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8/7/2019 Ch. 3 Data Transmission Part 2
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Analog and Digital Data
Transmission data
entities that convey information
signals
electric or electromagnetic representations of
data
signaling
physically propagates along a medium
transmission
communication of data by propagation andprocessing of signals
Acoustic Spectrum (Analog)
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Digital Data
Examples:
Text
Character
strings
IRA
Advantages & Disadvantagesof Digital Signals
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Audio Signals
frequency range of typical speech is 100Hz -7kHz
easily converted into electromagnetic signals
varying volume converted to varying voltage
can limit frequency range for voice channel to300-3400Hz
Conversion of PC Input toDigital Signal
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Analog Signals
Digital Signals
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Analog and
Digital
Transmission
Transmission Impairments
signal received may differ from signaltransmitted causing:
analog - degradation of signal quality
digital - bit errors
most significant impairments are
attenuation and attenuation distortion
delay distortion
noise
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ATTENUATION
Received signalstrength must be:
strong enough to bedetected
sufficiently higher thannoise to be receivedwithout error
Strength can beincreased using
amplifiers or repeaters.
Equalizeattenuation across
the band of frequencies usedby using loading
coils or amplifiers.
signal strength falls off with distance over any
transmission medium
varies with frequency
Attenuation Distortion
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Delay Distortion
occurs because propagation velocity of asignal through a guided medium varies
with frequency
various frequency components arrive at
different times resulting in phase shifts
between the frequencies
particularly critical for digital data since
parts of one bit spill over into others
causing intersymbol interference
Noiseunwanted signalsinserted betweentransmitter andreceiver
is the major limitingfactor incommunicationssystem performance
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Categories of Noise
Intermodulation noise
produced by nonlinearities in thetransmitter, receiver, and/or intervening transmission medium
effect is to produce signals at afrequency that is the sum or difference of the two originalfrequencies
Categories of NoiseCrosstalk:
a signal from one line ispicked up by another
can occur by electricalcoupling between nearbytwisted pairs or whenmicrowave antennas pickup unwanted signals
Impulse Noise: caused by external
electromagnetic interferences
noncontinuous, consisting of irregular pulses or spikes
short duration and highamplitude
minor annoyance for analogsignals but a major source of error in digital data
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Channel Capacity
Maximum rate at which data can be transmitted over agiven communications channel under given conditions
data rate
in bits per second
bandwidth
in cyclesper
second or Hertz
noise
averagenoise levelover path
error rate
rate of corrupted
bits
limitationsdue to
physicalproperties
mainconstraint
onachievingefficiencyis noise
Nyquist Bandwidth
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Shannon Capacity Formula
considering the relation of data rate, noise and
error rate: faster data rate shortens each bit so bursts of noise
corrupts more bits
given noise level, higher rates mean higher errors
Shannon developed formula relating these to
signal to noise ratio (in decibels)
SNRdb=10 log10 (signal/noise)
capacityC = B log2(1+SNR)
theoretical maximumcapacity
get much lower rates in practice
Summary
t ans ission concepts and te inology
guided/unguided edia
f equency, spect u and bandwidth
analog vs. digital signals
data ate and bandwidth elationship
t ans ission i pai ents
attenuation/delay disto tion/noise channel capacity
Nyquist/Shannon
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