Introduction to Information Technology

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Introduction to Information Technology

• LECTURE 8 TRANSMISSION TECHNOLOGY

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Communications Media

• Free Space
• Electromagnetic wave through free space via radio
  frequency and satellite systems.
• Satellite Communications
• Microwave Transmission
• Cellular
• Radio
• Copper Wire
• Electromagnetic variations through copper wires
• Cable Television (Coaxial Cable)
• Copper Cable Into Homes (Twisted Pair Cable)
• LANs, WANs, Telephone System
• Fiber Optic Cable
• Light transmitted through fiber-optic cable
  (glass)
• Backbone of Telephone System and WANs

Wireless
Wired
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Some Transmission Concepts

• Information Rate - The number of bits that can be
  sent per second at the transmission origin and
  received at the destination. Then whats
  bandwidth?
• Latency - The time delay involved in the movement
  of a message from one location to another.
• Real Time Data Transmission - Nothing arrives
  instantaneously, but some applications receive
  data in adequate time to be considered real
  time
• Even speech has a delay time, but is interpreted
  as real time.
• Application-specific
• Speed of Light - No information can be relayed at
  speeds that exceed the speed of light. (Einstein)
  3x108 m/s

Information rate versus latency Communication
channels can move data at a rate of billions of
bits per second, but still suffer huge latencies.
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Radio-Frequency Systems

• Radio communication has been in existence for
  more than 90 years.
• Were constantly surrounded by technology-generate
  d radio waves
• Entertainment - AM and FM radio, broadcast TV,
  satellite TV.
• Communications - Cellular, global long distance,
  CB radio, wireless LANs.
• Space exploration
• Garage door opener, baby monitor, wireless phone,
  car entry.
• Air traffic control, navigation systems, military
  applications, etc.
• Were constantly bathed in natural
  electromagnetic radiation
• The sun, lightning flashes, radioactive material
  in the earth, etc.

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Radio Waves

• A radio wave travels from a transmitter to a
  receiver - What is it?
• Continuous sine waves are used to transmit
  information
• Electromagnetic wave
• Travels at the speed of light
• Unlike sound waves, which require air, radio
  waves propagate through air or empty space, just
  like light travels to the earth from distant
  stars.
• Visible light, radio, the X-Ray, ultraviolet
  light, etc. travel in a similar manner

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Radio Waves

• Each radio signal uses a different sine wave
  frequency
• Frequency Modulation (FM) 88 MHz to 108 MHz
• Amplitude Modulation (AM) 535 kHz to 1,700 kHz
• Television stations
• 54 to 88 MHz for channels 2 through 6
• 174 to 220 MHz for channels 7 through 13
• Garage door openers, alarm systems 40 MHz
• Cordless phones 40 to 50 MHz, 900 MHz
• Radio controlled cars Around 75 MHz
• Air traffic control radar 960 to 1,215 MHz

Electromagnetic Spectrum
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Calculating Wavelength
Each frequency has an associated wavelength
calculated as follows
l wavelength c speed of light (3x108 m/s) f
frequency in Hz
l c/f
What is the wavelength for a frequency of 8 MHz?
l 3x108/8x106 37.5 m
Electromagnetic Spectrum
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Calculating Wavelength
l wavelength c speed of light (3x108 m/s) f
frequency in Hz
l c/f
What is the wavelength for a frequency of 5 MHz?

• 3x108/5x106 60 m
• 300,000,000/5,000,000 60 m

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How Can Waves Carry Information?

• Through modulation the process of controlling
  the properties of a signal so that it contains
  the information patterns to be transmitted.
• Three types of modulation
• Amplitude Modulation (AM)
• Frequency Modulation (FM)
• Phase Modulation (PM)

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Amplitude Modulation (AM)

• Uses a single frequency to convey information
• Varies only its amplitude to convey information
• Vulnerable to interference

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Frequency Modulation

• Frequency Modulation (FM)
• Information represented as changes in frequency
  rather than amplitude.
• Less vulnerable to interference.
• Requires more of the frequency spectrum.
• Phase Modulation (PM)
• A change of phase (direction) represents a change
  from one state to another, such as from a 0 to a
  1.

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Basic Parameters in the Design of RF systems

• Transmitter power
• Transmitter frequency Maintained by FCC
• Receiver sensitivity
• Desired bandwidth and/or bit rate
• Limitations on antenna size, locations
• Desired transmission distance

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Examples

• Baby Monitor
• Modulation Amplitude Modulation
• Frequency range 49 MHz
• Transmitter power .25 Watts
• Cell Phone
• Modulation Frequency Modulation
• Frequency range 824 to 849 MHz
• Transmitter power 3 Watts

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Radio Communications Example - GPS
GPS GLOBAL POSITIONING SYSTEM

• Determines the position of a GPS receiver
  anywhere on (or above) the Earth
• GPS has many applications
• Aircraft and Marine Navigation
• Military Applications
• Hiking, Hunting, Other Outdoor Activities
• Driving (Route Finding, Emergencies)
• Surveying
• Position defined in terms of latitude, longitude,
  and elevation above sea level
• Limitation Receiver must have a relatively
  clear view of the sky
• 3 Major Components
• Satellites, Receivers, and System Control Center
• Who owns, operates, and funds the GPS system?

