Communication and Networks Assignment
Communications and Networks
version 1.0
Diploma in Information Technology
Copyright © 2020 by Singapore Institute of Management Pte Ltd. All rights reserved.
Lesson 9: Wired Media
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Lesson 9 Learning Outcomes
Distinguish between guided and unguided transmission media
Identify the different classification of transmission media based on forms of energy
Understand the motivation for using different forms of wired media
Distinguish the three different types of twisted pair copper wiring
Explain the motivation for shielding copper wiring
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Lesson 9 Learning Outcomes
Identify the categories of twisted pair cables
Compare and contrast optical fibre with copper wiring
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Lesson 9 Outline
Wired Transmission Media
Electrical Energy Transmission
Light Energy Transmission
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Transmission Media Classes
There are two broad approaches:
Type of path: communication can follow an exact path such as a wire, or can have no specific path, such as a radio transmission
Form of energy: electrical energy is used on wires, radio transmission is used for wireless, and light is used for optical fiber
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Guided Media
Guided: constrain the propagation of signals into a solid cable
Copper wiring or optical fibers provide a specific path
Signals do not easily stray from the cable
Energy requirements are relatively low
Relatively secured from eavesdropping
Good error performance
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Unguided Media
Unguided: little to no constraint to the propagation of signals
At higher frequencies, signals tends to follow line of sight but still strays
Require more energy
Vulnerable to eavesdropping
Subject to interference and errors
Often fail in certain atmospheric conditions
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Taxonomy by Forms of Energy
Taxonomy: the practice and science of classification
classification are not perfect and exceptions exist
Example:
Space station in orbit around the earth might employ non-terrestrial communication that does not involve a satellite
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Energy Types
Source: Douglas, C (2016) Computer Networks and Internets
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Practice 9.1
What are the TWO (2) ways to classify transmission media?
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Lesson 9 Outline
Wired Transmission Media
Electrical Energy Transmission
Light Energy Transmission
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Electrical Energy Transmission
Electrical current flows along a complete circuit
All transmissions of electrical energy need two wires to form a circuit
One wire to receiver
One wire back to the sender
Simplest form of wiring consists of a cable that contains two copper wires
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Insulating Wires
Each wire used for electrical energy transmission is wrapped in plastic coating
To insulates the wires electrically
The outer coating on the cable holds related wires together to make it easier for humans who connect equipment
Prevent electrical shock
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Interferences and Noise (1/2)
Random electromagnetic radiation, called noise, permeates the environment
Most communication systems generate minor amounts of noise as side-effect under normal operation
When noise hits metal, electromagnetic radiation induces a small signal
interfere with signals used for communication
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Interferences and Noise (2/2)
Metal absorbs radiation, acting as shield
Placing enough metal between source of noise and communication medium can prevent noise from interfering
Source: Bing, licensed under CC BY-SA
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Parallel Wiring
If two wires are parallel, high probability that one is closer to radiation than other
One wire act as shield, absorbs radiation
Second wire receives less radiation
Total of 32 units of radiation strikes
Top wire absorbs 20 units,
Bottom wire absorbs 12, producing difference of 8
Source: Douglas, C (2016) Computer Networks and Internets
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Twisted Pair Wiring
Twisting two wires makes them less susceptible to electrical noise than leaving them parallel
Total of 32 units of radiation strikes
Each is on top half the time
Each absorbs same amount of radiation
Source: Douglas, C (2016) Computer Networks and Internets
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Why Wires are Twisted
Source: https://www.youtube.com/watch?v=P7WfY9P2uNY
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Practice 9.2
What is the rational to twisting wires?
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Unshielded Twisted Pair (UTP)
Otherwise known as telephone cables
Pairs of metal cables twisted around each other in a regular fashion
Each metal conductor is surrounded by a plastic sheath to insulate it from other conductors
Several (usually 6) twisted pairs are then surrounded by an outer plastic sheath for protection
Source: Douglas, C (2016) Computer Networks and Internets
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Twisted Pair Problems
Although immune to most background radiation, twisted pair wiring does not solve all problems
Twisted pair tends to have problems with:
Strong electrical noise when close physical proximity to the source of noise
High frequency communication
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Electrical Noise Source
Electrical noise source:
factory that uses electric arc welding equipment
Cable runs above the ceiling in an office building on top of a florescent light fixture
Difficult to build equipment that can distinguish between valid signals and noise
even small amount of noise can cause interference when high frequencies are used
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Cable Television Wiring
One familiar form of wiring that have extra metal shielding is the wiring for cable television
known as coaxial cable (coax)
It has a thick metal shield formed from braided wires that surround a center (inner) wire that carries the signal
Provides barrier to electromagnetic radiation from any direction
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Coaxial Cable (Coax)
Early 10Mbit/s Ethernets and long-distance telecommunication circuits used Coax
Provide excellent performance
Outer plastic sheath for protection
But bulky, heavy and expensive
Difficult to install
Source: Douglas, C (2016) Computer Networks and Internets
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Coax Benefits
Metallic shield prevents signals on inner wire from radiating electromagnetic energy
that could affect other wires
Coax can be:
Placed adjacent to sources of electrical noise and other cables
Can be used for high frequencies
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Shielding of Coax
Source: Douglas, C (2016) Computer Networks and Internets
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Shielding Wires
Using braided wire instead of solid metal shield keeps coaxial cable flexible but making it heavy
less flexible than twisted pair wiring
Variations of shielding have been invented that provide a compromise:
cable is more flexible, but has slightly less immunity to electrical noise
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Making Shielded Cable Flexible
One popular shielded variation is known as Shielded Twisted Pair (STP)
STP has a thinner, more flexible metal shield surrounding one or more twisted pairs of wires
In most STP cable, the shield consists of metal foil like aluminum foil used in a kitchen
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Shielded Twisted Pair (STP)
Like UTP except outer sheath contains a metal mesh connected to the earth
Compare to UTP:
Pairs inside are protected from electromagnetic interference
Less subject to impulse noise
Higher speed or longer distance
More expensive
Heavier and harder to install
Better performance
Source: Douglas, C (2016) Computer Networks and Internets
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Standards for Twisted Pair
Standards organizations worked together to create standards for twisted pair cables used in computer networks
Mostly American orgnisations:
American National Standards Institute (ANSI)
Telecommunications Industry Association (TIA)
Electronic Industries Alliance (EIA)
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Categories of Twisted Pair
Source: Douglas, C (2016) Computer Networks and Internets
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Practice 9.3
Group the following type of media based on firstly, COST and secondly, immunity NOISE, in ascending order.
