Module B
Panko and Panko
Business Data Networks and Security, 9th Edition
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
Modulation converts a digital computer signal
into a form that can travel down an ordinary
analog telephone line.
Binary Data Analog Modulated
Signal
Client A
33.6 Telephone
kbps
Modem
Telephone
Modem
Server A
PSTN
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

There is a carrier wave.
This carrier wave is modulated (changed) to
carry information.
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There are several forms of modulation.
◦ Amplitude modulation
◦ Frequency modulation
◦ Phase modulation
◦ Quadrature amplitude modulation (QAM), which
combines amplitude and phase modulation
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Amplitude is the intensity of the signal.
◦ Loud or soft
Amplitude
(power)
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Low
Amplitude
(0)
Amplitude (low)
Amplitude (high)
High
Amplitude
(1)
1
0
Amplitude
Modulation
(1011)
1
1
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Frequency of a wave
◦ The number of complete cycles per second
◦ Called Hertz
◦ kHz, MHz, GHz, THz
Frequency (Hz)
Cycles in One Second
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Low
Frequency
(0)
Wavelength
Wavelength
High
Frequency
(1)
0
Frequency
Modulation
(1011)
1
1
1
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
Two signals can have the same frequency and
amplitude but have different phases—be at
different points in their cycles at a given
moment.
Basic
Signal
180 degrees
out of phase
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In Phase
(0)
180 degrees
out of phase
(1)
Frequency
Modulation
(1011)
0
1
1
1
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
Human hearing is largely insensitive to phase.
◦ So, harder to understand than AM or FM

But equipment is very sensitive to phase
changes.
◦ PM is used in all recent forms of modulation for
telephone modems and all forms of radio
transmission.
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Phase shift keying
◦ Another term for phase modulation

Binary phase shift keying (BPSK)
◦ Phase modulation with two phases

Quadrature phase shift keying (QPSK)
◦ Phase modulation with four phases
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Quadrature Amplitude Modulation (QAM)
◦ Uses two carrier waves: sine and cosine (90o out
of phase), both amplitude-modulated
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
Suppose each carrier wave has four
possible amplitude levels
◦ In each clock cycle, there are 16 combined
possibilities.
◦ In each clock cycle, can send 4 bits (24 = 16).
Sine Wave Amplitude
Cosine
(Quadrature)
Wave
Amplitude
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
QAM Alternatives
◦
◦
◦
◦
◦

16 QAM
32 QAM
64 QAM
128 QAM
256 QAM
Trade-off
◦ More states allow more bits to be sent per clock
cycle although with diminishing returns.
◦ But more states means that states are closer
together, so digital error resistance is reduced.
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