Lecture – 4

Data Communication and Computer Network

CS221

Ishtiyaq Qureshi
Assistant Professor, CSE, CGU
PhD, IIT Mandi
Data and Signals

Alice and Bob are two computers,

and Alice wants to download some
e-books from Bob.
Data and Signals

How the protocol

layering will look
like for this
communication?
Data and Signals

Communication at the physical layer

• Communication at the physical layer is
“physical” (“Not logical”). (Blue arrows)
• Alice and Bob need to exchange data,
communication at the physical layer
means exchanging signals.
• Both data and the signals that represent
them can be either analog or digital in
form.
Data and Signals
• Analog and Digital Data: The term analog data refers to information
that is continuous; digital data refers to information that has discrete
states.
• Analog and Digital Signals: An analog signal has infinitely many
levels of intensity over a period of time. A digital signal, on the other
hand, can have only a limited number of defined values.
Data and Signals

• Periodic Signals: A periodic signal completes a pattern within a

measurable time frame, called a period, and repeats that pattern over
subsequent identical periods.
• Non Periodic Signals: A nonperiodic signal changes without
exhibiting a pattern or cycle that repeats over time.
• In data communications, we commonly use periodic analog signals
(PAS) and nonperiodic digital signals (NDS).
Periodic Analog Signals
• A simple periodic analog signal, a sine wave, cannot be decomposed
into simpler signals. Composite signals composed of multiple sine
waves.
• Parameters: Peak amplitude, the frequency, and the phase.
Periodic Analog Signals
Periodic Analog Signals
Peak amplitude vs RMS amplitude: Peak value is equal to 2 × rms value
Periodic Analog Signals
Period and Frequency:
• Period refers to the amount of
time, in seconds, a signal needs to
complete 1 cycle.
• The completion of one full pattern
is called a cycle.
• Frequency refers to the number
of periods in 1 s.
• Frequency is the rate of change
with respect to time.
1
• Relation between the two: 𝑓 =
𝑇
• The period of a signal is 100 ms.
What is its frequency in kilohertz?
Periodic Analog Signals
Period and Frequency:
• Period refers to the amount of
time, in seconds, a signal needs to
complete 1 cycle.
• The completion of one full pattern
is called a cycle.
• Frequency refers to the number
of periods in 1 s.
• Frequency is the rate of change
with respect to time.
1
• Relation between the two: 𝑓 =
𝑇
• The period of a signal is 100 ms.
What is its frequency in kilohertz?
Periodic Analog Signals
Period and Frequency:
• Period refers to the amount of
time, in seconds, a signal needs to
complete 1 cycle.
• The completion of one full pattern
is called a cycle.
• Frequency refers to the number
of periods in 1 s.
• Frequency is the rate of change
with respect to time.
1
• Relation between the two: 𝑓 =
𝑇
• The period of a signal is 100 ms.
What is its frequency in kilohertz?
Periodic Analog Signals
Period and Frequency:
• Period refers to the amount of
time, in seconds, a signal needs to
complete 1 cycle.
• The completion of one full pattern
is called a cycle.
• Frequency refers to the number
of periods in 1 s.
• Frequency is the rate of change
with respect to time.
1
• Relation between the two: 𝑓 =
𝑇
• The period of a signal is 100 ms.
What is its frequency in kilohertz?
Periodic Analog Signals
Period and Frequency:
• Period refers to the amount of
time, in seconds, a signal needs to
complete 1 cycle.
• The completion of one full pattern
is called a cycle.
• Frequency refers to the number
of periods in 1 s.
• Frequency is the rate of change
with respect to time.
1
• Relation between the two: 𝑓 =
𝑇
• The period of a signal is 100 ms.
What is its frequency in kilohertz?
Periodic Analog Signals
Period and Frequency:
• Period refers to the amount of
time, in seconds, a signal needs to
complete 1 cycle.
• The completion of one full pattern
is called a cycle.
• Frequency refers to the number
of periods in 1 s.
• Frequency is the rate of change
with respect to time.
1
• Relation between the two: 𝑓 =
𝑇
• The period of a signal is 100 ms.
What is its frequency in kilohertz?
Periodic Analog Signals
Period and Frequency:
• 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.
?
• If a signal does not change at
all, its frequency is zero.
?
• If a signal changes
instantaneously, its frequency is
infinite.
?
Periodic Analog Signals
Period and Frequency:
• 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.
?
• If a signal does not change at
all, its frequency is zero.
?
• If a signal changes
instantaneously, its frequency is
infinite.
?
Periodic Analog Signals
Period and Frequency:
• 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.
• If a signal does not change at
all, its frequency is zero.
?
• If a signal changes
instantaneously, its frequency is
infinite.
?
Periodic Analog Signals
Period and Frequency:
• 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.
• If a signal does not change at
all, its frequency is zero.
• If a signal changes
instantaneously, its frequency is
infinite.
?
Periodic Analog Signals
Period and Frequency:
