Communication and Networks Assignment
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0_Lesson13Modulation.pptxCommunications and Networks
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Diploma in Information Technology
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Lesson 13: Modulation
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Lesson 13 Learning Outcomes
Explain carriers, frequency and propagation
Explain the three different types of modulation
Explain the shift keying
Understand what modulation and demodulation means
Describe the different types of modems used for modulation and demodulation
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Lesson 13 Outline
Analog Modulation
Digital Modulation
Modulation and Demodulation Hardware
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Carriers
Long-distance communication use an oscillating electromagnetic wave called carrier
Makes small changes to carrier that represent information being sent
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Frequency and Propagation
Frequency of electromagnetic energy determines how the energy propagates
One motivation for the use of carriers arises from the desire to select a frequency that will propagate well
Independent of the rate that data is being sent
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Analog Modulation
Modulation: changes made to a carrier according to the information being sent
Takes two inputs
Carrier
Information signal
Sender must change one of the fundamental characteristics of the wave
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Analog Modulation Schemes
Three primary techniques that modulate an electromagnetic carrier according to a signal:
Amplitude modulation
Frequency modulation
Phase shift modulation
First two methods of modulation are most familiar and used extensively
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What is Amplitude Modulation
Source: https://www.youtube.com/watch?v=fGf_ng7qljI
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Modulation Concept
Diagram show the concept of modulation with two inputs
Source: Douglas, C (2016) Computer Networks and Internets
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Amplitude Modulation (AM)
AM: varies amplitude of a carrier in proportion to the information being sent (according to a signal)
Carrier continues oscillating at fixed frequency, but amplitude of the wave varies
Only amplitude of the sine wave is modified
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AM Illustration
(a) unmodulated carrier wave
(b) analog information signal
(c) resulting AM carrier
Source: Douglas, C (2016) Computer Networks and Internets
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AM and Shannon’s Theorem
Modulation only changes amplitude of a carrier slightly, depending on a constant, modulation index
Practical systems do not allow for a modulated signal to approach zero
In Shannon's Theorem, signal-to-noise ratio will approach zero as the signal approaches zero
Keeping carrier wave near maximum ensures signal-to-noise ratio remains as large
Permits the transfer of more bits per second
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Frequency Modulation (FM)
FM: amplitude of the carrier remains fixed but frequency changes according to signal:
Signal stronger, carrier frequency increase slightly
Signal weaker, carrier frequency decrease slightly
FM is more difficult to visualize as slight changes in frequency are not as clearly visible
But can notice that modulated wave has higher frequencies when signal used for modulation is stronger
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FM Illustration
Analog Information Signal
Resulting FM Carrier
Source: Douglas, C (2016) Computer Networks and Internets
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Sine Wave Phase Properties
One of the properties of sine wave is its phase, the offset from reference time which sine wave begins
Possible to use changes in phase to represent signals
The term phase shift characterise such changes
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Phase Modulation (PM)
If phase changes after cycle k, next sine wave will start slightly later than the time at which cycle k completes
Slight delay resembles a change in frequency
PM: a special form of frequency modulation
Phase shifts are important when a digital signal is used to modulate a carrier
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Practice 13.1
Define modulation and describe THREE (3) different ways a signal can be transformed by a modulator into a resulting carrier that is transmitted over a medium.
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Lesson 13 Outline
Analog Modulation
Digital Modulation
Modulation and Demodulation Hardware
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Digital Modulation
Modifications to the modulation schemes are needed for digital modulation
Instead of modulation that is proportional to a continuous signal, digital schemes use discrete values
Digital modulation is referred to as shift keying
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Shift Keying
Shift keying operates like analog modulation
But instead of continuous values, digital shift keying has a fixed set
For example, AM allows amplitude of carrier to vary by small amounts in response to change in signal
Amplitude Shift Keying (ASK): uses a fixed set of possible amplitudes
Frequency Shift Keying (FSK): uses a fixed set of possible frequencies
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Shift Keying Illustration
Carrier wave
Digital input signal
Amplitude Shift Keying
Frequency Shift Keying
Source: Douglas, C (2016) Computer Networks and Internets
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Phase Shift Keying (PSK)
AM and FM require at least one cycle of a carrier wave to send a single bit
unless a special encoding scheme is used
Number of bits sent per unit time can be increased
if encoding scheme permits multiple bits to be encoded in a single cycle of the carrier
PSK: changes phase of the carrier wave abruptly
Each such change is called a phase shift
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PSK Illustration
There are three abrupt phase changes
A phase shift is measured by angle of the change
Left most portion of sine wave changes its phase by π/2 radians or 180o
Second phase change corresponds to 180 shift
Abrupt changes
Source: Douglas, C (2016) Computer Networks and Internets
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Encoding Data Using Phase Shifts
A sender and receiver can agree on the number of bits per second
