Thinnet types.
§
BNC Cable connector, BNC T Connector, BNC barrel connector, BNC
Terminator is used for connecting thinnet cable.
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Fibre optics transfer data in the form of light and it is immune to Electromagnetic
Inference (EMI). Fibre optics comes as Single Mode and Multimode Fibre.
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Unguided or Unbounded media is Electromagnetic waves in form of Radio or
Microwave signals.
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The communication satellite consists of a radio transmitter and receiver called
transponders. Communication satellites used where cable is difficult to layout.
§
Terrestrial Microwave communication is used in adverse geographical situation
like across rivers, mountains and jungle over short distance up to 50 km.
§
Radio communication is used for public broadcast of radio and TV as well as for
wireless communication.
§
Most of the consumer device likes mobile phones, Computers and PDA uses
infrared communication over short distance to communicate with each other.
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3
Networking Principles
3.5 Exercise
Fill in the blanks
1. When selecting transmission media, important facts have to be noted are ____________,
____________ and ____________________.
3
2. When “cabling” system is used to guide the data transmission along a specific path, such
media is called _____________
3. Example of an unguided transmission media is ________________
4.____________ cable transmits data in forms of light rather than as electric signals.
5. Two types of twisted pairs are ___________and ___________________
6. In twisted pair, wires are twisted together in order to reduce ________________
7. Twisted pair cables have normally ___________ to __________twists per inch
8. STP is better than
____________interference.
UTP
because
metal
shielding
protect
signal
from
9. Category 1 twisted pair is used for ____________ only.
10. Category 3 twisted pair can carry data up to __________ Mbps and used
for_____________
11. Coaxial cable can carry a signal at much _______________frequencies than twisted pairs.
12. ________cable can be used in places with high electrical and electromagnetic
interferences.
Questions:
1. What do you mean by guided transmission media?
2. List different types of guided transmission media.
3. What is difference between UTP and STP?
4. List different categories of twisted pairs indicate their usages.
5. Compare different types of cables with their advantages and disadvantages.
6. What are different forms of unguided transmission and their uses?
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