This document discusses signals and transmission in data communication networks. It covers topics like analog vs digital signals, sine waves, bandwidth, transmission impairments like attenuation and noise. Specific chapters cover digital/analog transmission, multiplexing, transmission media, circuit switching and high speed digital access. Signals must be transformed into electromagnetic waves to be transmitted. Key signal characteristics discussed include frequency, amplitude, phase, bandwidth, throughput, propagation speed and wavelength.

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Physical Layer
PART II

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Position of the physical layer

3. Services

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Chapters
Chapter 3 Signals
Chapter 4 Digital Transmission
Chapter 5 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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Chapter 3
Signals

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To be transmitted, data must be
transformed to electromagnetic
signals.
Note:

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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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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.
Note:

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Figure 3.1 Comparison of analog and digital signals

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In data communication, we commonly
use periodic analog signals and
aperiodic digital signals.
Note:

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3.2 Analog Signals
Sine Wave
Phase
Examples of Sine Waves
Time and Frequency Domains
Composite Signals
Bandwidth

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Figure 3.2 A sine wave

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Figure 3.3 Amplitude

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Frequency and period are inverses of
each other.
Note:

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Figure 3.4 Period and frequency

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Table 3.1 Units of periods and frequencies
Unit Equivalent Unit Equivalent
Seconds (s) 1 s hertz (Hz) 1 Hz
Milliseconds (ms) 10–3 s kilohertz (KHz) 103 Hz
Microseconds (ms) 10–6 s megahertz (MHz) 106 Hz
Nanoseconds (ns) 10–9 s gigahertz (GHz) 109 Hz
Picoseconds (ps) 10–12 s terahertz (THz) 1012 Hz

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Frequency is the rate of change with
respect to time. Change in a short span
of time means high frequency. Change
over a long span of time means low
frequency.
Note:

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Phase describes the position of the
waveform relative to time zero.
Note:

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Figure 3.5 Relationships between different phases

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Figure 3.6 Sine wave examples

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Figure 3.6 Sine wave examples (continued)

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Figure 3.6 Sine wave examples (continued)

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An analog signal is best represented in
the frequency domain.
Note:

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Figure 3.7 Time and frequency domains

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Figure 3.7 Time and frequency domains (continued)

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Figure 3.7 Time and frequency domains (continued)

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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.
Note:

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When we change one or more
characteristics of a single-frequency
signal, it becomes a composite signal
made of many frequencies.
Note:

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According to Fourier analysis, any
composite signal can be represented as
a combination of simple sine waves
with different frequencies, phases, and
amplitudes.
Note:

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Figure 3.9 Three harmonics

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Figure 3.10 Adding first three harmonics

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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.
Note:

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In this book, we use the term
bandwidth to refer to the property of a
medium or the width of a single
spectrum.
Note:

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Figure 3.13 Bandwidth

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Figure 3.17 Bit rate and bit interval

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Table 3.12 Bandwidth Requirement
Bit
Rate
Harmonic
1
Harmonics
1, 3
Harmonics
1, 3, 5
Harmonics
1, 3, 5, 7
1 Kbps 500 Hz 2 KHz 4.5 KHz 8 KHz
10 Kbps 5 KHz 20 KHz 45 KHz 80 KHz
100 Kbps 50 KHz 200 KHz 450 KHz 800 KHz

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The bit rate and the bandwidth are
proportional to each other.
Note:

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3.6 Transmission Impairment
Attenuation
Distortion
Noise

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Figure 3.20 Impairment types

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Figure 3.21 Attenuation

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Figure 3.22 Example 14
dB = –3 + 7 – 3 = +1

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Figure 3.23 Distortion

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Figure 3.24 Noise

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3.7 More About Signals
Throughput
Propagation Speed
Propagation Time
Wavelength

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Figure 3.25 Throughput

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Figure 3.26 Propagation time

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Figure 3.27 Wavelength