© 2009 Pearson Education, Inc. Publishing as Prentice Hall More on Modulation Module B Updated...
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Transcript of © 2009 Pearson Education, Inc. Publishing as Prentice Hall More on Modulation Module B Updated...
© 2009 Pearson Education, Inc. Publishing as Prentice Hall
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
© 2009 Pearson Education, Inc. Publishing as Prentice HallB-2
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
© 2009 Pearson Education, Inc. Publishing as Prentice Hall
Carrier Wave
• There is a carrier wave
• This carrier wave is modulated (changed) to carry information
B-3
© 2009 Pearson Education, Inc. Publishing as Prentice HallB-4
Modulation
• There are several forms of modulation
– Amplitude modulation
– Frequency modulation
– Phase modulation
– Quadrature amplitude modulation (QAM), which combines amplitude and phase modulation
© 2009 Pearson Education, Inc. Publishing as Prentice HallB-5
Amplitude Modulation (AM)
• Amplitude is the intensity of the signal
– Loud or soft
Amplitude(power)
© 2009 Pearson Education, Inc. Publishing as Prentice HallB-6
Amplitude Modulation (AM)
LowAmplitude
(0)
HighAmplitude
(1)
AmplitudeModulation
(1011)
Amplitude (low)
Amplitude (high)
1
0 1
1
© 2009 Pearson Education, Inc. Publishing as Prentice HallB-7
Waves
• Frequency of a wave
– The number of complete cycles per second
– Called Hertz
– kHz, MHz, GHz, THz
Frequency (Hz)
Cycles in One Second
© 2009 Pearson Education, Inc. Publishing as Prentice HallB-8
Figure B-1: Frequency Modulation (FM)
LowFrequency
(0)
HighFrequency
(1)
FrequencyModulation
(1011)
Wavelength
Wavelength
1
0
1
1
© 2009 Pearson Education, Inc. Publishing as Prentice HallB-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
BasicSignal
180 degreesout of phase
© 2009 Pearson Education, Inc. Publishing as Prentice HallB-10
Figure B-2: Phase Modulation (PM)
In Phase(0)
180 degreesout of phase
(1)
FrequencyModulation
(1011) 1
0 1
1
© 2009 Pearson Education, Inc. Publishing as Prentice HallB-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
© 2009 Pearson Education, Inc. Publishing as Prentice HallB-12
Figure B-3: QAM
• Quadrature Amplitude Modulation (QAM)
– Uses two carrier waves: sine and cosine (90o out of phase), both amplitude-modulated
© 2009 Pearson Education, Inc. Publishing as Prentice HallB-13
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
© 2009 Pearson Education, Inc. Publishing as Prentice Hall B-14
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Copyright © 2009 Pearson Education, Inc. Copyright © 2009 Pearson Education, Inc. Publishing as Prentice HallPublishing as Prentice Hall