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Transmission MediaTransmission MediaUnguided MediaUnguided Media

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Unguided MediaUnguided Media

wireless communicationwireless communication No physical conductorNo physical conductor Signals are broadcast through airSignals are broadcast through air

– Available to anyone who has a device Available to anyone who has a device capable of receiving themcapable of receiving them

Radio Frequency AllocationRadio Frequency Allocation– Eight rangesEight ranges

VLF (very low frequency) to Extremely VLF (very low frequency) to Extremely high frequency (EHF)high frequency (EHF)

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Propagation of Radio WavesPropagation of Radio Waves

Radio wave transmission utilizes 5 Radio wave transmission utilizes 5 types of propagationtypes of propagation– SurfaceSurface– TroposphericTropospheric

aprox. 30 miles from the earth’s surfaceaprox. 30 miles from the earth’s surface– IonosphericIonospheric

above Tropospheric, below the spaceabove Tropospheric, below the space– Line-of-sightLine-of-sight– SpaceSpace

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Radio Frequency AllocationRadio Frequency Allocation

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Types of PropagationTypes of Propagation

Surface Propagation

Troposphere Propagation

Ionosphere Propagation

Line-of-sight Propagation

Space Propagation

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Surface Propagation

Radio waves travel through the lowest Radio waves travel through the lowest portion of the atmosphereportion of the atmosphere

Signals emanate in all directionsSignals emanate in all directions– From transmitting antennaFrom transmitting antenna– Follow the curvature of the planetFollow the curvature of the planet

Distance depends on the amount of Distance depends on the amount of power in signalpower in signal

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Troposhperic Propagation

Works in two waysWorks in two ways– A signal is sent from antenna to antenna A signal is sent from antenna to antenna

(line-of-sight)(line-of-sight)

– A signal is broadcast at an angle into the A signal is broadcast at an angle into the upper layers of the troposphere, then it is upper layers of the troposphere, then it is reflected back down to the earth’s surfacereflected back down to the earth’s surface

Longer distance to coverLonger distance to cover

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Ionosphere PropagationIonosphere Propagation

Signals radiate upward into the Signals radiate upward into the ionosphereionosphere

Reflected back to earthReflected back to earth The density difference between the The density difference between the

troposphere and the ionosphere cause troposphere and the ionosphere cause each radio wave to speed up and each radio wave to speed up and change direction, bending back to change direction, bending back to earthearth

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Line-of-Sight PropagationLine-of-Sight Propagation

Very high frequency signals are Very high frequency signals are transmitted in straight lines directly transmitted in straight lines directly from antenna to antennafrom antenna to antenna

Antennas facing each other, tall Antennas facing each other, tall enough enough

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Space PropagationSpace Propagation

Use satellite relays instead of antennaUse satellite relays instead of antenna Signals are received by satelliteSignals are received by satellite Satellite rebroadcasts the signal back Satellite rebroadcasts the signal back

to earthto earth

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Radio Frequency RangesRadio Frequency Ranges

Very Low Frequency (VLF) wavesVery Low Frequency (VLF) waves– Propagated as surface wave, through airPropagated as surface wave, through air– Do not suffer much attenuation in Do not suffer much attenuation in

transmissiontransmission– Susceptible to the high levels of Susceptible to the high levels of

atmosphere noise (heat and electricity) atmosphere noise (heat and electricity) – Used mostly for long-range radio Used mostly for long-range radio

navigationnavigation– Frequency range : 3-30 KHzFrequency range : 3-30 KHz

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Radio Frequency RangesRadio Frequency Ranges

Low Frequency (LF) wavesLow Frequency (LF) waves– Propagated as surface wavesPropagated as surface waves– Used for long-range radio navigationUsed for long-range radio navigation– Attenuation is greater during the Attenuation is greater during the daytimedaytime

(absorption of waves by natural (absorption of waves by natural obstacles increases)obstacles increases)

– Frequency range : 30-300 KHzFrequency range : 30-300 KHz

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Radio Frequency RangesRadio Frequency Ranges

Middle Frequency (MF) wavesMiddle Frequency (MF) waves

– Propagated in the tropospherePropagated in the troposphere– These frequencies are absorbed by the ionosphere These frequencies are absorbed by the ionosphere – Absorption increases during the daytimeAbsorption increases during the daytime– Most of MF transmission rely on line-of-sight Most of MF transmission rely on line-of-sight

antennas to increase control and avoid absorption antennas to increase control and avoid absorption problemproblem

