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CS 313 Introduction to Computer Networking & Telecommunication

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

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  1. CS 313 Introduction to Computer Networking & Telecommunication Physical Layer – Transmission Media Chi-Cheng Lin, Winona State University

  2. Topics • Guided Transmission Media • Wireless Transmission • Communication Satellites

  3. 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

  4. Guided Media • Twisted pair • Coaxial cable • Fiber optics

  5. 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

  6. Twisted Pair (a) Category 3 UTP. (b) Category 5 UTP.

  7. 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)

  8. Fiber Optics • Light • Electromagnetic energy traveling at 3108 m/s • Refraction • Critical angle • Reflection

  9. 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) cladding core cladding I (critical angle) (More dense)

  10. Fiber Cables (a) Side view of a single fiber. (b) End view of a sheath with three fibers.

  11. 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

  12. Modes • The McGraw-Hill Companies, Inc., 2004

  13. 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

  14. 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

  15. 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

  16. Electromagnetic Spectrum Ground Sky Line-of-sight

  17. Propagation methods • The McGraw-Hill Companies, Inc., 2004

  18. Radio Transmission • Easy to generate • Travel long distance • Penetration • Interference

  19. 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

  20. Infrared and Millimeter Waves • Remote control • Directional, cheap, easy to build • Cannot pass through solid walls • Good or bad? • Limited use on desktop

  21. 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

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

  23. 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

  24. 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

  25. Communication Satellites VSATs using a hub. VSATs: Very Small Aperture Terminals

  26. 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

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

  28. Satellites Vs. Fiber • Availability • Mobility • Broadcasting • Geographically issue • Right of way • Rapid deployment • Future?

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