Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude...

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Computer Communication & Networks Lecture # 16

Transcript of Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude...

Page 1: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

Computer Communication & Networks

Lecture # 16

Page 2: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

Today’s Menu

Ϟ Modulation/Demodulation

Ϟ Amplitude Modulation

Ϟ Frequency Modulation

Ϟ Phase Modulation

Ϟ Quadrature Amplitude Modulation

Page 3: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 4: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 5: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 6: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 7: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 8: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 9: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 10: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 11: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 12: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 13: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 14: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 15: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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

Page 16: Computer Communication & Networks Lecture # 16. Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature.

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