1

Dr. Tom HicksComputer Science Department

Trinity University

2

Some Material From Chapter 3

Some Material From Chapter 5I think it fits in here better!

3

Position of the Physical Layer

3.4

Communication at the Physical Layer

5

Services

6

Signals

7

To be Transmitted, Data

Must be Transformed to

Electromagnetic Signals.

8

Physical Layer

Concerns

9

Physical Major Concern

Major Concern of Physical Layer – Moving

Information, In The Form Of Electromagnetic Signals,

Across A Transmission Media.

Information

Voice

Image

Numeric Data

Character Data

Binary Data

Etc.

10

Physical Layer Must Encode

Information Usable To A Person Can Not Be

Directly Transmitted Across A Network; The

Information Must Be Encoded As 0’s and 1’s

11

Analog Signals

&

Digital Signals

12

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:

13

Analog/Digital

Analog – Continuous Signal – A Set Of Points

With All Points In-Between

Digital – Discrete Signal – A Set Of Points With

NoPoints In-Between

14

Graph Of An Analog Signal

The term Analog Data refers to information that

is Continuous

15

16

Digital Data refers to Information that has Discrete

States

Graph Of A Digital Signal

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Graph Of A Digital Signal

Acceptable Values Might Range

From

-1 to 1

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Analog vs. Digital Signals

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Periodic Signals

&

Aperiodic Signals

20

In Data Communication, we

commonly use

Periodic Analog Signals

&

Aperiodic Digital Signals.

Note:

21

Sine Wave

A Periodic Signal completes a pattern within a

measurable time frame, called a Period, and

repeats that pattern over subsequent identical

periods.

The Sine Wave is a Periodic Signal. The graph

below has 3 Periods.

The Sine Wave is the Most Fundamental Form

of a Periodic Analog Signal.

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Amplitude

23

Amplitude

Signal Amplitude – the value of the

signal at any point along the wave

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Can Vary The Amplitude

Amplitude Can Be Changed by varying

the current! [volts, amperes, watts]

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Two Signals – Two Amplitudes

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Frequency and

Period are

Inverses of Each

Other.

Note:

27

Frequency

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Periods In A Second Hz

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Units of Period & Frequency

The voltage of a battery is a constant; this constant

value can be considered a sine wave, as we will

see later.

For example, the peak value of an AA battery is

normally 1.5 V.

Example 3.2

Express a period of 100 ms in microseconds.

Example 3.4

Solution

From Table 3.1 we find the equivalents of 1 ms (1 ms

is 10–3 s) and 1 s (1 s is 106 μs).

We make the following substitutions:

Period = 100 ms = _________________ μs105

The period of a signal is 100 ms. What is its frequency

in kilohertz?

Example 3.5

Solution

First we change 100 ms to seconds, and then we

calculate the frequency from the period (1 Hz = 10–3

kHz).

Frequency = _________________ kHz10-2

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Periods

&

Cycles

34

Periodic Signals

Periodic Signal – completes a pattern

within a measurable time frame, called a

Period

35

Period

Period – the time required for a periodic

signal to complete one complete Cycle of

the pattern.

3.36

The power we use at home has a frequency of 60 Hz

(50 Hz in Europe). The period of this sine wave can be

determined as follows:

Example 3.3

Period = _________________ ms

This means that the period of the power for our

lights at home is 0.0116 s, or 16.6 ms.

Our eyes are not sensitive enough to distinguish

these rapid changes in amplitude.

16.6

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Why The Sine

Wave?

38

39

Frequency

&

Periods

40

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Units of Period & Frequency

43

Practice

Problems

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45

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50

High & Low

Frequency

51

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.

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If a Signal Does Not Change at

all, its Frequency is Zero.

If a Signal Changes

Instantaneously, its Frequency is

Infinite.

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Phase

54

Phase Describes the

Position of the Waveform

Relative to Time Zero.

55

Relationships Between Phases

56

About Phase

Phase Is

Measured

In

Degrees Or

Radians

Phase – describes the position of the

wave form relative to time 0.

57

Sine 3 Different Phases

3.57

A sine wave is offset 1/6 cycle with respect to time 0.

What is its phase in degrees and radians?

Example 3.6x

Phase = _________________ Degrees60

Solution

We know that 1 complete cycle is 360°.

A sine wave is offset 1/6 cycle with respect to time 0.

What is its phase in degrees and radians?

Example 3.6

Solution

We know that 1 complete cycle is 360°. Therefore,

1/6 cycle is

Phase = _________________ rad1.046

60

Time Domain &

Frequency

Domain Plots

61

Wavelength

Wavelength is another characteristic of a signal

traveling through a transmission medium.

