1 Transmission Media Unguided Media. 2 Unguided Media wireless communication wireless communication...
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Transcript of 1 Transmission Media Unguided Media. 2 Unguided Media wireless communication wireless communication...
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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
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AntennasAntennas
Two typesTwo types– Parabolic dishParabolic dish– HornHorn
Parabolic dish Parabolic dish antennaantenna
Horn antennaHorn antenna
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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
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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