SLIIT Academy
BSc (IT) – Year 01, Semester 01
Data Communications and Computer Networks - I
Lecture 1 – Communication Fundamentals I
Ms. Dilushinie Fernando
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Learning Outcomes
▪ LO2: To differentiate the characteristics of analog and digital signal,
execute calculations based on Signal to Noise Ratio (SNR) and Power
Ratio, basic characteristics of a sinusoidal signal, Pulse Code
Modulation (PCM), Analog and Digital Modulation.
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Communication
Fundamentals
Part I – Characteristics of analog and
digital signal
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Data and Signals
▪ Sending a picture from a design workstation to another PC means the
transmission of data across network connections.
▪ But the data usable to a person or application are not in a form that
can be transmitted over a network.
▪ So, the data must be changed to a form that can be transmitted.
▪ One of the major functions of the physical layer is to move data in the
form of electromagnetic signals across a transmission medium.
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Data and Signals (cont.)
▪ Data can be analog or digital.
▪ Analog data- information that is continuous
▪ Analog data, such as the sounds made by a human voice, take on continuous values.
▪ Digital data - information that has discrete states
▪ Digital data are stored in computer memory in the form of 0s and 1s. They can be
converted to a digital signal or modulated into an analog signal for transmission across a
medium.
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Data and Signals (cont.)
▪ A signal is an electromagnetic or electrical current that carries data
from one system or network to another.
▪ In electronics, a signal is often a time-varying voltage that is also an
electromagnetic wave carrying information, though it can take on other forms,
such as current.
▪ Signals can be either analog or digital.
▪ Analog signal has infinitely many levels of values over a period of time.
▪ Digital signal can have only a limited number of defined values.
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Data and Signals (cont.)
▪ The simplest way to show signals is by plotting them on a pair of
perpendicular axes.
▪ Comparison of analog and digital signals
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Digital Signal
▪ A digital signal, can have only a limited number of defined values.
▪ Although each value can be any number, it is often as simple as 1 and 0.
▪ Digital signals are used in all digital electronics, including computing equipment
and data transmission devices.
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Digital Signal (cont.)
▪ Most digital signals are non-periodic, and thus period and frequency
are not appropriate characteristics.
▪ Bit rate (instead of frequency) is used to describe digital signals.
The bit rate is the number of bits sent in 1s,
expressed in bits per second (bps).
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Digital Signal (cont.)
▪ The digital signal also gets mixed with noise.
▪ The centre location of the digital signal can be identified by using a special bit
pattern – clock signal
▪ Regeneration of the signal – By checking the level at the centre location of
each bit it can be decided whether it is a 0 or 1.
▪ Noise can be eliminated.
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Digital Signal (cont.)
▪ Digital signals can convey information with less noise, distortion, and
interference.
▪ Due to high noise, there is a possibility to change the bit from 1 to 0 or 0 to 1 –
error
▪ Digital signal quality will not depend on the distance travelled by the
signal.
But, a higher bandwidth is required for digital
communication when compared to analog
transmission of the same information.
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Analog Signal
▪ Analog signals can have an infinite number of values in a range.
▪ When someone speaks, an analog wave is created in the air.
▪ This can be captured by a microphone and converted to an analog signal
or sampled and converted to a digital signal.
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Sinusoidal/ Sine Wave
▪ The simplest form of the periodic analog signal.
▪ The completion of one full pattern is called a cycle.
▪ When plotted on a time domain spectrum, an analog signal should
produce a smooth and continuous curve.
▪ A sine wave can be represented by three parameters:
▪ Peak amplitude (a)
▪ Frequency (f)
▪ Phase (θ)
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Sinusoidal/ Sine Wave (cont.)
▪ Peak amplitude – the absolute value of its highest intensity,
proportional to the energy it carries. (measured in volts)
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Sinusoidal/ Sine Wave (cont.)
Two signals with the
same phase and
frequency, but different
amplitudes.
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Sinusoidal/ Sine Wave (cont.)
▪ Period and Frequency
𝑓=
▪ Period – the amount of time, in
seconds, a signal needs to
complete 1 cycle. (expressed in
seconds)
1
𝑇
and 𝑇 =
1
𝑓
▪ Frequency – the number of periods
in 1 s. (expressed in Hz)
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Sinusoidal/ Sine Wave (cont.)
Two signals with the
same amplitude and
phase, but different
frequencies.
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Sinusoidal/ Sine Wave (cont.)
Units of period and frequency
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Sinusoidal/ Sine Wave (cont.)
Three sine waves with the same amplitude and
frequency, but different phases.
▪ Phase – the position of the
waveform relative to time 0.
(measured in degrees or radians)
▪ It indicates the status of the first
cycle.
360° = 2𝜋 rad
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Sinusoidal/ Sine Wave (cont.)
▪ Wavelength – the distance a
simple signal can travel in 1
period. (measured in micrometer
▪ depends on both the frequency
and the medium.
Wavelength = propagation speed x period
𝛌 = 𝑐𝑇
𝑐 = 𝑓𝛌
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Summary
▪ Identify the difference between Digital Signals and Analog Signals.
▪ Understanding basic characteristics of a sine wave.
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References
▪ B.A. Forouzan, "Chapter 3: Introduction to Physical Layer", Data
Communications and Networking, 5th Edition.
▪ W. Stallings, "Chapter 3: Data Transmission", Data and Computer
Communications, 8th Edition.
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