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More on Modulation

Module BUpdated January 2009

Raymond Panko’sBusiness Data Networks and Telecommunications, 7th edition

May only be used by adopters of the book

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Modulation

• Modulation converts an digital computer signal into a form that can travel down an ordinary analog telephone line

PSTNClient A

Server A Telephone

Telephone33.6kbps

Modem

Binary Data Analog ModulatedSignal

Modem

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Carrier Wave

• There is a carrier wave

• This carrier wave is modulated (changed) to carry information

B-3

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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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Amplitude Modulation (AM)

• Amplitude is the intensity of the signal

– Loud or soft

Amplitude(power)

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Amplitude Modulation (AM)

LowAmplitude

(0)

HighAmplitude

(1)

AmplitudeModulation

(1011)

Amplitude (low)

Amplitude (high)

1

0 1

1

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Waves

• 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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Figure B-1: Frequency Modulation (FM)

LowFrequency

(0)

HighFrequency

(1)

FrequencyModulation

(1011)

Wavelength

Wavelength

1

0

1

1

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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

BasicSignal

180 degreesout of phase

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Figure B-2: Phase Modulation (PM)

In Phase(0)

180 degreesout of phase

(1)

FrequencyModulation

(1011) 1

0 1

1

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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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Figure B-3: QAM

• Quadrature Amplitude Modulation (QAM)

– Uses two carrier waves: sine and cosine (90o out of phase), both amplitude-modulated

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Figure B.3: 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 (2^4=16)

Sine Wave

Cosine(Quadrature)

Wave

High/High1111

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