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Part II Data TransmissionCh 4 Transmission Media

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Transmission of Information

Transmission of Information

All computer comm. Involves encoding data in a form of energy, and sending the energy across a transmission media

Electric current wireLaser fiberElectromagnetic wave air

From physicsEnergyElectromagnetic wave propagation

From mathematicsCoding theory

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Transmission MediaTransmission MediaCopper ( 銅 ) wire

Twisted pair ( 雙絞線 )(to minimize interference)UTP (Unshielded Twisted Pair)STP (Shielded Twisted Pair)

Coaxial cable ( 同軸線 )

Copper conductor

Insulator

Metal shieldOuter cover

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Pin12345678

Color綠白綠橘白藍藍白橘棕白棕

EIA/TIA-568A

RJ-45

Transmission MediaTransmission Media

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Transmission MediaTransmission MediaOptical fiber

FlexibleLight “stays in”

Air / spaceUsed for electromagnetic transmission

光纖 (Core)外圍材料 (Cladding)

保護外皮 (Protective Sheath)

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Transmission MediaTransmission MediaTwisted pair

Two insulated copper wires are twisted around each other

Twisting minimizes electromagnetic interference between pairs

Digital signal data rates vary with distance

100 Mbps over a segment length of 100m2400 bps for up to 10 miles

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Transmission MediaTransmission MediaCoaxial cable

Bandwidth of about 500 MbpsMore expensive than twisted pairTransmits information

baseband mode>bandwidth devoted to a single stream of data

broadband mode>bandwidth is divided into ranges>each range typically carries separate coded

information>allows the transmission of multiple data

streams over the same cable simultaneously>eg. Cable TV

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Transmission MediaTransmission MediaOptical fiber

Use optical signals instead of electrical signals

Light sources : LED (Light Emitting Diode, 發光二極體）Laser Diode

Detector: Photoelectric Diodes

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Transmission MediaTransmission MediaOptical fiber (ctn.)

Advantages over wiresNo electrical interferenceA fiber can carry a pulse of light much fartherLight can carry more information than electrical signal

DisadvantagesInstalling a fiber requires special equipmentMore expensive

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Transmission MediaTransmission MediaOptical fiber (ctn.)

Stepped Index MultimodeMultiple propagation paths exist Modal dispersion phenomenon

>different path length (time) to traverse the fiber

A

B

C

C

A

B

可接受入射角度

Very pure glass corecladdingProtective cover

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Transmission MediaTransmission MediaOptical fiber (ctn.)

Graded Index Multimode gradually less dense from the core

Cladding with variable

refractive index

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Forms of Energy Used To Transmit Data (1)

Forms of Energy Used To Transmit Data (1)

Audible ( 聽得見的 ) soundsElectric currentOmni-directional electromagnetic waves

Radio Frequency (RF)Infrared

X

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Forms of Energy Used to Transmit Data (2)

Forms of Energy Used to Transmit Data (2)

Directional electromagnetic wavesPoint-to-point satellite channelLimited broadcast (spot beam)MicrowaveLaser beam

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Types of SatellitesTypes of Satellites

Geosynchronous Earth Orbit (GEO)Low Earth Orbit (LEO)

Array neededEarth

22300 miles

Kepler’s third law : P2=K D3

P : time to rotate around another planetary (period)D : distance between satellite and the planet’s centerK : constant depending on gravitational forces

Geosynchronous orbit P= 24-hour D=(Radius+22300) miles

Site x A eart

h

Site y

200 – 400 miles

At least 66 LEOs