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Transcript
Module 3.0: Data Transmission
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Terminology and fundamental concepts
Harmonics
Bandwidth
Transmission impairments
Channel Capacity and Nyquist’s Theorem
K. Salah
1
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
K. Salah
2
Fundamental Concepts
•
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
K. Salah
3
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
K. Salah
4
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.
K. Salah
5
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
•
Bandwidth (in digital communications and networking)
– The theoretical capacity of communications channel expressed
in bits per second (bps), which is called data rate.
K. Salah
6
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.
K. Salah
7
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.
K. Salah
8
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.
K. Salah
9
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
K. Salah
10
Analog Signal
Phase Change
K. Salah
11
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.
K. Salah
12
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
K. Salah
13
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
K. Salah
14
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
K. Salah
15
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.
K. Salah
16
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.
K. Salah
17
Channel Capacity
•
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
»
M is the values used per signaling state
– Shannon’s formula (only thermal noise)
 MDR = w log2 (1 + SNR)
K. Salah
18