CS 313 Introduction to Computer Networking

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CS 313 Introduction to Computer Networking
Telecommunication

• Physical Layer Transmission Media

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Topics

• Guided Transmission Media
• Wireless Transmission
• Communication Satellites

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Transmission Media

• Physical layer Transport a raw bit stream
• Physical media
• Guided media
• Information transmitted on wires by varying some
  physical property such as voltage or current
• Copper wire, fiber optics
• Unguided media
• Information transmitted wirelessly by
  electromagnetic waves
• Radio, lasers

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Guided Media

• Twisted pair
• Coaxial cable
• Fiber optics

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Twisted Pair Cable

• Oldest, but still most common
• Two twisted insulated copper wires
• Why twisted?
• To reduce electrical interference
• Telephone system
• Repeater needed for longer distances
• Repeater device that extends the distance a
  signal can travel by regenerating the signal
• Adequate performance at low cost

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Twisted Pair

• (a) Category 3 UTP.
• (b) Category 5 UTP.

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Coaxial Cable

• Better shielding than twisted pairs
• Span longer distances at higher speeds
• Lower error rate
• Widely used for
• Cable TV
• WAN (Internet over cable)

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Fiber Optics

• Light
• Electromagnetic energy traveling at 3?108 m/s
• Refraction
• Critical angle
• Reflection

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Fiber Optics
(Less dense)
cladding
core
I (critical angle)
cladding
(More dense)

• (a) Three examples of a light ray from inside a
  silica fiber impinging on the air/silica boundary
  at different angles.
• (b) Light trapped by total internal reflection.

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Fiber Cables

• (a) Side view of a single fiber.
• (b) End view of a sheath with three fibers.

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Fiber Optics

• Optical transmission system
• Light source LED or lasers
• Transmission medium fiber optic cable
• Detector converting detected light to electrical
  pulse
• Propagation modes
• Multimode
• Step-index
• Grade-index
• Single mode

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Modes
Core diameter 200 micrometers 50 - 100
micrometers 10 micrometers

• The McGraw-Hill Companies, Inc., 2004

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Single Mode

• All beams received together and signal can be
  combined with little distortion
• Widely used for longer distance (over 550 m)
• More expensive
• Currently 50 Gbps for 100 km w/o amplification

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Fiber Optics Vs. Copper Wire

• Pros
• Higher bandwidth
• Less attenuation ? less repeater needed (about
  every 50 km, copper 5 km)
• Noise resistance no interference, surge, ...
• Thin and lightweight
• Excellent security, as wiretapping is harder
• Cons
• Fiber interface costs more
• Fragility

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Wireless Transmission

• Electromagnetic Spectrum
• Electron movement creates electromagnetic wave
• Frequency number of oscillations per second of a
  electromagnetic wave measured in Hertz (Hz)
• Wavelength distance between two consecutive
  maxima (or minima)
• Speed of light C 3 ? 108 m/sec
• C wavelength ? frequency, i.e., C ?f

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Electromagnetic Spectrum
Ground Sky Line-of-sight
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Propagation methods

• The McGraw-Hill Companies, Inc., 2004

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Radio Transmission

• Easy to generate
• Travel long distance
• Penetration
• Interference

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Microwave Transmission

• MCI?
• Straight line travel
• Higher towers for longer distances
• Multipath fading problem, absorption by rain
• Advantages
• Right of way not needed
• Inexpensive
• Industrial/Scientific/Medical (ISM) bands
• No license needed
• Garage door opener, cordless phone, etc
• Bluetooth, 802.11 wireless LANs

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Infrared and Millimeter Waves

• Remote control
• Directional, cheap, easy to build
• Cannot pass through solid walls
• Good or bad?
• Limited use on desktop

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Applications of Wireless Media

• Radio waves
• Multicast communications
• Radio, television, and paging systems
• Microwaves
• Unicast communication
• Cellular telephones, satellite networks, and
  wireless LANs.
• Infrared signals
• Short-range communication in a closed area using
  line-of-sight propagation
• Wireless keyboards, mice, printers

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Lightwave Transmission

• Lasers
• High bandwidth, low cost, easy to install
• Aiming is hard
• No penetration through rain or thick fog

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Communication Satellite

• Big microwave repeater in the sky
• Transponders, each
• Listens to some portion of spectrum
• Earth to satellite Uplink
• Amplifies incoming signal
• Rebroadcast it at another frequency
• Satellite to Earth Downlink
• ? Bent pipe mode

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Communication Satellites
(Geostationary Earth Orbit)
(Medium Earth Orbit), app. GPS
(Low Earth Orbit), voice/data communication

• Communication satellites and some of their
  properties, including altitude above the earth,
  round-trip delay time and number of satellites
  needed for global coverage.

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Communication Satellites

• VSATs using a hub.

VSATs Very Small Aperture Terminals
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Communication Satellite

• Low-Earth Orbit Satellites
• Iridium 66 satellites
• Goal
• Provide worldwide telecommunication service using
  hand-held devices that communicates directly with
  the Iridium satellites
• Current status?
• Broke, auctioned, restarted
• Globalstar 48 LEOs using bent-pipe design
• Teledisc
• Goal provide Internet users with high bandwidth
  using VSAT-like antenna

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Iridium vs. Globalstar

• (a) Iridium Relaying in space.
• (b) Globalstar Relaying on the ground.

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Satellites Vs. Fiber

• Availability
• Mobility
• Broadcasting
• Geographically issue
• Right of way
• Rapid deployment
• Future?