KM2008 Predavanje3 Ang

8/11/2019 KM2008 Predavanje3 Ang

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Data Transmission


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Transmission Terminologydata transmission occurs between atransmitter & receiver via some mediumguided medium

eg. twisted pair, coaxial cable, optical fiber

unguided / wireless mediumeg. air, water, vacuum


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Transmission Terminologydirect link

no intermediate devices

point-to-pointdirect linkonly 2 devices share link

multi-pointmore than two devices share the link


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Transmission Terminologysimplex

one direction eg. television

half duplexeither direction, but only one way at a time

eg. police radio

full duplexboth directions at the same time

eg. telephone


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Frequency, Spectrum and Bandwidth

time domain conceptsanalog signal

various in a smooth way over timedigital signal

maintains a constant level then changes to anotherconstant level

periodic signal pattern repeated over time

aperiodic signal pattern not repeated over time


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Analogue & Digital Signals


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PeriodicSignals


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Addition of

FrequencyComponents(T=1/f)

c is sum of f & 3f


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Spectrum & Bandwidthspectrum

range of frequencies contained in signal

absolute bandwidthwidth of spectrum

effective bandwidthoften just bandwidth narrow band of frequencies containing mostenergy

DC Componentcomponent of zero frequency


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Data Rate and Bandwidthany transmission system has a limited band offrequenciesthis limits the data rate that can be carriedsquare have infinite components and hencebandwidthbut most energy in first few components

limited bandwidth increases distortionhave a direct relationship between data rate &bandwidth


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Analog and Digital DataTransmission

dataentities that convey meaning

signals & signallingelectric or electromagnetic representations ofdata, physically propagates along medium

transmissioncommunication of data by propagation andprocessing of signals


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Digital Dataas generated by computers etc.has two dc components

bandwidth depends on data rate


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Analog Signals


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Digital Signals


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Channel Capacitymax possible data rate on communicationchannelis a function of

data rate - in bits per secondbandwidth - in cycles per second or Hertznoise - on comms link

error rate - of corrupted bitslimitations due to physical propertieswant most efficient use of capacity


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OverviewGuided - wireUnguided - wireless

Characteristics and quality determinedby medium and signalFor guided, the medium is moreimportantFor unguided, the bandwidth producedby the antenna is more important

Key concerns are data rate and distance


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Design FactorsBandwidthHigher bandwidth gives higher data rate

Transmission impairments Attenuation

Interference

Number of receiversIn guided mediaMore receivers (multi-point) introduce

more attenuation


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Guided Transmission MediaTwisted Pair

Coaxial cableOptical fiber


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Twisted Pair


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Twisted Pair - ApplicationsMost common medium

Telephone networkBetween house and local exchange(subscriber loop)

Within buildingsTo private branch exchange (PBX)

For local area networks (LAN)10Mbps or 100Mbps


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Twisted Pair - Pros and ConsCheap

Easy to work withLow data rateShort range


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Twisted Pair - Transmission Characteristics Analog

Amplifiers every 5km to 6km

DigitalUse either analog or digital signalsrepeater every 2km or 3km

Limited distanceLimited bandwidth (1MHz)Limited data rate (100MHz)Susceptible to interference and noise


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Unshielded and Shielded TPUnshielded Twisted Pair (UTP)

Ordinary telephone wire

CheapestEasiest to installSuffers from external EM interference

Shielded Twisted Pair (STP)Metal braid or sheathing that reduces interferenceMore expensiveHarder to handle (thick, heavy)


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Coaxial Cable


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Coaxial Cable ApplicationsMost versatile mediumTelevision distribution

Ariel to TVCable TV

Long distance telephone transmission

Can carry 10,000 voice calls simultaneouslyBeing replaced by fiber optic

Short distance computer systems links

Local area networks


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Coaxial Cable - TransmissionCharacteristics

Analog Amplifiers every few kmCloser if higher frequencyUp to 500MHz

DigitalRepeater every 1kmCloser for higher data rates


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Optical Fiber


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Optical Fiber - BenefitsGreater capacity

Data rates of hundreds of Gbps

Smaller size & weightLower attenuation

Electromagnetic isolationGreater repeater spacing

10s of km at least


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Optical Fiber - ApplicationsLong-haul trunks

Metropolitan trunksRural exchange trunksSubscriber loops

LANs


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Optical Fiber - Transmission Characteristics Act as wave guide for 10 14 to 10 15 HzPortions of infrared and visible spectrum

Light Emitting Diode (LED)CheaperWider operating temp rangeLast longer

Injection Laser Diode (ILD)More efficientGreater data rate

Wavelength Division Multiplexing


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Optical Fiber Transmission Modes


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Wireless TransmissionUnguided mediaTransmission and reception via antennaDirectional

Focused beam

Careful alignment requiredOmnidirectionalSignal spreads in all directions

Can be received by many antennae


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Frequencies2GHz to 40GHzMicrowaveHighly directionalPoint to pointSatellite

30MHz to 1GHz

OmnidirectionalBroadcast radio

3 x 10 11 to 2 x 10 14 InfraredLocal


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Terrestrial MicrowaveParabolic dishFocused beamLine of sightLong haul telecommunications

Higher frequencies give higher datarates


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Satellite MicrowaveSatellite is relay stationSatellite receives on one frequency, amplifiesor repeats signal and transmits on anotherfrequencyRequires geo-stationary orbit

Height of 35,784kmTelevisionLong distance telephonePrivate business networks


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Broadcast RadioOmnidirectionalFM radioUHF and VHF televisionLine of sight

Suffers from multipath interferenceReflections


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InfraredModulate noncoherent infrared lightLine of sight (or reflection)Blocked by wallse.g. TV remote control, IRD port

KM2008 Predavanje3 Ang

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