Communication and Networks Assignment
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0_Lesson10WirelessMedia.pptxCommunications and Networks
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Diploma in Information Technology
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Lesson 10: Wireless Media
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Lesson 10 Learning Outcomes
Understand the motivation for infrared communication
Explain infrared communication
Explain point-to-point laser communication
Explain radio communication
Describe the different types of satellites
Distinguish the different types of satellite orbit
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Lesson 10 Learning Outcomes
Compare and contrast wireless media with wired media
Use appropriate measures to measure performance of media
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Lesson 10 Outline
Wireless Transmission
Radio Communication
Satellite Communication
Measuring Media Performance
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Energy Types
Source: Douglas, C (2016) Computer Networks and Internets
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Infrared Data Association (IrDA)
Infrared Data Association (IrDA): founded in 1993
To develop and promote a standard for infra-red data communications
Earliest members were HP, IBM and Sharp
Designed for short-range digital communications between PC and devices
Keyboard, digital cameras, smartphones
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InfraRed(IR) Communication
InfraRed (IR): uses same type of energy as a TV remote control
Behaves like visible light but falls outside the range that is visible to a human eye
Like visible light, infrared disperses quickly
Infrared signals can reflect from a smooth, hard surface
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IR Limitations
Opaque object can block the signal
Thin sheet of paper or moisture in atmosphere
Up to 1 meter in normal light conditions within a 30o cone from the transmitter
Speed ranging from 9.6 kbit/s to 4 Mbit/s
IR commonly used to connect to a nearby peripheral
Attractive for laptop users as can move around
Now mostly uses Bluetooth technology
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IR Technologies
Source: Douglas, C (2016) Computer Networks and Internets
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Point-to-Point Communication
Pair of devices with beam that follows line-of-sight
IR is a type of point-to-point communication
Other point-to-point communication technologies also exist
One form of point-to-point communication uses a beam of coherent light produced by a laser
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Laser Communication
Laser communication requires unobstructed path between the communicating sites
Laser beam does not cover broad area and is only a few centimeters wide
Sender and receiver must be aligned precisely to ensure that sender beam hits sensor in the receiver
Suitable for outdoors and span great distances
Useful in cities to transmit from building to building
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Bluetooth
Low-cost, low power short-range radio developed by Bluetooth Special Interest Group made up initially of Ericsson, Intel, Nokia and Toshiba
Now include Microsoft and many others
Designed to carry voice and data between devices within noisy radio environment
Smartphones, hands-free devices, laptops
In unlicensed 2.4 GHz frequency band
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Bluetooth Piconet
Piconet: network of 8 active Bluetooth devices
One act as master
Communication between master and slave
2 slaves cannot communicate directly
Piconet supports up to 255 non-active devices
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Bluetooth Scatternet
Scatternet: network of ten piconets
Slave of a piconet becomes a master of another
Each bridging traffic between two piconets
Severe interference is likely to reduce data rate
but will not stop Bluetooth from working together
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Service Discovery
Bluetooth includes service discovery protocols
Allows applications to discover what functions are supported by Bluetooth devices
Creates a database of trusted devices
following authentication
Pair up devices by entering PIN numbers
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Bluetooth and WiFi
Bluetooth uses same frequency range as WiFi
Likely to cause interference when used near WiFi hotspots
IEEE has set up a task group to make recommendations that will help these two coexist
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Practice 10.1
For each of the following, say whether an obstructed direct path will prevent communication entirely and give an example.
InfraRed (IR) communication
Laser communication
Bluetooth communication
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Lesson 10 Outline
Wireless Transmission
Radio Communication
Satellite Communication
Measuring Media Performance
18
Radio Communication
Most common form of unguided communication consists of wireless networking technologies
Electromagnetic energy in the Radio Frequency (RF) range
RF transmission has distinct advantage over light
Can traverse long distances and penetrate objects like walls of a building
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Radio Frequencies
Properties of electromagnetic energy depends on frequency
Spectrum: range of possible frequencies
Organizations allocate frequencies for specific purposes
Federal Communications Commission (FCC) sets rules for how frequencies are allocated
Limits on amount of power that communication equipment can emit at each frequency
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Electromagnetic Spectrum
One part of the spectrum corresponds to IR
Spectrum used for RF communications spans frequencies from approximately 3KHz-300GHz
Includes frequencies allocated to radio and television broadcast, satellite and microwave communications
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Electromagnetic Frequencies
Source: Douglas, C (2016) Computer Networks and Internets
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Signal Propagation
Amount of information an electromagnetic wave can represent depends on the frequency
Frequency of electromagnetic wave determines how wave propagates
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Radio Waves Types
Surface waves follows surface of earth
Space (or tropospheric) waves follows line of sight paths but can also be reflected off the ground and other large objects.
Sky (or Ionospheric) waves are reflected off the ionosphere and can be carried over long distance
Scattered waves: broadcast and scattered it in all directions
some are scattered back to earth and picked up, often used in off-shore oil platforms
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Signal Frequency Types
Lowest frequencies: electromagnetic radiation follow the earth's surface
If terrain is relatively flat, possible to place receiver beyond horizon of transmitter
Medium frequencies: signal can bounce off the ionosphere to travel
transmitter and receiver can be farther apart
Highest frequencies: signal propagates in a straight line
Path between transmitter and receiver must be free from obstructions
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Signal Frequencies Summary
Source: Douglas, C (2016) Computer Networks and Internets
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Wireless Propagation Types
Terrestrial: Communication uses equipment like radio or microwave transmitters that is close to earth surface
Typical locations include tops of hills, man-made towers and tall buildings
Non-terrestrial: some of the communication is outside the earth's atmosphere
satellite in orbit around earth
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Frequency & Power
Frequency and amount of power used can affect following:
Speed at which data can be sent
Maximum distance for communication to occur
Characteristics like if signal can penetrate solid objects
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Practice 10.2
There are two kinds of wireless propagation types. For each of the following, say what kind of propagation type does it use.
