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EE354 : Communications System I

EE354 : Communications System I. Lecture 13-15: Transmission media Aliazam Abbasfar. Outline. Transmission media. Communications systems. Wireline (wired) Telephony (voice, fax, modem, DSL) Ethernet/LAN Cable TV Backplane copper links Wireless (Electromagnetic)

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EE354 : Communications System I

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  1. EE354 : Communications System I Lecture 13-15: Transmission media Aliazam Abbasfar

  2. Outline • Transmission media

  3. Communications systems • Wireline (wired) • Telephony (voice, fax, modem, DSL) • Ethernet/LAN • Cable TV • Backplane copper links • Wireless (Electromagnetic) • Over the air communication • Radio and TV broadcast • WLAN • Cellular • Radar • Fiber optics • High speed long haul data communication • High traffic data transfer

  4. Transmission media • Open wire • Twisted pair • Coaxial cable • Optical fiber • Air

  5. Open-wire • On utility poles • share power line routes • Interference limited (EMI) • Limits the BW too • Transposition to reduce interference • Early twisting scheme • 4 twists per Km • Environmental effects

  6. Twisted pair cable • Unshielded twisted pair (UTP) • Shielded twisted pair (STP)

  7. Transmission parameters • Lumped model • for a unit length • System response H(f) = e-gL g = a + j b = Loss (dB) = 20 log10e a L = a’ L phase change = b L • Loss increases with frequency

  8. Interference • Interference is the main limitation (BW) • Twisting reduces interference • Shielding further reduces interference • Cross talk (X-Talk) • Near end cross talk (NEXT) • Far end cross talk (FEXT) NEXT FEXT • Cable length  --  • Space between pairs   • NEXT and FEXT decreases with frequency • f1.5 and f2

  9. Applications • Communication networks • Subscriber lines • Analog (Voice) • Digital (ADSL, HDSL, VDSL) • Links (E1,T1) • Computer networks • LAN • Cat 3 < 10 Mbps • Cat 4 < 100 Mbps • Cat 5 < 150 Mbps • Cat 6 < 350 Mbps

  10. Coaxial cable • Low cross talk • Increases with frequency • High speed/Long links • Long distance communication links • Cable TV • WAN/MAN

  11. Wireline media comparison

  12. Optical Fiber • Structure • Core (8 µm) • Cladding (125 µm) • Buffer (250 µm) • Jacket (400 µm) • Optical cable

  13. Fiber optics • Optical propagation • Refraction index • ncore > ncladding • Fiber types • Step index • Graded index • Single mode • Multi mode

  14. Optical fiber • Loss • Absorption • Scattering • Connectors • Low loss : 0.2 dB/Km • Color dependent • Dispersion • Multi modes = different paths • Refraction index is frequency (color) dependent • High bandwidth ( > 1 GHz) • Wavelength division multiplexing (WDM) • DWDM • Data rate > TB/s in a single fiber !

  15. Wireless communication • Frequency allocation needed in shared environment • To avoid interference • Spectrum is a very valuable resource • Band allocation to applications • Government regulations and policies • ITU coordinates between nations • Freq band: • 3-30KHz Very low freq. (VLF) • 30-300KHz Low freq. (LF) • 300K-3MHz Medium freq. (MF) • 3-30MHz High freq (HF) • 30-300MHz Very high freq (VHF) • 300M-3G Ultra high freq (UHF) • 3-3GHz Super high freq. (SHF)

  16. Electromagnetic waves propagations • Ground waves travels along the surface of the earth • ( freq < 2 MHz) • Sky waves reflected by ionosphere • Very variable – seasonal • Angle and loss of reflection • Freq < 30 MHz • Line of sight (LOS) • No reflection or refraction • Non Line of sight • Local reflections/refractions

  17. Wireless issues • Path loss • Fading • Mobility • Interference

  18. Satellite systems • LEOs • Lower power • Smaller delay • Need many satellites • Shift towards LEOs in 1990 • Global domination • Compete with cellular systems • Failed miserably (Iridium ) • Big, power hungry mobile terminals • Global Positioning System (GPS) • Satellite signals used to pinpoint location • Popular in cars, cell phones, and navigation devices • Natural area for satellite systems is broadcasting • Now operate in 12GHz band • 100s of TV and radio channels • All over the world

  19. Reading • Carlson Ch. 1 • Proakis Ch. 1

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