No phase shift denote logical 0
Presence of a phase shift to denote a logical 1
A system can use a 180o phase shift to represent presence of a phase shift
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Constellation Diagram
Constellation diagram: to express exact assignment of data bits to specific phase changes
A possible phase shift mechanism can permit sender to transfer one bit at a time
Called Binary Phase Shift Keying (BPSK) or 2-PSK
2-PSK is also used to denote the two possible values
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Constellation Diagram Illustration
A constellation diagram showing logical 0 as 0o phase shift and logical 1 as a 180o phase shift
Source: Douglas, C (2016) Computer Networks and Internets
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Detecting Specific Phase Shift
Receiver can measure amount a carrier shifted during a phase change
Can devise system that recognizes a set of phase shifts each to represent specific values of data
Systems are designed to use power of 2 possible shifts
Sender can use bits of data to select among shifts
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Specific Phase Shift Illustration
Diagram shows a system that uses 4-PSK
At each stage of transmission, sender uses 2-bits of data to select among 4-possible shift values
Source: Douglas, C (2016) Computer Networks and Internets
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Data Rate in Phase Shifts
Possible to increase the data rate by increasing the range of phase shifts
A 16-PSK mechanism can send twice as many bits per second as a 4-PSK mechanism
But in practice, noise and distortion limit the ability of hardware
to distinguish among minor differences in phase shifts
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Increasing Data Rate Further
Data rate can be increased further by using combination of modulation techniques
Change two characteristics of carrier at one time
Most sophisticated technology combines ASK and PSK
Known as Quadrature Amplitude Modulation (QAM) or Quadrature Amplitude Shift Keying (QASK)
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Quadrature Amplitude Modulation (QAM)
QAM: uses both change in phase and amplitude
To represent QAM on a constellation diagram
Use distance from the origin as a measure of amplitude
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QAM Illustration
Diagram shows a variant known as 16QAM with dark gray areas indicating the amplitudes
Source: Douglas, C (2016) Computer Networks and Internets
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Practice 13.2
What is digital modulation known as? Distinguish between analog and digital modulation.
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Lesson 13 Outline
Analog Modulation
Digital Modulation
Modulation and Demodulation Hardware
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What is a Modem
Source: https://www.youtube.com/watch?v=T4oeyn9Pxsc
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Modulator and Demodulator
Modulator: accepts a sequence of data bits, applies modulation to carrier wave according to the bits
Demodulator: accepts a modulated carrier wave and recreates sequence of data bits
Most communication systems are full-duplex
Each location needs a modulator to send and a demodulator to receive data
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Modem
To keep cost low and make the pair of devices easy to install and operate
Manufacturers combine modulation and demodulation into a device: modem (modulator and demodulator)
Modems are also designed to provide communication over long distances
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Modem Illustration
Diagram shows how a pair of modems use a 4-wire connection to communicate
Source: Douglas, C (2016) Computer Networks and Internets
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Modem Variations
Modems can be used with other media like Radio Frequency (RF) transmission and optical fibers
RF modems and optical modems
Modems can use entirely different media, but the principle remains the same:
at sender, a modem modulates a carrier
at receiver, extract data from modulated carrier
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Dialup Modems
Dialup modem uses an audio tone
Like conventional modems, carrier is modulated at sender and demodulated at receiver
Uses data to modulate an audible carrier
which is transmitted to the phone system
Difference with conventional modems is the lower bandwidth of audible dialup modems
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Modern Telephone System
Modern telephone system used today is digital
Phone system digitises incoming audio
Transports a digital form internally
Converts the digitised version back to analog audio for delivery
Receiver demodulates analog carrier
Extracts original digital data
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Internal & External Modems
Dialup modem can also be embedded in a computer
Internal modem: denote an embedded device
External modem: denote separate physical device
Source: Douglas, C (2016) Computer Networks and Internets
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QAM on Dialup Modem
QAM is used with dialup modems to maximise data rate
Most telephone connections transfer frequencies 300-3000Hz but a given connection may not handle the extremes well
To guarantee better reproduction and lower noise, dialup modems use 600-3000Hz
Available bandwidth = 3000-600= 2400 Hz
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QAM on Dialup Modem Illustration
Source: Douglas, C (2016) Computer Networks and Internets
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V.32 and V.32bis Modem
V.32 modem that uses 32 combinations of ASK and PSK to achieve a data rate of 9600 bps in each direction
V.32bis modem uses 128 combinations of ASK and PSK to achieve 14400 bps in each direction
Sophisticated signal analysis is needed to detect the minor change that occurs
from a point in the constellation to a neighboring point
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V.32 and V.32bis Constellations
Source: Douglas, C (2016) Computer Networks and Internets
V.32 Dialup Modem
V.32Bis Dialup Modem
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Practice 13.3
How many modems are required in a full-duplex communication system?
What are optical modems and RF modem? Describe one similarity between them.
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Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 10
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End of Lesson
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