Transcript Chapter 1 - Introduction

Computer Networks and Internets, 5e
By Douglas E. Comer
Lecture PowerPoints
By Lami Kaya, [email protected]
© 2009 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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Chapter 10
Modulation
And
Modems
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Topics Covered
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10.1 Introduction
10.2 Carriers, Frequency, And Propagation
10.3 Analog Modulation Schemes
10.4 Amplitude Modulation
10.5 Frequency Modulation
10.6 Phase Shift Modulation
10.7 Amplitude Modulation And Shannon's Theorem
10.8 Modulation, Digital Input, And Shift Keying
10.9 Phase Shift Keying
10.10 Phase Shift And A Constellation Diagram
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Topics Covered
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10.11
10.12
10.13
10.14
10.15
10.16
Quadrature Amplitude Modulation
Modem Hardware For Modulation And Demodulation
Optical And Radio Frequency Modems
Dialup Modems
QAM Applied To Dialup
V.32 and V.32bis Dialup Modems
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10.1 Introduction
• This chapter
– focuses on the use of high-frequency signals to carry information
– discusses how information is used to change a high-frequency
electromagnetic wave
– explains why the technique is important
– describes how analog and digital inputs are used
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10.2 Carriers, Frequency, And
Propagation
• Many long-distance communication systems use a
oscillating electromagnetic wave called a carrier
• The system makes small changes to the carrier that
represent information being sent
• The 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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10.3 Analog Modulation Schemes
• We use the term modulation to refer to changes made in a carrier
– according to the information being sent
• Modulation takes two inputs
– a carrier
– and a signal
• Then it generates a modulated carrier as output, as in Figure 10.1
• In essence, a sender must change one of the fundamental
characteristics of the wave
• There are three primary techniques that modulate an electromagnetic
carrier according to a signal:
– Amplitude modulation
– Frequency modulation
– Phase shift modulation
• The first two methods of modulation are the most familiar and have been
used extensively
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10.3 Analog Modulation Schemes
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10.4 Amplitude Modulation (AM)
• AM varies the amplitude of a carrier in proportion to the
information being sent (i.e., according to a signal)
– The carrier continues oscillating at a fixed frequency, but the
amplitude of the wave varies
• Figure 10.2 illustrates
– an unmodulated carrier wave
– an analog information signal
– and the resulting amplitude modulated carrier
• As it is seen from the figure:
– only the amplitude (i.e., magnitude) of the sine wave is modified
– a time-domain graph of a modulated carrier has a shape similar to
the signal that was used
– imagine an envelope consisting of a curve that connects the peaks of
the sine wave in Figure 10.2c
• the resulting curve has the same shape as the signal in Figure 10.2b
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10.4 Amplitude Modulation
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10.5 Frequency Modulation (FM)
• In FM, the amplitude of the carrier remains fixed
• In FM, frequency changes according to the signal:
– when the signal is stronger, the carrier frequency increases slightly,
– and when the signal is weaker, the carrier frequency decreases
slightly
• Figure 10.3 illustrates an example of FM for an info signal
• FM is more difficult to visualize
– because slight changes in frequency are not as clearly visible
– However, one can notice that the modulated wave has higher
frequencies when the signal used for modulation is stronger
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10.5 Frequency Modulation
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10.6 Phase Modulation (PM)
• One of the property of a sine wave is its phase, the offset
from a reference time at which the sine wave begins
• It is possible to use changes in phase to represent a signal
• We use the term phase shift to characterize such changes
• If phase changes after cycle k, the next sine wave will start
slightly later than the time at which cycle k completes
– A slight delay resembles a change in frequency
• PM can be thought of as a special form of frequency
modulation
– However, phase shifts are important when a digital signal is used to
modulate a carrier
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10.7 Amplitude Modulation And
Shannon's Theorem
• Figure 10.2c shows the amplitude varying from a maximum
to almost zero
• The figure is slightly misleading:
– in practice, modulation only changes the amplitude of a carrier
slightly, depending on a constant known as the modulation index
• Practical systems do not allow for a modulated signal to
approach zero
• Consider Shannon's Theorem
– assuming the amount of noise is constant
• the signal-to-noise ratio will approach zero as the signal approaches zero
• Keeping the carrier wave near maximum insures that the
signal-to-noise ratio remains as large as possible
– This permits the transfer of more bits per second
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10.8 Modulation, Digital Input, And Shift
Keying
• How can digital input be used in modulation?
• Modifications to the modulation schemes described above
are needed:
– instead of modulation that is proportional to a continuous signal,
digital schemes use discrete values
• To distinguish between analog and digital modulation
– we use the term shift keying rather than modulation
• Shift keying operates similar to analog modulation
– Instead of a continuum of possible values, digital shift keying has a
fixed set
– For example, AM allows the amplitude of a carrier to vary by
arbitrarily small amounts in response to a change in the signal
• In contrast, amplitude shift keying uses a fixed set of possible amplitudes
• Figure 10.4 illustrates concept for ASK and FSK
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Figure 10.4
Illustration of
(a) a carrier wave
(b) a digital input
signal
(c) amplitude shift
keying
(d) frequency shift
keying
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10.9 Phase Shift Keying (PSK)
• Amplitude and frequency modulations require at least one
cycle of a carrier wave to send a single bit
– unless a special encoding scheme is used
• The number of bits sent per unit time can be increased
– if the encoding scheme permits multiple bits to be encoded in a
single cycle of the carrier
• Data communications systems often use techniques that
can send more bits
• PSK changes the phase of the carrier wave abruptly
– each such change is called a phase shift
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10.9 Phase Shift Keying (PSK)
• Figure 10.5 illustrates how phase shifts affects a sine wave
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There are three (abrupt) phase changes
A phase shift is measured by the angle of the change
The left most portion of sine wave changes its phase by π/2 radians or 180
The second phase change corresponds to a 180 shift
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10.10 Phase Shift and a Constellation
Diagram
• How can data be encoded using phase shifts?