• 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.
• If a signal does not change at
all, its frequency is zero.
• If a signal changes
instantaneously, its frequency is
infinite.
Periodic Analog Signals
Phase:
• The term phase, or phase shift,
describes the position of the
waveform relative to time 0.
• Unit: Degree or Radian
• Fig a. A sine wave with a phase of 0°
starts at time 0 with a zero amplitude.
The amplitude is increasing.
• b. A sine wave with a phase of 90°
starts at time 0 with a peak
amplitude. The amplitude is
decreasing
• A sine wave with a phase of 180°
starts at time 0 with a zero amplitude.
The amplitude is decreasing.
Periodic Analog Signals
Phase:
• The term phase, or phase shift,
describes the position of the
waveform relative to time 0.
• Unit: Degree or Radian
• Fig a. A sine wave with a phase of 0°
starts at time 0 with a zero amplitude.
The amplitude is increasing.
• b. A sine wave with a phase of 90°
starts at time 0 with a peak
amplitude. The amplitude is
decreasing
• A sine wave with a phase of 180°
starts at time 0 with a zero amplitude.
The amplitude is decreasing.
Periodic Analog Signals
Phase:
• The term phase, or phase shift,
describes the position of the
waveform relative to time 0.
• Unit: Degree or Radian
• Fig a. A sine wave with a phase of 0°
starts at time 0 with a zero amplitude.
The amplitude is increasing.
• b. A sine wave with a phase of 90°
starts at time 0 with a peak
amplitude. The amplitude is
decreasing
• A sine wave with a phase of 180°
starts at time 0 with a zero amplitude.
The amplitude is decreasing.
Periodic Analog Signals
Phase:
• The term phase, or phase shift,
describes the position of the
waveform relative to time 0.
• Unit: Degree or Radian
• Fig a. A sine wave with a phase of 0°
starts at time 0 with a zero amplitude.
The amplitude is increasing.
• b. A sine wave with a phase of 90°
starts at time 0 with a peak
amplitude. The amplitude is
decreasing
• A sine wave with a phase of 180°
starts at time 0 with a zero amplitude.
The amplitude is decreasing.
Periodic Analog Signals
Phase:
• The term phase, or phase shift,
describes the position of the
waveform relative to time 0.
• Unit: Degree or Radian
• Fig a. A sine wave with a phase of 0°
starts at time 0 with a zero amplitude.
The amplitude is increasing.
• b. A sine wave with a phase of 90°
starts at time 0 with a peak
amplitude. The amplitude is
decreasing
• A sine wave with a phase of 180°
starts at time 0 with a zero amplitude.
The amplitude is decreasing.
Periodic Analog Signals
Phase:
• The term phase, or phase shift,
describes the position of the
waveform relative to time 0.
• a. A sine wave with a phase of 0° is
not shifted.
• b. A sine wave with a phase of 90° is
shifted to the left by cycle. However,
note that the signal does not really
exist before time 0.
• c. A sine wave with a phase of 180°
is shifted to the left by cycle.
However, note that the signal does
not really exist before time 0.
Periodic Analog Signals
Phase:
• The term phase, or phase shift,
describes the position of the
waveform relative to time 0.
• a. A sine wave with a phase of 0° is
not shifted.
• b. A sine wave with a phase of 90° is
shifted to the left by cycle. However,
note that the signal does not really
exist before time 0.
• c. A sine wave with a phase of 180°
is shifted to the left by cycle.
However, note that the signal does
not really exist before time 0.
Periodic Analog Signals
Phase:
• The term phase, or phase shift,
describes the position of the
waveform relative to time 0.
• a. A sine wave with a phase of 0° is
not shifted.
• b. A sine wave with a phase of 90° is
shifted to the left by cycle. However,
note that the signal does not really
exist before time 0.
• c. A sine wave with a phase of 180°
is shifted to the left by cycle.
However, note that the signal does
not really exist before time 0.
Periodic Analog Signals
Phase:
• The term phase, or phase shift,
describes the position of the
waveform relative to time 0.
• a. A sine wave with a phase of 0° is
not shifted.
• b. A sine wave with a phase of 90° is
shifted to the left by cycle. However,
note that the signal does not really
exist before time 0.
• c. A sine wave with a phase of 180°
is shifted to the left by cycle.
However, note that the signal does
not really exist before time 0.
Periodic Analog Signals
Periodic Analog Signals
Periodic Analog Signals
Periodic Analog Signals
Periodic Analog Signals
Wavelength
• While the frequency of a signal is independent of the medium, the wavelength
depends on both the frequency and the medium.
Periodic Analog Signals
Wavelength
• While the frequency of a signal is independent of the medium, the wavelength
depends on both the frequency and the medium.
Periodic Analog Signals
Wavelength
• While the frequency of a signal is independent of the medium, the wavelength
depends on both the frequency and the medium.
Periodic Analog Signals
Wavelength
• While the frequency of a signal is independent of the medium, the wavelength
depends on both the frequency and the medium.
Periodic Analog Signals
Periodic Analog Signals
Periodic Analog Signals
Periodic Analog Signals
Periodic Analog Signals