– Used in AM radio, maritime radioUsed in AM radio, maritime radio– Frequency range : 300 KHz – 3 MHzFrequency range : 300 KHz – 3 MHz– (AM Radio : 535 KHz – 1605 MHz)(AM Radio : 535 KHz – 1605 MHz)

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Radio Frequency RangesRadio Frequency Ranges

High Frequency (HF) wavesHigh Frequency (HF) waves– Use ionosphere propagationUse ionosphere propagation– Reflected back to earthReflected back to earth– Used inUsed in

amateur radio (ham radio), amateur radio (ham radio), international broadcasting, international broadcasting, military communication, military communication, LLong-distance aircraftong-distance aircraft Ship communicationShip communication Telephone, telegraphTelephone, telegraph

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Radio Frequency RangesRadio Frequency Ranges

Others Others – VHF (Very High Frequency)VHF (Very High Frequency)– UHF (Ultra High Frequency)UHF (Ultra High Frequency)– SHF (Super High Frequency)SHF (Super High Frequency)– EHF (Extremely High Frequency)EHF (Extremely High Frequency)

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Terrestrial MicrowaveTerrestrial Microwave

Do not follow the curvature of the earthDo not follow the curvature of the earth Require line-of-sight signal transmission and Require line-of-sight signal transmission and

rereception equipmentception equipment Distance depends of the height of the Distance depends of the height of the

antennasantennas– The taller antenna, the longer distanceThe taller antenna, the longer distance

Microwave signals propagate in one direction Microwave signals propagate in one direction at a timeat a time– Two frequencies are required for two-way Two frequencies are required for two-way

communicationcommunication

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RepeatersRepeaters

Repeaters are used Repeaters are used to increase the to increase the distance by distance by terrestrial terrestrial microwavemicrowave

Signal received by Signal received by one antenna is one antenna is relayed to the next relayed to the next antennaantenna

Used in telephone Used in telephone system worldwidesystem worldwide

1818

AntennasAntennas

Two typesTwo types– Parabolic dishParabolic dish– HornHorn

Parabolic dish Parabolic dish antennaantenna

Horn antennaHorn antenna

1919

Satellite CommunicationSatellite Communication

Similar to line-of-Similar to line-of-sight microwave sight microwave transmissiontransmission

Satellites acting as Satellites acting as super tall antennas super tall antennas and repeatersand repeaters

2020

Cellular TelephonyCellular Telephony

Designed to provide stable Designed to provide stable communications connections between communications connections between tow moving devices,tow moving devices,

Or between one mobile unit and one Or between one mobile unit and one stationary (land) unitstationary (land) unit

A service provider must be able to A service provider must be able to – track a callertrack a caller– Assign a channel to the callAssign a channel to the call– Transfer the signal from channel to channel Transfer the signal from channel to channel

as the caller moves out the range of one as the caller moves out the range of one channel into the range of anotherchannel into the range of another

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Cellular TelephonyCellular Telephony

To make tracking To make tracking possiblepossible– Cellular service area Cellular service area

is divided into small is divided into small regional called cellsregional called cells

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Transmission ImpairmentTransmission Impairment

noise

impairment

distortion

attenuation

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Transmission ImpairmentTransmission Impairment

Attenuation Attenuation – Loss of energyLoss of energy

Some of the energy is converted to heatSome of the energy is converted to heat– Amplifiers are used to amplify the signalAmplifiers are used to amplify the signal

DistortionDistortion– Signal changes its form or shapeSignal changes its form or shape

Frequencies changeFrequencies change NoiseNoise

– Thermal noiseThermal noise– Induced noiseInduced noise– Crosstalk, etc.Crosstalk, etc.

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PerformancePerformance

Three concepts to measure performanceThree concepts to measure performance– ThroughputThroughput– Propagation speedPropagation speed– Propagation timePropagation time

ThroughputThroughput– How fast data can pass through a pointHow fast data can pass through a point

Propagation speedPropagation speed– The distance a signal or bit can travel through a The distance a signal or bit can travel through a

medium in one secondmedium in one second– Depends on the medium and the frequency of the Depends on the medium and the frequency of the

signalsignal

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PerformancePerformance

Propagation timePropagation time– Measure the time required for a signal to Measure the time required for a signal to

travel from one point of the transmission travel from one point of the transmission medium to anothermedium to another

– Calculated by dividing the distance by the Calculated by dividing the distance by the propagation speedpropagation speed

Propagation time = distance/propagation Propagation time = distance/propagation speedspeed