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CS 313 Introduction to Computer Networking & Telecommunication. Physical Layer – Transmission Media. Topics. Guided Transmission Media Wireless Transmission Communication Satellites. Transmission Media. Physical layer: Transport a raw bit stream Physical media Guided media

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cs 313 introduction to computer networking telecommunication

CS 313 Introduction to Computer Networking & Telecommunication

Physical Layer – Transmission Media

Chi-Cheng Lin, Winona State University

  • Guided Transmission Media
  • Wireless Transmission
  • Communication Satellites
transmission media
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
guided media
Guided Media
  • Twisted pair
  • Coaxial cable
  • Fiber optics
twisted pair cable
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
twisted pair
Twisted Pair

(a) Category 3 UTP.

(b) Category 5 UTP.

coaxial cable
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)
fiber optics
Fiber Optics
  • Light
    • Electromagnetic energy traveling at 3108 m/s
    • Refraction
    • Critical angle
    • Reflection
fiber optics9
Fiber Optics

(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.

(Less dense)







(More dense)

fiber cables
Fiber Cables

(a) Side view of a single fiber.

(b) End view of a sheath with three fibers.

fiber optics11
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


  • The McGraw-Hill Companies, Inc., 2004
single mode
Single Mode
  • All beams received “together” and signal can be combined with little distortion
  • Widely used for longer distance
  • More expensive
  • Currently 50 Gbps for 100 km w/o amplification
fiber optics vs copper wire
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
  • Cons
    • Fiber interface costs more
    • Fragility
    • Unidirectional
wireless transmission
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 = λf
electromagnetic spectrum
Electromagnetic Spectrum

Ground Sky Line-of-sight


Propagation methods

  • The McGraw-Hill Companies, Inc., 2004
radio transmission
Radio Transmission
  • Easy to generate
  • Travel long distance
  • Penetration
  • Interference
microwave transmission
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
infrared and millimeter waves
Infrared and Millimeter Waves
  • Remote control
  • Directional, cheap, easy to build
  • Cannot pass through solid walls
    • Good or bad?
  • Limited use on desktop
applications of wireless media
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
lightwave transmission
Lightwave Transmission
  • Lasers
  • High bandwidth, low cost, easy to install
  • Aiming is hard
  • No penetration through rain or thick fog
communication satellite
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
      • Earth to satellite: Downlink

 Bent pipe mode

communication satellites
Communication Satellites

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

(Geostationary Earth Orbit)

(Medium Earth Orbit), app.: GPS

(Low Earth Orbit), voice/data communication

communication satellites25
Communication Satellites

VSATs using a hub.

VSATs: Very Small Aperture Terminals

communication satellite26
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
iridium vs globalstar
Iridium vs. Globalstar
  • (a) Iridium: Relaying in space.
  • (b) Globalstar: Relaying on the ground.
satellites vs fiber
Satellites Vs. Fiber
  • Availability
  • Mobility
  • Broadcasting
  • Geographically issue
  • Right of way
  • Rapid deployment
  • Future?