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