Satellite dishes on top of buildings
Satellite in space
Antennas on top of a hill
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Lesson 10 Outline
Wireless Transmission
Radio Communication
Satellite Communication
Measuring Media Performance
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How Satellite Internet Works
Source: https://www.youtube.com/watch?v=QpO0FwN9Png
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Laws of Physics
Kepler's Law govern the motion of an object that orbits the earth
Satellite
Period: time required for a complete orbit
Depends on the distance from the earth
Communication satellites are classified according to their distance from the earth
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Satellite Networks
Satellites have been used for communications since early days of space travel in 1960s
Provide high bandwidth between distant points on earth
Transmitter and receivers can be large or small dishes or even portable satellite telephones
Transponder: signal relay equipment on satellite (transmitter/responder)
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Satellite Footprint
Each satellite can transmit and receive signals from an area of earth surface known as satellite footprint
Size of footprint depend on height of orbit and to what degree the satellite focuses the beam of its signal
Source: Douglas, C (2016) Computer Networks and Internets
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GEO Satellites
Geostationary Earth Orbit (GEO): orbital period is the same as the rate that earth rotates
If positioned above equator, GEO satellite always remains in the same location over earth surface
Stationary satellite position means that once a ground station has been aligned with the satellite
Equipment never needs to move
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GEO Communication
Source: Douglas, C (2016) Computer Networks and Internets
GEO satellite and ground stations are permanently aligned
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GEO Distance from Earth
Distance required for GEO is 35,785 km
about 1/10 the distance to the moon
Implication:
Consider radio wave traveling to GEO and back
At speed of light, 3x108 m/s , trip takes:
Source: Douglas, C (2016) Computer Networks and Internets
Distance:
Speed:
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Delays from GEO
Delay of about 0.2s can be significant for some applications
For electronic transactions like stock exchange offering a limited set of bonds
Delaying an offer by 0.2s may mean difference between successful and unsuccessful offer
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Constraints in GEO
Limited space available in GEO above equator
Satellites using same frequency must be separated to avoid interference
Separation distance depends on power of transmitters
As technology evolves, possible allocate more satellites on orbit
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Coverage of the Earth
Three satellites are needed to cover the earth
Positioned around the equator (120o)
Source: Douglas, C (2016) Computer Networks and Internets
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Medium Earth Orbit (MEO)
Medium Earth Orbit (MEO): between 8,000-20,000Km above earth, orbital period of 2-12hours
Global Positioning System (GPS): uses 24 satellites in six MEOs
Triangulation: receivers measures delays from 4 GPS satellites in MEO to calculate their position on earth
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Low Earth Orbit (LEO)
Low Earth Orbit (LEO): altitudes up to 2000Km
Satellite must be placed above fringe of the atmosphere to avoid drag produced by encountering gases
LEO satellites are typically placed at altitudes of 500-600Km or higher
LEO offers short delays, typically 1 to 4 ms
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Problems with LEO
Orbit does not match the rotation of the earth
LEO satellite appears to move across the sky
Ground station must have an antenna that can rotate to track the satellite
Tracking is difficult because satellites move rapidly
Lowest altitude LEO satellites orbit the earth in approximately 90 minutes
Higher LEO satellites require several hours
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Satellite Cluster
LEO satellites in clustering or array deployment
Group of LEO satellites designed to work together
Satellite in groups can communicate with one another
Members of the group stay in communication and agree to forward messages as needed
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Satellite Cluster Example
Suppose a user in Europe sends a message to a user in USA
A ground station in Europe transmits message to satellite currently overhead (above it)
A cluster of satellites communicate to forward message to satellite in cluster that is currently over a ground station in USA
Finally, satellite currently over USA transmits the message to a ground station
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Practice 10.3
What is a satellite footprint?
For Global Positioning System (GPS), how many satellites are needed and what kind of orbits are these satellites on?
Explain the idea that GPS uses to calculate the position on earth.
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Lesson 10 Outline
Wireless Transmission
Radio Communication
Satellite Communication
Measuring Media Performance
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Media Choice Factors (1/2)
Cost: materials, installation, operation, and maintenance
Data rate: number of bits per second that can be sent
Delay: time required for signal propagation or processing
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Media Choice Factors (2/2)
Affect on signal: attenuation and distortion
Environment: susceptibility to interference and electrical noise
Security: susceptibility to eavesdropping
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Quantitative Characteristics (1/2)
Propagation delay: time required for a signal to traverse the medium
Queuing delay: time required for a signal to wait for its turn to be transmitted
Channel capacity: maximum data rate that the medium can support
Shannon’s Law, Nyquist Theorem
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Quantitative Characteristics (2/2)
Attenuation: loss of signal power over distance
Utilisation: proportion of time that channel is fully occupied
Throughput: number of bits carried by the channel per second as experienced by end user
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Practice 10.4
Describe each of the following:
Attenuation
Utilisation
Throughput
Security
Channel capacity
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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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