Transmission Media

Transmission Media

1. Design Factors

   • bandwidth
     • higher bandwidth gives higher data rate
   • transmission impairments
     • eg. attenuation
   • interference
   • number of receivers in guided media

2. Transmission Characteristics of Guided Media

   • Twisted pair (with loading)
   • Twisted pairs (multi-pair cables)
   • Optical fiber
     • Optical Fiber - Benefits s
       • greater capacity
       • smaller size & weight
       • lower attenuation
       • electromagnetic isolation
       • greater repeater spacing
     • Optical Fiber - Transmission Characteristics
       • uses total internal reflection to transmit light
       • can use several different light sources
       • relation of wavelength, type & data rate
   • Coaxial cable
     • Coaxial Cable - Transmission Characteristics
       • superior frequency characteristics to TP
       • performance limited by attenuation & noise
       • analog signals
       • digital signals

3. Unshielded vs Shielded TP

   • unshielded Twisted Pair (UTP)
     • ordinary telephone wire
     • cheapest
     • suffers from external EM interference
     • easiest to install
   • shielded Twisted Pair (STP)
     • metal braid or sheathing that reduces interference
     • more expensive
     • harder to handle (thick, heavy)

4. Wireless Transmission Frequencies

   • 2GHz to 40GHz
     • microwave
     • highly directional
     • point to point
     • satellite
   • 30MHz to 1GHz
     • omnidirectional
     • broadcast radio
   • 3 x 10^11 to 2 x 10^14
     • nfrared
     • local

5. Antennas

   • electrical conductor used to radiate or collect electromagnetic energy
   • transmission antenna
   • reception antenna
   • same antenna is often used for both purposes

6. Radiation Pattern

   • power radiated in all directions
   • an isotropic antenna is a (theoretical) point in space
   • not same performance in all directions

7. Antenna Gain

   • measure of directionality of antenna
   • measured in decibels (dB)
   • results in loss in power in another direction

8. Terrestrial Microwave

   • used for long haul telecommunications
   • and short point-to-point links
   • requires fewer repeaters but line of sight
   • 1-40GHz frequencies
   • also interference

9. Satellite Microwave

   • satellite is relay station
   • receives on one frequency, amplifies or repeats signal and transmits on another frequency
   • typically requires geo-stationary orbit
   • ypical uses

10. Broadcast Radio

    • radio is 3kHz to 300GHz
    • use broadcast radio, 30MHz - 1GHz, for:  FM radio  UHF and VHF television
    • is omnidirectional
    • still need line of sight
    • suffers from multipath interference

11. Infrared

    • modulate noncoherent infrared light
    • end line of sight (or reflection)
    • are blocked by walls
    • no licenses required
    • typical uses

12. Refraction

    • velocity of electromagnetic wave is a function of density of material
    • speed changes as move between media
    • Index of refraction (refractive index)
    • have gradual bending if medium density varies

13. Line of Sight Transmission

    • Free space loss
    • Atmospheric Absorption
    • Multipath
    • Refraction