– A sender and receiver can agree on the number of bits per second
– It can use no phase shift to denote logical 0, and the presence of a
phase shift to denote a logical 1
– For example, a system might use a 180 phase shift
• A constellation diagram is used to express the exact
assignment of data bits to specific phase changes
• Figure 10.6 illustrates the concept
• Many variations of PSK exist
– A phase shift mechanism such as the one illustrated in Figure 10.6
permits a sender to transfer one bit at a time
• It is called Binary Phase Shift Keying (BPSK) or 2-PSK
– Notation of 2-PSK is also used to denote the two possible values
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10.10 Phase Shift and a Constellation
Diagram
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10.10 Phase Shift And A Constellation
Diagram
• We can do more than detect the presence of a phase shift!
– A receiver can measure the amount a carrier shifted during a phase
change
– It is possible to devise a system that recognizes a set of phase shifts
• and use each particular phase shift to represent specific values of data
• Systems are designed to use a power of 2 possible shifts
– which means a sender can use bits of data to select among the shifts
• Figure 10.7 shows the constellation diagram for a system
that uses 4 possible phase shifts
– At each stage of transmission, a sender uses 2-bits of data to select
among the 4-possible shift values
– This is known as 4-PSK mechanism
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10.10 Phase Shift and a Constellation
Diagram
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10.10 Phase Shift and a Constellation
Diagram
• In theory, it is possible to increase the data rate by
increasing the range of phase shifts
• Thus, a 16-PSK mechanism can send twice as many bits
per second as a 4-PSK mechanism
• In practice, noise and distortion limit the ability of hardware
to distinguish among minor differences in phase shifts
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10.11 Quadrature Amplitude Modulation
• How can the data rate be increased further?
• A combination of modulation techniques that change two
characteristics of a carrier at the same time
• The most sophisticated technology combines ASK and PSK
• One of such is known as Quadrature Amplitude Modulation
(QAM) or Quadrature Amplitude Shift Keying (QASK)
– the approach uses both change in phase and in amplitude
• To represent QAM on a constellation diagram
– We use distance from the origin as a measure of amplitude
– Figure 10.8 shows the constellation diagram for a variant known as
16QAM with dark gray areas indicating the amplitudes
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10.11 Quadrature Amplitude Modulation
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10.12 Modem Hardware For Modulation
And Demodulation
• A mechanism that accepts a sequence of data bits and
applies modulation to a carrier wave according to the bits is
called a modulator
• A mechanism that accepts a modulated carrier wave and
recreates the sequence of data bits that was used to
modulate the carrier is called a demodulator
• Transmission of data requires a modulator at one end of the
transmission medium and a demodulator at the other
• Most communication systems are full duplex
– which means each location needs both a modulator to send data,
and a demodulator to receive data
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10.12 Modem Hardware For Modulation
And Demodulation
• Users would like keep cost low and make the pair of devices
easy to install and operate
• Manufacturers combine modulation and demodulation
mechanisms into a single device
– called a modem (modulator and demodulator)
• Figure 10.9 illustrates how a pair of modems use a 4-wire
connection to communicate
• Modems are designed to provide communication over long
distances
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10.12 Modem Hardware For Modulation
And Demodulation
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10.13 Optical And Radio Frequency
Modems
• Modems are also used with other media
– including Radio Frequency (RF) transmission and optical fibers
• A pair of RF modems can be used to send data via radio
• A pair of optical modems can be used to send data across a
pair of optical fibers
• Modems can use entirely different media, but the principle
remains the same:
– at the sending end, a modem modulates a carrier
– at the receiving end, data is extracted from the modulated carrier
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10.14 Dialup Modems
• A dialup modem uses an audio tone
– As with conventional modems, the carrier is modulated at the
sending end and demodulated at the receiving end
• A dialup modem uses data to modulate an audible carrier
– which is transmitted to the phone system
• The chief difference between dialup and conventional
modems arises from the lower bandwidth of audible dialup
modems
• Interior of a modern telephone system used today is digital
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The phone system digitizes the incoming audio
Transports a digital form internally
Converts the digitized version back to analog audio for delivery
The receiving modem demodulates the analog carrier
Extracts the original digital data
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10.14 Dialup Modems
• Figure 10.10 illustrates the ironic use of analog and digital
signals by dialup modems
– A dialup modem is usually embedded in a computer
• Term internal modem to denote an embedded device
• Term external modem to denote a separate physical device
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10.15 QAM Applied To Dialup
• QAM is also used with dialup modems as a way to
maximize the rate at which data can be sent
• Figure 10.11 shows the bandwidth available on a dialup
connection
• Most telephone connections transfer frequencies between
300 and 3000 Hz
• A given connection may not handle the extremes well
– Thus, to guarantee better reproduction and lower noise, dialup
modems use frequencies between 600 and 3000 Hz
– It means the available bandwidth is 2400 Hz
• A QAM scheme can increase the data rate dramatically
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10.15 QAM Applied To Dialup
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10.16 V.32 and V.32bis Dialup Modems
• Consider the V.32 and V.32bis standards
• Figure 10.12 illustrates the QAM constellation for
– a V.32 modem that uses 32 combinations of ASK and PSK
• to achieve a data rate of 9600 bps in each direction
– A V.32bis modem uses 128 combinations of ASK and PSK
• to achieve a data rate of 14,400 bps in each direction
• Figure 10.13 illustrates the constellation
• 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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QAM for V.32 Dialup Modems
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QAM for V.32bis Dialup Modems
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