data and computer communications
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Data and Computer Communications. Chapter 3 – Data Transmission . Ninth Edition by William Stallings. Data Transmission. What we've got here is failure to communicate. Paul Newman in Cool Hand Luke. Data Transmission. The successful transmission of data depends on two factors:. - PowerPoint PPT PresentationTRANSCRIPT
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Data and Computer Communications
Ninth Editionby William Stallings
Chapter 3 – Data Transmission
Data and Computer Communications, Ninth Edition by William Stallings, (c) Pearson
Education - Prentice Hall, 2011
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Data Transmission
What we've got here is failure to communicate.
Paul Newman in Cool Hand Luke
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Data Transmission
The successful transmission of data depends on two factors: 1) quality of the signal
being transmitted (garbage in, garbage out)
2) characteristics of the transmission medium
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Transmission Terminology
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Transmission Terminology
Data transmission occurs between transmitter and receiver over some transmission medium.
Communication is in the form of electromagneti
c waves.
Guided media twisted pair,
coaxial cable, optical
fiber
Unguided media
(wireless)
air, vacuum, seawater
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Transmission Terminology
• no intermediate devices
Direct link
• direct link • only 2 devices share link
Point-to-point
• more than two devices share the link
Multi-point
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Transmission Terminology Simplex
signals transmitted in one direction• eg. Television
Half duplex both stations transmit, but only one at a time
• eg. police radio
Full duplex simultaneous transmissions
• eg. telephone
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Frequency, Spectrum and Bandwidth
analog signal• signal intensity varies smoothly with no breaks
digital signal• signal intensity maintains a constant level and
then abruptly changes to another level periodic signal
• signal pattern repeats over time aperiodic signal
• pattern not repeated over time
Time Domain Concepts
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Analog and Digital Signals
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PeriodicSignals
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Sine Wave
peak amplitude (“A” in the previous slide) maximum strength of signal typically measured in volts
frequency (f) rate at which the signal repeats Hertz (Hz) or cycles per second period (T) is the amount of time for one repetition T = 1/f
phase () relative position in time within a single period of signal
(periodic continuous signal)
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Varying Sine Wavess(t) = A sin(2ft +)
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Wavelength ()the wavelength of
a signal is the distance occupied by a single cycle
can also be stated as the distance between two
points of corresponding phase of two consecutive
cycles
assuming signal velocity v, then the
wavelength is related to the period as = vT
or equivalently
f = v
especially when v=c• c = 3*108 ms-1 (speed of light
in free space)• For copper: v=0.55c to 0.77c
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Frequency Domain Concepts signals are made up of many frequencies components are sine waves Fourier analysis can show that any signal
is made up of components at various frequencies, in which each component is a sinusoid
can plot frequency domain functions
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Addition of Frequency
Components(T=1/f)
(c) is the sum of f & 3f
“Internet Explorer” only: http://www.falstad.com/fourier/
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FrequencyDomain
Representations frequency domain
function of Fig 3.4c (previous slide)
frequency domain function of single square pulse
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Spectrum & Bandwidthspectrum• range of frequencies contained in signal (ex: 100 to 250MHz)
absolute bandwidth• width of spectrum (ex: 150MHz)
effective bandwidth• often just bandwidth (ex: 100MHz)• narrow band of frequencies containing most energy (Figure)
dc component• component of zero frequency
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Signal with dc Component
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Data Rate and Bandwidthany transmission
system has a limited band of
frequencies
this limits the data rate that can be carried on the transmission
medium
square waves have infinite
components and hence an infinite
bandwidth
most energy in first few
components
limiting bandwidth
creates distortions
If only two frequencies get through the transmission system, the wave form shown in the previous slide would be received.
There is a direct relationship between data rate and bandwidth.
