More on Modulation

Module B

Panko and PankoBusiness Data Networks and Security, 9th Edition© 2013 Pearson Education, Inc. Publishing as Prentice Hall

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

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Modulation

PSTNClient A

Server A Telephone

Telephone33.6kbps

Modem

Binary Data Analog ModulatedSignal

Modem

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There is a carrier wave. This carrier wave is modulated (changed) to

carry information.

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Carrier Wave

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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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Modulation

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Amplitude is the intensity of the signal.◦ Loud or soft

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B.1: Amplitude Modulation (AM)

Amplitude(power)

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B.1: Amplitude Modulation (AM)

LowAmplitude

(0)

HighAmplitude

(1)

AmplitudeModulation

(1011)

Amplitude (low)

Amplitude (high)

1

0 1

1

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Frequency of a wave◦ The number of complete cycles per second◦ Called Hertz◦ kHz, MHz, GHz, THz

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Waves

Frequency (Hz)

Cycles in One Second

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B.2: Frequency Modulation (FM)

LowFrequency

(0)

HighFrequency

(1)

FrequencyModulation

(1011)

Wavelength

Wavelength

1

0

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.

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Phase

BasicSignal

180 degreesout of phase

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B.3: Phase Modulation (PM)

In Phase(0)

180 degreesout of phase

(1)

FrequencyModulation

(1011) 1

0 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 Modulation (PM)

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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

Phase Shift Keying

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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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B.4: QAM

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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).

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B.4: QAM

Sine Wave Amplitude

Cosine(Quadrature)

WaveAmplitude

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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.

QAM

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© 2009 Pearson Education, Inc.  Publishing as Prentice Hall© 2013 Pearson Education, Inc. Publishing as Prentice Hall