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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 l.jpg

CS 313 Introduction to Computer Networking & Telecommunication

Physical Layer – Transmission Media

Chi-Cheng Lin, Winona State University

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  • 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

(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)

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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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  • 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

  • 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

  • Cons

    • Fiber interface costs more

    • Fragility

    • Unidirectional

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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 = λf

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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

      • Earth to satellite: Downlink

         Bent pipe mode

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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

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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?