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Chapter 4 – Transmission Media Chapter 4 – Transmission Media

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OverviewOverview

guided – copper twisted pair, coaxial cable optical fiber

unguided – wireless; through air, vacuum, or water

characteristics and quality determined by medium and transmitted signalin unguided media - bandwidth of signal

produced by the antenna is more importantin guided media - medium is more important

key concerns are data rate and distance

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Design FactorsDesign Factors

A number of design factors relating to the transmission media and the signal determine the data rate and distance

1. bandwidth higher bandwidth gives higher data rate

2. transmission impairments e.g. attenuation limit the distance

3. Interference from other signals in overlapping frequency bands

4. number of receivers in guided media more receivers introduces more attenuation

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Electromagnetic SpectrumElectromagnetic Spectrum

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Transmission Characteristics of Transmission Characteristics of Guided mediaGuided media

186 to 370 THz

0.2 to 0.5 dB/km

  Frequency Range

Typical Attenuatio

n

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 5 µs/km 40 km

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Twisted pairTwisted 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

Twisted Pair: Transmission Twisted Pair: Transmission CharacteristicsCharacteristics

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Analog Transmissionneeds amplifiers every 5km to 6km

Digital Transmissioncan use either analog or digital signalsneeds a repeater every 2-3km

Limited distanceLimited bandwidth (1MHz)Limited data rate (few Mbps)Susceptible to interference and noise

e.g., 50/60 Hz power line frequencyLess expensive that coaxial cables and fiber

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Attenuation vs frequency for the guided media

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Unshielded vs Shielded twisted pairsUnshielded vs Shielded twisted pairs

Unshielded Twisted Pair (UTP)ordinary telephone wirecheapesteasiest to installsuffers from external Electromagnetic (EM) interference

Shielded Twisted Pair (STP)metal braid or sheathing that reduces interferencemore expensiveharder to handle (thick, heavy)

In a variety of categories - see the Electronic Industry Association standard EIA-568

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Twisted Pair Categories and Classes

Category 3 Class C

Category 5 Class D

Category 5E Category 6 Class E

Category 7 Class F

Bandwidth 16 MHz 100 MHz 100 MHz 200 MHz 600 MHz

Cable Type UTP UTP/FTP UTP/FTP UTP/FTP SSTP

Link Cost (Cat 5 =1)

0.7 1 1.2 1.5 2.2

UTP = Unshielded Twisted Pair

FTP = Foil Twisted Pair (Foil قصدير)

SSTP = Shielded Screen Twisted Pair

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Comparison Between Unshielded and Shielded Twisted Pair

Attenuation (dB per 100 m)

Frequency(MHz)

Category 3UTP

Category 5UTP 150-ohm STP

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

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Coaxial Cable

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Coaxial Cable: Transmission characteristics

superior frequency characteristics to TPperformance limited by attenuation &

noiseanalog signals

amplifiers every few Kmcloser when using higher frequenciesup to 500MHz

digital signalsrepeater every 1kmcloser for higher data rates

Coaxial Cable : Applications

Television Distribution (cable TV)Long distance telephone transmissionShort-run computer system linksLocal area networks

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Optical Fiber

Optical Fiber: Benefits

greater capacitydata rates of hundreds of Gbps

smaller size & weightlower attenuationelectromagnetic isolationgreater repeater spacing

10s of km at least

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Optical Fiber: Transmission Characteristics

uses total internal reflection to transmit lighteffectively acts as wave guide for 1014 to 1015 Hz

can use several different light sourcesLight Emitting Diode (LED)

cheaper, wider operating temp range, lasts longerInjection Laser Diode (ILD)

more efficient, has greater data rate

relation of wavelength, type & data rate

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Optical Fiber Transmission Modes

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Frequency Utilization for Fiber Application

Wavelength (invacuum) range

(nm)

FrequencyRange (THz)

BandLabel

Fiber Type Application

820 to 900 366 to 333 Multimode LAN

1280 to 1350 234 to 222 S Single mode Various

1528 to 1561 196 to 192 C Single mode WDM

1561 to 1620 192 to 185 L Single mode WDM

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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)

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Optical Fiber : Applications