Transcript module_30

Module 3.0: Data Transmission
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Terminology and fundamental concepts
Harmonics
Bandwidth
Transmission impairments
Channel Capacity and Nyquist’s Theorem
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Terminology
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Medium
– Guided medium
 e.g. twisted pair, optical fiber
– Unguided medium
 e.g. air, water, vacuum
Simplex
– One direction
 e.g. Television
Half duplex
– Either direction, but only one way at a time
 e.g. police radio
Full duplex
– Both directions at the same time
 e.g. telephone
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Fundamental Concepts
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Time domain concepts
– Continuous signal (Analog)
 Various in a smooth
way over time
 e.g. sound, video
– Discrete signal (Digital)
 Maintains a constant
level then changes to
another constant
level
 e.g., text, integers
– Periodic signal
 Pattern repeated
over time
– Aperiodic signal
 Pattern not repeated
over time
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Sine Wave (Harmonic) Components
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Peak Amplitude (A)
– maximum strength of
signal
– volts
Frequency (f)
– Rate of change of signal
– Hertz (Hz) or cycles per
second
– Period = time for one
repetition (T)
– T = 1/f
Phase ()
– Relative position in time
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Wavelength
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Length of wave or Distance occupied by one cycle
Meaning the distance a simple signal can travel in one period
Distance between two points of corresponding phase in two
consecutive cycles

Assuming signal velocity v
–  = vT
– f = v
– c = 3*108 m/s (speed of light in free space) or  = 3*108
meters.
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Bandwidth
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Bandwidth (in analog communications)
– The total capacity of communications channel measured in
hertz(Hz).
– It is the difference between the highest and lowest
frequencies capable of being carried over a channel.
– Any transmission system has a limited band of frequencies
– This limits the data rate that can be carried
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Bandwidth (in digital communications and networking)
– The theoretical capacity of communications channel expressed
in bits per second (bps), which is called data rate.
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Propagation Delay
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Propagation time measures the time required for a signal (or a
bit) to travel from one point of the transmission medium to
anoterh.
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Analog and Digital Data Transmission
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Signals
– Electric or electromagnetic or light representations of data.
Means of propagating Data.
Analog Signal
 Represent data with continuously varying sine wave.
 Continuously variable. Various media, e.g. wire, fiber optic,
space
 Speech bandwidth 100Hz to 7kHz, Telephone bandwidth
300Hz to 3400Hz, Video bandwidth 4MHz
Digital Signal
 Represent data with sequence of voltage pulses
 Use two DC components and vary it over time. Component
of zero frequency.
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Digital Signal
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In RS-232, voltage that varies between -15 and -5 volts is
interpreted as a binary 0, and voltage that varies between +5 and +15
volts is interpreted as a binary 1. Voltage that varies between -5 and
+5 volts is ignored; that is, no interpretation is made.
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Analog Signal
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A single frequency
with fixed A and  has
0 bandwidth. It
carries no information.
However, when it is
combined with other
frequencies or its
components become
variable, the
bandwidth gets
changed.
Amplitude Change
Frequency Change
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Analog Signal
Phase Change
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Modem Modulation Techniques
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Usually use digital
signals for digital data
and analog signals for
analog data
Can use analog signal
to carry digital data
– Modem
Digital signals have a
wide spectrum and
thus are subject to
strong attenuation
and delay distortion.
DC is good for short
distances.
Analog signals are
used instead with
different modulation
of components.
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Analog vs. Digital Transmission
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Transmission
– Communication of data by propagation and processing of signals
Analog Transmission:
– Analog signal transmitted without regard to content
– May be analog or digital data
– Attenuated over distance
– Use amplifiers to boost signal
– Also amplifies noise
Digital Transmission:
– Concerned with content
– Integrity endangered by noise, attenuation etc.
– Repeaters used
– Repeater
 receives signal
 extracts bit pattern
 retransmits
– Attenuation is overcome
– Noise is not amplified
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Transmission Impairments
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Signal received may differ from signal transmitted
Analog - degradation of signal quality
Digital - bit errors
Caused by
– Attenuation and attenuation distortion
– Delay distortion
– Noise
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Attenuation
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Attenuation:
– Signal strength falls off with distance
– Depends on medium
– Received signal strength:
 must be enough to be detected
 must be sufficiently higher than noise to be received without
error
– Attenuation is an increasing function of frequency
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Delay Distortion
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Delay Distortion:
– Only in guided media
– Propagation velocity varies with frequency
– Called also “Intersymbol Interference”. Due to delay
distortions, some of the signal components of one bit
position will spill over into other bit positions.
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Noise
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Additional signals inserted between transmitter and receiver
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Crosstalk
– A signal from one line is picked up by another
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Thermal
– Due to thermal agitation of electrons
– Uniformly distributed
– White noise
Impulse
– Irregular pulses or spikes
 Short duration
 High amplitude
– e.g. External electromagnetic interference such as lightning
or cut in cable or fault in communication equipment.
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Channel Capacity
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Data rate = Capacity
– In bits per second
– Rate at which data can be communicated
– Nyquist’s Theorem
 In 1920, Henry Nyquist discovered that sampling rate must be at
least twice the highest frequency, in order to construct the original
signal.
 Called also Sampling Rate theorem.
 See nice animation at
http://www.cs.brown.edu/exploratories/freeSoftware/repository/edu/brown/
cs/exploratories/applets/nyquist/nyquist_limit_java_browser.html
 Also, maximum data rate (how much data you can represent in one
signal) of any signal of frequency w is
 MDR (noiseless) = 2 w log2 M
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M is the values used per signaling state
– Shannon’s formula (only thermal noise)
 MDR = w log2 (1 + SNR)
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