Computer Communication & Networks

Lecture # 16

Today’s Menu

Ϟ Modulation/Demodulation

Ϟ Amplitude Modulation

Ϟ Frequency Modulation

Ϟ Phase Modulation

Ϟ Quadrature Amplitude Modulation

Modulation

Ϟ Process of changing one of the characteristic of analog signal based on some digital input

Ϟ By changing one aspect of a simple electrical signal back and forth, we can use it to represent digital data

Ϟ Example: using internet over the telephone line

Modulation

There are three characteristics of a sine wave, so we can change

each of them to modulate a signal

Ϟ Amplitude Modulation

Ϟ Frequency Modulation

Ϟ Phase Modulation

We can also combine them to get another method knows as QAM

Quadrature Amplitude Modulation

Bit Rate VS Baud Rate

Bit rate

Number of bits transmitted during one second

Baud rate

Number of signal units/elements per second that are required to

represent that bit

Ϟ To measure the efficiency of computer we use bit rate

Ϟ To measure the efficiency of a data communication

system we use baud rate

Bit Rate VS Baud Rate

Lets use the transportation example we used to understand the

bandwidth concepts here to better understand bit/baud rate

Think of a baud as a car and bit as a passenger

Ϟ If 100 cars travel from one place to another carrying only one passenger (driver only), than 100 passenger are transported

Ϟ However if each car carry 4 passengers than 400 passengers are transported using 100 cars

Ϟ Note the number of cars (bauds) not the number of passengers (bits) determine the traffic and therefore need for wider highways

Bit Rate VS Baud Rate

An analog signal carries 4 bits in each signal element. If 1000 signal

elements are sent per second, find the baud rate and bit rate?

Baud rate = number of signal elements per second

Baud rate = 1000 bauds

Bit rate = baud rate * number of bits per signal element

Bit rate = 1000 * 4 = 4000 b/s

Amplitude Modulation

Ϟ Amplitude of the signal is varied to represent binary 1 or 0Ϟ Both frequency and phase remains constant, while the

amplitude changes

Ϟ Highly vulnerable to the noise interferenceϞ Noise usually affects the amplitude

Amplitude Modulation

1 bit (1) 1 bit (1) 1 bit (0) 1 bit (1) 1 bit (0)

1 baud 1 baud 1 baud 1 baud 1 baud

1 Second

Frequency Modulation

Ϟ Frequency of the signal is varied to represent binary 1 or 0Ϟ Both amplitude and phase remains constant, while the

frequency changes

Ϟ Avoids most of the noise problemsϞ We are looking for specific frequency changes over a given

number of periods, it can ignore voltage spikes

Frequency Modulation

1 bit (1) 1 bit (1) 1 bit (0) 1 bit (1) 1 bit (0)

1 baud 1 baud 1 baud 1 baud 1 baud

1 Second

Phase Modulation

Ϟ Phase of the signal is varied to represent binary 1 or 0Ϟ Both amplitude and frequency remains constant, while the

phase changes

Ϟ It is not susceptible to the noise degradationϞ Higher degree variations are not easily detected by the receiver

Phase Modulation

1 bit(1) 1 bit(1) 1 bit(0) 1 bit(1) 1 bit(0)

1 baud 1 baud 1 baud 1 baud 1 baud

1 Second

Phase Modulation

2 Phase Modulation

Ϟ There are multiple variations of phase modulation

Ϟ The version we just studied was 2 phase modulation

Ϟ The reason is that it as two phase shifts 0o and 180o

Ϟ We can see it in the constellation diagram

Constellation diagram

Bit Phase

0 0

1 180

Phase Modulation

4 Phase Modulation

Ϟ As noise does not affect this type of modulation, why not utilize it

more by adding more phases and representing more bits over

one phase

Constellation diagram

Baud rate = 4 (remember it is the number of signal elements)

Bit rate = 8 (remember it is the number of bit elements)

Bit Phase

00 0

01 90

10 180

11 270

Phase Modulation

8 Phase Modulation

Constellation diagram

Baud rate = 8

Bit rate = 24

Bit Phase

000 0

001 45

010 90

011 135

100 180

101 225

110 270

111 315