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Chapter 4 – Transmission Media. 1/21. Overview. guided – copper twisted pair, coaxial cable optical fiber unguided – wireless; through air, vacuum, or water characteristics and quality determined by medium and transmitted signal

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  1. Chapter 4 – Transmission Media 1/21

  2. Overview • guided – copper twisted pair, coaxial cable optical fiber • unguided – wireless; through air, vacuum, or water • characteristics and quality determined by medium and transmitted signal • in unguided media - bandwidth of signal produced by the antenna is more important • in guided media - medium is more important • key concerns are data rate and distance 2/21

  3. Design Factors • A number of design factors relating to the transmission media and the signal determine the data rate and distance • bandwidth • higher bandwidth gives higher data rate • transmission impairments • e.g. attenuation limit the distance • Interference • from other signals in overlapping frequency bands • number of receivers in guided media • more receivers introduces more attenuation 3/21

  4. Electromagnetic Spectrum 4/21

  5. Transmission Characteristics of Guided media Frequency Range Typical Attenuation Typical Delay Repeater Spacing Twisted pairs (multi-pair cables) 0 to 1 MHz 0.7 dB/km @ 1 kHz 5 µs/km 2 km Coaxial cable 0 to 500 MHz 7 dB/km @ 10 MHz 4 µs/km 1 to 9 km Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km 5/21

  6. Twisted pair • Twisting tends to decrease the crosstalk interference between adjacent pairs in a cable. • Neighboring pairs in a bundle typically have somewhat different twist lengths to reduce the crosstalk interference • On long-distance links, the twist length varies from 5 to 15 cm 6/21

  7. Twisted Pair: Transmission Characteristics • Analog Transmission • needs amplifiers every 5km to 6km • Digital Transmission • can use either analog or digital signals • needs a repeater every 2-3km • Limited distance • Limited bandwidth (1MHz) • Limited data rate (few Mbps) • Susceptible to interference and noise • e.g., 50/60 Hz power line frequency • Less expensive that coaxial cables and fiber 7/21

  8. Attenuation vs frequency for the guided media 8/45

  9. Unshielded vs Shielded twisted pairs • Unshielded Twisted Pair (UTP) • ordinary telephone wire • cheapest • easiest to install • suffers from external Electromagnetic (EM) interference • Shielded Twisted Pair (STP) • metal braid or sheathing that reduces interference • more expensive • harder to handle (thick, heavy) • In a variety of categories - see the Electronic Industry Association standard EIA-568 9/21

  10. Twisted Pair Categories and Classes UTP = Unshielded Twisted Pair FTP = Foil Twisted Pair (Foil قصدير) SSTP = Shielded Screen Twisted Pair 10/21

  11. Comparison Between Unshielded and Shielded Twisted Pair A t t e n u a ti o n (d B p e r 100 m ) F re qu e n cy Ca t egory 3 Ca t egory 5 ( M H z ) UT P UT P 150 - oh m S T P 1 2 . 6 2 . 0 1 . 1 4 5 . 6 4 . 1 2 . 2 16 13 . 1 8 . 2 4 . 4 25 — 10 . 4 6 . 2 100 — 22 . 0 12 . 3 300 — — 21 . 4 11/21

  12. Coaxial Cable 12/21

  13. Coaxial Cable: Transmission characteristics • superior frequency characteristics to TP • performance limited by attenuation & noise • analog signals • amplifiers every few Km • closer when using higher frequencies • up to 500MHz • digital signals • repeater every 1km • closer for higher data rates 13/21

  14. Coaxial Cable : Applications • Television Distribution (cable TV) • Long distance telephone transmission • Short-run computer system links • Local area networks 14/21

  15. Optical Fiber 15/21

  16. Optical Fiber: Benefits • greater capacity • data rates of hundreds of Gbps • smaller size & weight • lower attenuation • electromagnetic isolation • greater repeater spacing • 10s of km at least 16/21

  17. Optical Fiber: Transmission Characteristics • uses total internal reflection to transmit light • effectively acts as wave guide for 1014 to 1015 Hz • can use several different light sources • Light Emitting Diode (LED) • cheaper, wider operating temp range, lasts longer • Injection Laser Diode (ILD) • more efficient, has greater data rate • relation of wavelength, type & data rate 17/21

  18. Optical Fiber Transmission Modes 18/21

  19. Frequency Utilization for Fiber Application 19/21

  20. Optical Fiber : Applications • Long-haul trunks (1500Km, high capacity: typically 20000-60000 voice channels) • Metropolitan trunks (12 Km, 100000 voice channel in a trunk group) • Rural exchange trunks (40-160 Km, 5000 voice channels) • Subscriber loops (replace twisted pair and coaxial cables, carry voice, data, image and video) • Local area networks (100Mbps-10Gbps) 20/21

  21. Optical Fiber : Applications 21/21

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