Coaxial cable (Coax)
Unshielded Twisted Pair (UTP)
Shielded coaxial Cable (STP)
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Lesson 9 Outline
Wired Transmission Media
Electrical Energy Transmission
Light Energy Transmission
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How Fiber Optics is Made
Source: https://www.youtube.com/watch?v=6CqT4DuAVxs
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Using Light Energy Transmission
Three media use light energy to carry information:
Optical fibers
InfraRed transmission
Point-to-point lasers
Most important and widely used type is optical fiber
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Optical Fiber
Each fiber consists of a thin strand of glass or transparent plastic encased in a plastic cover
One end connects to a laser or Light Emitting Diode (LED) used to transmit light
Other end connects to a photosensitive device used to detect incoming light
Two fibers are needed for two-way communication
One to carry information in each direction
Source: Douglas, C (2016) Computer Networks and Internets
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Traveling Round a Bend in Fiber
When light encounters boundary between two substances, its behavior depends on:
density of the two substances
angle at which the light strikes the boundary
Given a pair of substances, there exists a critical angle, theta θ
measured with respect to a line that is perpendicular to boundary
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Angle of Incidence
Angle of incidence: angle between the transmitted light and perpendicular line to the boundary
angle of incidence < θ, light crosses the boundary and is refracted
angle of incidence = θ, light travels along the boundary
angle of incidence > θ, light is reflected as if boundary was a mirror
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Angle of Incidence Illustration
Angle of incidence < θ
Angle of incidence = θ
Angle of incidence > θ
Source: Douglas, C (2016) Computer Networks and Internets
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Reflection in Fiber
Reflection in optical fiber is not perfect
It absorbs a small amount of energy
If a photon takes a zig-zag path that reflects from the walls of the fiber many times
This photon will travel slightly longer distance than another that takes straight path
Is dispersed (stretched) over time
Serious problem for long optical fibers
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Dispersion Illustration
Source: Douglas, C (2016) Computer Networks and Internets
Light pulse sent and received over an optical fiber
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Types of Fiber
Multimode, Step Index: least expensive and used when performance is unimportant
Reflect frequently and high dispersion
Multimode, Graded Index: slightly more expensive
Reduced reflection and lowered dispersion
Single Mode: most expensive and least dispersion
used for long distances and higher bit rates
has smaller diameter and other properties that help reduce reflection
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Types of Fiber Illustration
Source: Douglas, C (2016) Computer Networks and Internets
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Transmitting Light
Single mode fiber and the equipment used at each end are designed to focus light
Pulse of light can travel long distances without becoming dispersed
Minimal dispersion helps increase the rate at which bits can be sent
A pulse corresponding to one bit does not disperse into the pulse that corresponds to a successive bit
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Receiving Light
Devices used for transmission must match the fiber
Transmission: LED or Injection Laser Diode (ILD)
Reception: photo-sensitive cell or photodiode
Multimode fiber generally use LEDs and photo-sensitive cells
Single mode fiber generally use ILDs and photodiodes
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Fiber Optic Transmission
Source: Douglas, C (2016) Computer Networks and Internets
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Optical Fiber Properties
Optical fiber:
immune to electrical noise
higher bandwidth
Ends of an optical fiber must be polished before they can be used
Requires special equipment and expertise for installation
Can easily break if accidentally pulled or bent
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Optical Fiber vs Copper Wiring
Source: Douglas, C (2016) Computer Networks and Internets
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The Internet’s Undersea World
Source: https://visual.ly/community/Infographics/technology/internets-undersea-world
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Practice 9.4
For each of the following requirements, each suggest ONE type of media that can be used.
Electrical energy transmission, lowest cost
Light energy transmission, high bandwidth
Electrical energy transmission, flexible cabling
Radio transmission, non-terrestrial propagation
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Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 7
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End of Lesson
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