1. More on Modulation Module B
Panko’s Business Data Networks and Telecommunications, 6th edition Copyright 2007 Prentice-Hall May only be used by adopters of the book

2. Modulation
Modulation converts an 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

3. Modulation There are several forms of modulation
Amplitude modulation (in Chapter 3)
Frequency modulation
Phase modulation
Quadrature amplitude modulation (QAM), which combines amplitude and phase modulation

4. Amplitude Modulation (AM)
Amplitude is the intensity of the signal
Loud or soft
Amplitude
(power)

5. Amplitude Modulation (AM) (from Chapter 3)
Amplitude (low)
Low
Amplitude
(0)
Amplitude (high)
High
Amplitude
(1)
Amplitude
Modulation
(1011)

6. Waves Frequency of a wave The number of complete cycles per second
Called Hertz
kHz, MHz, GHz, THz
Frequency (Hz)
Cycles in One Second

7. Figure B-1: Frequency Modulation (FM)
Wavelength
Low
Frequency
(0)
Wavelength
High
Frequency
(1) 1
Frequency
Modulation
(1011) 1 1

8. 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

9. Figure B-2: Phase Modulation (PM)
In Phase
(0)
180 degrees
out of phase
(1)
Frequency
Modulation
(1011)

10. Phase Modulation (PM) Human hearing is largely insensitive to phase
So harder to understand than FM and AM
But equipment is very sensitive to phase changes
PM is used in all recent forms of modulation for telephone modems

11. Figure B-3: QAM Quadrature Amplitude Modulation (QAM)
Uses two carrier waves: sine and cosine (out of phase), both amplitude-modulated

12. 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
High/High
1111
Cosine
(Quadrature)
Wave