Computer Communication & Networks Lecture # 16

Today’s Menu ϞModulation/Demodulation ϞAmplitude Modulation ϞFrequency Modulation ϞPhase Modulation ϞQuadrature Amplitude Modulation

Modulation ϞProcess of changing one of the characteristic of analog signal based on some digital input ϞBy changing one aspect of a simple electrical signal back and forth, we can use it to represent digital data ϞExample: 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 ϞAmplitude Modulation ϞFrequency Modulation ϞPhase 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 ϞTo measure the efficiency of computer we use bit rate ϞTo 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 ϞIf 100 cars travel from one place to another carrying only one passenger (driver only), than 100 passenger are transported ϞHowever if each car carry 4 passengers than 400 passengers are transported using 100 cars ϞNote 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 ϞAmplitude of the signal is varied to represent binary 1 or 0 ϞBoth frequency and phase remains constant, while the amplitude changes ϞHighly vulnerable to the noise interference ϞNoise 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 ϞFrequency of the signal is varied to represent binary 1 or 0 ϞBoth amplitude and phase remains constant, while the frequency changes ϞAvoids most of the noise problems ϞWe 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 ϞPhase of the signal is varied to represent binary 1 or 0 ϞBoth amplitude and frequency remains constant, while the phase changes ϞIt is not susceptible to the noise degradation ϞHigher 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 ϞThere are multiple variations of phase modulation ϞThe version we just studied was 2 phase modulation ϞThe reason is that it as two phase shifts 0o and 180o ϞWe can see it in the constellation diagram Constellation diagram BitPhase

Phase Modulation 4 Phase Modulation ϞAs 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) BitPhase

Phase Modulation 8 Phase Modulation Constellation diagram Baud rate = 8 Bit rate = 24 BitPhase