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
Modulation

Modulation converts an digital computer signal
into a form that can travel down an ordinary
analog telephone line
Binary Data Analog Modulated
Signal
Client A
33.6 Telephone
kbps
Modem
Telephone
Modem
Server A
PSTN
2
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
3
Amplitude Modulation (AM)

Amplitude is the intensity of the signal

Loud or soft
Amplitude
(power)
4
Amplitude Modulation (AM) (from Chapter 3)
Low
Amplitude
(0)
Amplitude (low)
Amplitude (high)
High
Amplitude
(1)
Amplitude
Modulation
(1011)
5
Waves

Frequency of a wave

The number of complete cycles per second

Called Hertz

kHz, MHz, GHz, THz
Frequency (Hz)
Cycles in One Second
6
Figure B-1: Frequency Modulation (FM)
Low
Frequency
(0)
Wavelength
Wavelength
High
Frequency
(1)
0
Frequency
Modulation
(1011)
1
1
1
7
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
8
Figure B-2: Phase Modulation (PM)
In Phase
(0)
180 degrees
out of phase
(1)
Frequency
Modulation
(1011)
9
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
10
Figure B-3: QAM

Quadrature Amplitude Modulation (QAM)

Uses two carrier waves: sine and cosine (out of
phase), both amplitude-modulated
11
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/High
1111
12