1. More on Modulation Module B Panko and Panko
Business Data Networks and Security, 9th Edition
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2. Modulation
Modulation converts a digital computer signal into a form that can travel down an ordinary analog telephone line.
Analog Modulated
Signal
Binary Data
Modem
Telephone
33.6
kbps
Modem
Telephone
Server A
Client A
PSTN
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3. Carrier Wave There is a carrier wave.
This carrier wave is modulated (changed) to carry information.
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4. Modulation 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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5. B.1: Amplitude Modulation (AM)
Amplitude is the intensity of the signal.
Loud or soft
Amplitude
(power)
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6. B.1: Amplitude Modulation (AM)
Amplitude (low)
Low
Amplitude
(0)
Amplitude (high)
High
Amplitude
(1) 1
Amplitude
Modulation
(1011) 1 1
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7. Waves Frequency of a wave Frequency (Hz) Cycles in One Second
The number of complete cycles per second
Called Hertz
kHz, MHz, GHz, THz
Frequency (Hz)
Cycles in One Second
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8. B.2: Frequency Modulation (FM)
Wavelength
Low
Frequency
(0)
Wavelength
High
Frequency
(1) 1
Frequency
Modulation
(1011) 1 1
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9. Phase
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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10. B.3: Phase Modulation (PM)
In Phase
(0)
180 degrees
out of phase
(1) 1
Frequency
Modulation
(1011) 1 1
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11. Phase Modulation (PM) 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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12. Phase Shift Keying Phase shift keying Binary phase shift keying (BPSK)
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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13. B.4: QAM Quadrature Amplitude Modulation (QAM)
Uses two carrier waves: sine and cosine (90o out of phase), both amplitude-modulated
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14. B.4: QAM 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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15. QAM QAM Alternatives Trade-off 16 QAM 32 QAM 64 QAM 128 QAM 256 QAM
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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16. © 2013 Pearson Education, Inc. Publishing as Prentice Hall