Physical Layer by mohammad nazir

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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