GPS Satellite
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How Does GPS Work?

• The principle of operation of the GPS system,
  triangulation, is very simple.
• If you knows your distance from a given point,
  you know you are somewhere on a circle with that
  point in the center.
• If, at the same time, you also know your distance
  from a different point, you know you are on
  another given circle that intersects.
• With a known distance from a third point, you can
  identify your exact location.
• GPS triangulation
• Satellite positions always known.
• Distance from satellite determined using speed of
  light.
• 24 GPS satellites in orbit at an altitude of
  11,000 miles with an orbital period of 12 hours.

General Example of Triangulation
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Triangulation
With signals from four (or more) satellites, the
altitude can also be known, so that the position
in 3D space may be determined.
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GPS Receiver

• 5 Major Elements
• Antenna
• Sensitive radio receiver
• Microprocessor
• Distance calculations, speed of travel, direction
  of travel, route to follow, time required.
• Database street address, maps
• Display

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Geosynchronous Satellites
Whats a geosynchronous satellite?

• A geosynchronous satellite appears to remain
  stationary to the earth.
• Orbits the Earth at a height of 22,300 miles
• At this height, the time for one orbit of the
  Earth is 24 hours.
• Because the Earth also rotates once every 24
  hours, the satellite appears to stay still over
  the same spot on Earth.
• At this distance, a signal travelling at the
  speed of light takes 270 milliseconds to reach
  the Earth.
• The delay between when you say something and when
  you hear the other persons response is 540
  milliseconds - over half a second.

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Historical Milestone SPUTNIK

• First satellite
• Launched by the Soviet Union in 1957
• Prompted a surge in U.S. investment in technology
• U.S. formed the Advanced Research Projects Agency
  (ARPA)
• Responsible for ARPANET, precursor to the
  Internet.

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Wire as a Transmission Medium
Copper wire is still the most common
communications medium. Wire based transmission
schemes guide electromagnetic waves, either
between a pair of separate wires, or inside a
coaxial arrangement.
TWISTED PAIR

• The most common type of cable is twisted-pair.
• Pair of wires twisted around each other.
• Each wire covered in jacket.
• Why twisted? Shields the cable from
  interference.
• The more twists the better the shield.

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General Types of Twisted Pair

• Two kinds of twisted pair
• Unshielded Twisted Pair (UTP)
• Found in walls of most buildings
• Local Loop, ISDN, DSL, LANs (10Base-T Ethernet),
  T1 lines (1.544 Mbps)
• Wiring connecting homes to the telephone
  companies local switch
• All over the George Mason Campus
• Shielded Twisted Pair (STP) - Special purpose
  cable
• Today the most common type of twisted pair
  installed is called CAT 6 or category 6
  twisted pair. (Also CAT 5)

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COAX
COAXIAL CABLE

• Another type of commonly used copper cable is
  coax.
• Main Components
• Center Conductor
• Shield Conductor
• Insulated material
• Cable TV
• 70 of U.S. homes have CATV
• High bandwidth

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Cable Characteristic - Attenuation

• Attenuation - Loss of energy and related
  reduction in the size of transmitted pulses
• Longer the cable, greater the attenuation
• Measured in dB

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Cable Susceptibility to Noise

• Cables tend to be routed next to each other and
  near sources of electromagnetic interference
• Unwanted noise affects all copper cables to
  various extents
• Parallel wire Greatly affected
• UTP Less affected because of the twists
• STP Less affected because of the metal
  shielding
• Coaxial cable Most resistant (out of copper
  cables) to loss and noise problems

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Fiber Optics
Transmission of LIGHT over GLASS or PLASTIC
fiber.

• Explosive, rapidly evolving technology
• Coaxial arrangement of glass
• The glass has to be extremely pure
• Imagine a 50 mile thick window you could see
  perfectly through.
• Main components
• Core - low index of refraction
• Cladding- higher index of refraction
• Teflon Jacket protects and stiffen the fiber
• Core and Cladding have different indices of
  refraction.
• Index of refraction is a measure of speed of
  light in a material, which affects the angle by
  which a light ray is bent on passing through the
  material

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Advantages of Fiber

• Much Higher Bandwidth (Gbps) - Thousands of
  channels can be multiplexed together over one
  strand of fiber.
• Immunity to Noise - Immune to electromagnetic
  interference (EMI).
• Safety - Doesnt transmit electrical signals,
  making it safe in environments like a gas
  pipeline.
• High Security - Difficult to tap into.
• Less Loss - Repeaters can be spaced 75 miles
  apart.
• Reliability - More resilient than copper in
  extreme environmental conditions.
• Size - Lighter and more compact than copper.
• Flexibility - Unlike impure, brittle glass, fiber
  is physically very flexible.

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Disadvantages of Fiber

• Cost of interfacing equipment necessary to
  convert electrical signals to optical signals.
  (optical transmitters, receivers)
• Splicing fiber optic cable is also more
  difficult.

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Two Types of Fiber Optic Cable

• Multimode fiber
• Transmits multiple signals per fiber
• Larger core 62.5 microns in diameter
• Single-mode fiber
• Transmits one signal per fiber
• Small core 9 microns in diameter

Industry Term DARK FIBER An important
related technology OPTICAL SWITCHING