!
Notes appended and modified by C. Rodgers
to those accompanying
"Data Communications and Networking" 3rd Ed.
Behrouz A. Forouzan
Position of the physical layer
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Physical Layer Services
Chapters related to the Physical Layer
Chapter 3
Signals
Chapter 4
Chapter 5
Digital Transmission
Analog Transmission
Chapter 6
Multiplexing
Chapter 7
Transmission Media
Chapter 8
Circuit Switching and Telephone Network
Chapter 9
High Speed Digital Access
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Note:
To be transmitted, data must be
transformed to electromagnetic
signals.
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3.1 Analog and Digital
Analog and Digital Data
Analog and Digital Signals
Periodic and Aperiodic Signals
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Note:
Signals can be analog or digital.
Analog signals can have an infinite
number of values in a range; digital
signals can have only a limited
number of values.
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Figure 3.1
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Comparison of analog and digital signals
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Note:
In data communication, we commonly
use periodic analog signals and
aperiodic digital signals.
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3.2 Analog Signals
Time and Frequency Domains
Composite Signals
Bandwidth
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Figure 3.7
Analog Signals in the Time and Frequency domains
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Note:
A single-frequency sine wave is not
useful in data communications; we
need to change one or more of its
characteristics to make it useful.
When we change one or more
characteristics of a single-frequency
signal, it becomes a composite signal
made of many frequencies.
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Note:
According to Fourier analysis, any
composite signal can be represented as
a combination of simple sine waves
with different frequencies, phases, and
amplitudes.
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Fourier Theory
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Fourier Harmonic Series
Fourier Components of the Square Wave
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Square Wave
Sawtooth Wave
For these waveforms, the ratio of harmonic amplitudes
to the fundamental amplitude are
The practice of calling the largest amplitude “1”
and scaling all other amplitudes in relation to it
See VEE “Fourier - Func Gen. in
Freq. & time domain ” simulation
See Fourier Links
Figure 3.12
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Signal corruption
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Note:
The bandwidth is a property of a medium:
It is the difference between the highest and
the lowest frequencies that the medium can
satisfactorily pass.
In this book, we use the term bandwidth to
refer to the property of a medium or the
width of a signal spectrum.
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Figure 3.13
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Bandwidth
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Example 3
If a periodic signal is decomposed into five sine waves
with frequencies of 100, 300, 500, 700, and 900 Hz,
what is the bandwidth?
Solution
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Figure 3.14
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Example 3
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