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William Stallings
Data and ComputerCommunications
Chapter 3Data Transmission
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Terminology (1)
TransmitterReceiverMedium
Guided mediume.g. twisted pair, optical fiber
Unguided mediume.g. air, water, vacuum
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Terminology (2)
Direct linkNo intermediate devices
Point-to-point
Direct linkOnly 2 devices share link
Multi-pointMore than two devices share the link
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Terminology (3)
SimplexOne direction
e.g. Television
Half duplexEither direction, but only one way at a time
e.g. police radio
Full duplex
Both directions at the same timee.g. telephone
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Frequency, Spectrum andBandwidth
Time domain conceptsContinuous signal
Various in a smooth way over time
Discrete signalMaintains a constant level then changes to another constantlevel
Periodic signalPattern repeated over time
Aperiodic signalPattern not repeated over time
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PeriodicSignals
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Sine Wave
Peak Amplitude (A)maximum strength of signalvolts
Frequency (f)Rate of change of signalHertz (Hz) or cycles per secondPeriod = time for one repetition (T)
T = 1/fPhase ( )
Relative position in time
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Varying Sine Waves
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Wavelength
Distance occupied by one cycleDistance between two points of correspondingphase in two consecutive cycles
Assuming signal velocity v
= vT
f = v
c = 3*10 8 ms -1 (speed of light in free space)
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Frequency Domain Concepts
Signal usually made up of many frequenciesComponents are sine wavesCan be shown (Fourier analysis) that any signal
is made up of component sine wavesCan plot frequency domain functions
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Addition ofFrequency
Components
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FrequencyDomain
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Spectrum & Bandwidth
Spectrumrange of frequencies contained in signal
Absolute bandwidth
width of spectrumEffective bandwidthOften just bandwidth Narrow band of frequencies containing most of theenergy
DC ComponentComponent of zero frequency
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Signal with DC Component
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Data Rate and Bandwidth
Any transmission system has a limited band offrequenciesThis limits the data rate that can be carried
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Analog and Digital DataTransmission
DataEntities that convey meaning
SignalsElectric or electromagnetic representations of data
TransmissionCommunication of data by propagation andprocessing of signals
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Data
AnalogContinuous values within some intervale.g. sound, video
DigitalDiscrete valuese.g. text, integers
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Acoustic Spectrum (Analog)
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Signals
Means by which data are propagated Analog
Continuously variable Various media
wire, fiber optic, space
Speech bandwidth 100Hz to 7kHzTelephone bandwidth 300Hz to 3400Hz
Video bandwidth 4MHzDigital
Use two DC components
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Data and Signals
Usually use digital signals for digital data andanalog signals for analog dataCan use analog signal to carry digital data
Modem
Can use digital signal to carry analog dataCompact Disc audio
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Analog Signals Carrying Analogand Digital Data
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Digital Signals Carrying Analogand Digital Data
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Analog Transmission
Analog signal transmitted without regard tocontentMay be analog or digital data
Attenuated over distanceUse amplifiers to boost signal Also amplifies noise
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Digital Transmission
Concerned with contentIntegrity endangered by noise, attenuation etc.Repeaters used
Repeater receives signalExtracts bit patternRetransmits
Attenuation is overcomeNoise is not amplified
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Advantages of DigitalTransmission
Digital technologyLow cost LSI/VLSI technologyData integrity
Longer distances over lower quality lines
Capacity utilizationHigh bandwidth links economicalHigh degree of multiplexing easier with digitaltechniques
Security & PrivacyEncryption
IntegrationCan treat analog and digital data similarly
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Transmission Impairments
Signal received may differ from signaltransmitted
Analog - degradation of signal quality
Digital - bit errorsCaused by Attenuation and attenuation distortionDelay distortionNoise
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Attenuation
Signal strength falls off with distanceDepends on mediumReceived signal strength:
must be enough to be detectedmust be sufficiently higher than noise to be receivedwithout error
Attenuation is an increasing function of
frequency
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Delay Distortion
Only in guided mediaPropagation velocity varies with frequency
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Noise (1)
Additional signals inserted between transmitterand receiverThermal
Due to thermal agitation of electronsUniformly distributedWhite noise
Intermodulation
Signals that are the sum and difference of originalfrequencies sharing a medium
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Noise (2)
Crosstalk A signal from one line is picked up by another
ImpulseIrregular pulses or spikese.g. External electromagnetic interferenceShort durationHigh amplitude
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Channel Capacity
Data rateIn bits per secondRate at which data can be communicated
BandwidthIn cycles per second of HertzConstrained by transmitter and medium
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