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Data Communication and Computer Networks
[EEE 314]
Mubashir Husain RehmaniMubashir Husain Rehmani
Assistant Professor
Department of Electrical Engineering
COMSATS Institute of Information Technology,
Wah Cantt, Pakistan
Physical
Layer
Physical Layer
Duties of Physical Layer
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
Signals
To be transmitted, data must be
transformed to electromagnetic signals.
Analog and Digital Data
Analog and Digital SignalsAnalog and Digital Signals
Periodic and Aperiodic Signals
Signals can be analog or digital.
Analog signals can have an infinite
number of values in a range; digital number of values in a range; digital
signals can have only a limited number
of values.
Comparison of Analog and
Digital Signal
In data communication, we commonly
use periodic analog signals and
aperiodic digital signals.aperiodic digital signals.
Analog Signals
Sine Wave
Phase
Examples of Sine WavesExamples of Sine Waves
Time and Frequency Domains
Composite Signals
Bandwidth
A Sine Wave
Amplitude
Frequency and period are inverses of
each other.
Period and Frequency
Units Units of periods and frequenciesof 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
Frequency is the rate of change with
respect to time. Change in a short span
of time means high frequency. Change 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.instantaneously, its frequency is infinite.
Phases
Time and Frequency Domain
Time and Frequency Domain
Time and Frequency Domain
A single-frequency sine wave is not
useful in data communications; we need
to change one or more of its to change one or more of its
characteristics to make it useful.
When we change one or more When we change one or more
characteristics of a singlecharacteristics of a single--frequency frequency
signal, it becomes a composite signal signal, it becomes a composite signal signal, it becomes a composite signal signal, it becomes a composite signal
made of many frequencies.made of many frequencies.
According to Fourier analysis, any
composite signal can be represented as
a combination of simple sine waves a combination of simple sine waves
with different frequencies, phases, and
amplitudes.
Square Wave
Signal Corruption
Bandwidth
The bandwidth is a property of a The bandwidth is a property of a
medium: It is the difference between the medium: It is the difference between the
highest and the lowest frequencies that highest and the lowest frequencies that highest and the lowest frequencies that highest and the lowest frequencies that
the medium can the medium can
satisfactorily pass.satisfactorily pass.
Bandwidth
Example 4Example 4
A signal has a bandwidth of 20 Hz. The highest frequency
is 60 Hz. What is the lowest frequency? Draw the
spectrum if the signal contains all integral frequencies of
the same amplitude.
SolutionSolutionSolutionSolution
B = B = ffhh −− ffll
20 = 60 20 = 60 −− ffll
ffll = 60 = 60 −− 20 = 40 Hz20 = 40 Hz
Digital Signal
Bit Interval and Bit Rate
As a Composite Analog Signal
Through Wide-Bandwidth MediumThrough Wide-Bandwidth Medium
Through Band-Limited Medium
Versus Analog Bandwidth
Higher Bit Rate
A Digital Signal
Example 6Example 6
A digital signal has a bit rate of 2000 bps. What is the
duration of each bit (bit interval)
SolutionSolution
The bit interval is the inverse of the bit rate.
Bit interval = 1/ 2000 s = 0.000500 s
= 0.000500 x 106 µs = 500 µs
Bit Rate and Bit Interval
The bit rate and the bandwidth are The bit rate and the bandwidth are
proportional to each other.proportional to each other.
Baseband transmission Baseband transmission means sending a means sending a
digital signal over a channel without changing digital signal over a channel without changing
the digital signal to analog form.the digital signal to analog form.
Baseband transmission requires that we have Baseband transmission requires that we have
a lowa low--pass channel. pass channel.
A lowA low--pass channel pass channel �� bandwidth that starts bandwidth that starts
from zero.from zero.
Broadband transmission or ModulationBroadband transmission or Modulation
means sending a digital signal over a channel means sending a digital signal over a channel
by changing the digital signal to analog form.by changing the digital signal to analog form.
Broadband transmission requires that we have Broadband transmission requires that we have Broadband transmission requires that we have Broadband transmission requires that we have
a banda band--pass channel. pass channel.
A bandA band--pass channel pass channel �� bandwidth that does bandwidth that does
not starts from zero.not starts from zero.
Low-Pass and Band-Pass
Channel
Transmission Impairments
Attenuation
Distortion
Noise
Throughput
Propagation Time
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