Fourier
Periodic Analog Signals

The advantage of the frequency domain is that we can immediately see

the values of the frequency and peak amplitude.
Periodic Analog Signals

FD Plots are more compact and useful when

we are dealing with more than one sine wave.
Periodic Analog Signals
Use of simple sine waves
• The power company sends a single sine wave with a frequency of 50 Hz
(in India) to distribute electric energy to houses and businesses.
• We can use a single sine wave to send an alarm to a security center when
a burglar opens a door or window in the house.
• In the first case, the sine wave is carrying energy; in the second, the sine
wave is a signal of danger.
• A single-frequency sine wave is not useful in data communications; we
need to send a composite signal, a signal made of many simple sine
waves.
Periodic Analog Signals
• A composite signal is made of
many simple sine waves.
• A periodic composite signal can
be decomposed into a series of
simple sine waves with discrete
frequencies integer values (1, 2, 3,
and so on).
• A nonperiodic composite signal
can be decomposed into a
combination of an infinite number
of simple sine waves with
continuous frequencies,
frequencies that have real values.
Periodic Analog Signals
• The amplitude of the sine wave
with frequency 𝑓 is almost the
same as the peak amplitude of the
composite signal.
Periodic Analog Signals
• The amplitude of the sine wave
with frequency 3𝑓 is one-third of
that of the first, and the amplitude
of the sine wave with frequency
9𝑓 is one-ninth of the first.
Periodic Analog Signals
• The frequency of the sine wave
with frequency 𝑓 is the same as
the frequency of the composite
signal; it is called the
fundamental frequency, or first
harmonic.
Periodic Analog Signals
• The sine wave with frequency 3f
has a frequency of 3 times the
fundamental frequency; it is
called the third harmonic. The
third sine wave with frequency 9f
has a frequency of 9 times the
fundamental frequency; it is
called the ninth harmonic.
• There are no frequencies such as
1.2f or 2.6f.
• Also, as it is composed of sine
waves, even harmonics become
all zero.
Aperiodic Analog Composite Signals
• In a time-domain representation of this composite signal, there are an infinite
number of simple sine frequencies.
• Although the number of frequencies in a human voice is infinite, the range is limited.
• A normal human being can create a continuous range of frequencies between 0 and 4
kHz.
• Note that the frequency decomposition of the signal yields a continuous curve.
• There are an infinite number of frequencies between 0.0 and 4000.0 (real values).
Periodic vs Aperiodic Signals

• The bandwidth of a composite

signal is the difference between the
highest and the lowest frequencies
contained in that signal.
• The bandwidth of the periodic
signal contains all integer
frequencies between 1000 and
5000 (1000, 1001, 1002, . . .).
• The bandwidth of the nonperiodic
signals has the same range, but the
frequencies are continuous.
References

1. Data Communications and Networking: Behrouz A. Forouzan, Tata

McGraw-Hill, 4th Edition.