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Analog and Digital Data Transmission
data entities that convey information
signals electric or electromagnetic representations of
data transmission
communication of data by propagation and processing of signals
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Acoustic Spectrum (Analog)-> définition du décibel
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Analog and Digital Transmission
(1080i vs 1080p : http://www.pcmag.com/article2/0,2817,2413044,00.asp)
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(Digital Data)
Examples:
Text
Character strings
IRA (International reference alphabet)
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Advantages & Disadvantages of Digital Signals
cheaperless susceptible to noise interference
suffer more from attenuationdigital now preferred choice
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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 to
300-3400Hz
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Video Signals
to produce a video signal a TV camera is used
USA standard is 483 lines per frame, at a rate of 30 complete frames per second
actual standard is 525 lines but 42 lost during vertical retrace
horizontal scanning frequency is 525 lines x 30 scans = 15750 lines per second (63.5 μs per line but 11 μs is lost during horizontal retrace -> 52.5 μs )
max frequency reached if line alternates black and white (450 columns)
max frequency of 4.2MHz (450/2 cycles in 52.5 μs)
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Video Signals
http://en.wikipedia.org/wiki/1080p
2.1 megapixels X frame rate
1 pixel needs 3 X 8 bits = 24 bits
24 fps => 2.1 X 24 X 24 Mbits/s 50 fps => 2.1 X 24 X 50 Mbits/s 60 fps => 2.1 X 24 X 60 Mbits/s
Compression techniques are often used.
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Analog Signals
Modulator/demodulator
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Digital Signals
Similar to modembut for voice signal
Coder/decoder
Transmitter/receiver
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(Analog and Digital
Transmission)
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Transmission Impairments signal received may differ from signal
transmitted causing: In analog systems -> degradation of signal quality In digital systems-> bit errors
most significant impairments are attenuation attenuation distortion (some freq. are more attenuated than others) delay distortion (velocity of waves depends on their frequencies) noise
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ATTENUATION
Received signal strength must be:•strong enough to be detected
•sufficiently higher than noise to be received without error
Strength can be increased using
amplifiers or repeaters.
Equalize attenuation
across the band of frequencies used by using loading coils or
amplifiers.
signal strength falls off with distance over any transmission medium
varies with frequency
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Attenuation Distortion
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Delay Distortion occurs because propagation velocity of a
signal 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
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Delay Distortion
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Noiseunwanted signals inserted between transmitter and receiver
=> is the major limiting factor in communications system performance
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Categories of NoiseThermal noise
• due to thermal agitation of electrons
• uniformly distributed across bandwidths
• referred to as white noiseIntermodulation noise
• produced by nonlinearities in the transmitter, receiver, and/or intervening transmission medium
• effect is to produce signals at a frequency that is the sum or difference of the two original frequencies (sosmat.com)
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Categories of NoiseCrosstalk:
a signal from one line is picked up by another
can occur by electrical coupling between nearby twisted pairs or when microwave antennas pick up unwanted signalsImpulse Noise:
caused by external electromagnetic interferences
noncontinuous, consisting of irregular pulses or spikes
short duration and high amplitude
minor annoyance for analog signals but a major source of error in digital data
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Channel Capacity
Maximum rate at which data can be transmitted over a given communication channel under given conditions
Channel characteristics :
data rate
in bits per second
bandwidth
in cycles per
second or Hertz
noise
average noise level over path
error rate
rate of corrupted
bits
limitations due to
physical properties
main constraint
on achieving efficiency is noise
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Nyquist BandwidthIn the case of a channel that is noise free: if rate of signal transmission is 2B then can carry
signal with frequencies no greater than B given bandwidth B, highest signal rate is 2B
for binary signals, 2B bps needs bandwidth B Hz can increase rate by using M signal levels Nyquist Formula is: C = 2B log2M data rate can be increased by increasing M
however this increases burden on receiver noise & other impairments limit the value of M
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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 the signal-to-noise ratio -> SNR= (signal power) / (noise power)
capacity C = B log2(1+SNR) (ex. SNR=0 -> C=0)
theoretical maximum capacity get much lower rates in practice
SNRdb=10 log10 (SNR)
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Summary transmission concepts and terminology
guided/unguided media frequency, spectrum and bandwidth analog vs. digital signals data rate and bandwidth relationship transmission impairments
attenuation/delay distortion/noise channel capacity
Nyquist/Shannon