Wavelength Binds the Period or the Frequency

of a Simple Sine Wave to the Propagation Speed

of the Medium

Wavelength

Direction ofpropagation

63

An Analog Signal is

Best Represented in the

Frequency Domain.

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65

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The Frequency Domain is more compact and useful

when we are dealing with more than one sine wave.

Below, we see three sine waves, each with different

amplitude and frequency. All can be represented by

three spikes in the frequency domain.

Example 3.7

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Decomposition

&

Fourier Analysis

Harmonics

69

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.

70

When We Change One or More

Characteristics of a Single-

Frequency signal, it Becomes a

Composite Signal made of

many frequencies.

71

According to Fourier analysis, Any

Composite Signal Can be

Represented as a Combination of

Simple Sine Waves with Different

Frequencies, Phases, and

Amplitudes.

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75

76

77

Square Wave

78

Three Separate Harmonics

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Harmonics

80

Frequency Spectrum Comparison

81

Decomposition Of Signal - 1 The analysis of this signal below can give us a good

understanding of how to Decompose Signals.

It is Very Difficult to Manually Decompose this Signal into

a series of simple sine waves.

Time

Amplitude

• • •

There are Tools, both hardware and software, that can help

us do the job of Decomposing Signals.

I am not concerned about how it is done; I am only

interested in the result.

The Result of Decomposing this Signal in the Time

Domains

Decomposition Of Signal - 2

There are Tools, both hardware and software, that can help

us do the job of Decomposing Signals.

The Result of Decomposing this Signal in the Frequency

Domains

Decomposition Of Signal - 3

84

NonPeriodic

Composite Signal

Speaking a Word, or Two, into a Microphone,

or Telephone, would create a Nonperiodic

Composite Signal.

This composite signal Cannot Be Periodic; this

would only happen if we repeated the same

word or words with exactly the same tone.

Nonperiodic Signal

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Signal

Corruption

87

Signal Corruption

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Bandwidth

89

The Bandwidth is a property of a

Medium:

The Bandwidth is the Difference

Between the Highest and the

Lowest Frequencies that the

medium can satisfactorily pass.

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91

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If a Periodic Signal is decomposed into five sine waves with

frequencies of 100, 300, 500, 700, and 900 Hz, what is its

bandwidth? Draw the spectrum, assuming all components have

a maximum amplitude of 10 V.

Example 3.10

Solution

Let fh be the highest frequency, fl the lowest frequency, and B

the bandwidth. Then

B = _________________ Hz800

A Periodic 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 frequencies of the same

amplitude.

Example 3.11

The spectrum contains all integer frequencies.

Solution

Let fh be the highest frequency, fl the lowest frequency, and B

the bandwidth. Then

B = _________________ Hz40

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Digital Signals

Bit Rate

&

Bit Interval

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98

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Bit Rate & Bit Interval

101

Transmission Of

Digital Signals

Based on Fourier Analysis (See Appendix E), a

Digital Signal is a Composite Analog Signal.

With a Digital Signal, the Bandwidth is Infinite.

We can intuitively come up with this concept when we consider a digital

signal. A digital signal, in the time domain, comprises connected vertical

and horizontal line segments. A vertical line in the time domain means a

frequency of infinity: a horizontal line in the time domain means a

frequency of zero. Going from a frequency of zero to a frequency of

infinity implies all frequencies in between are part of the domain.

Digital Signal Is A Composite Analog Signal

3.103

Baseband Transmission

A Baseband Transmission means sending a

Digital Signal over a Channel Without Changing

the Digital Signal to an Analog Signal

A Baseband Transmission requires a Low-Pass

Channel with a Bandwidth that starts at 0.

Low-Pass Channel

Illustrated above is a Low-Pass Channel that has a Wide

Bandwidth and one that has a Narrow Bandwidth.

In order to preserve the Exact Form of a Nonperiodic

Digital Signal with vertical segments vertical and horizontal

segments horizontal, we need to Send The Entire Spectrum

the continuous range of frequencies between zero and

infinity.

Case 1 - Low-Pass Channel that has a Wide Bandwidth-1

This would require a Dedicated Medium with an Infinite

Bandwidth between sender & receiver.

NOT POSSIBLE!

Fortunately, the Amplitudes of the Frequencies at the

Borders of the Bandwidth are SO SMALL that they can BE

IGNORED!

Case 1 - Low-Pass Channel that has a Wide Bandwidth-2

If we have Coax of Fiber Optic Cables with a Very Wide

Bandwidth, two stations Can Communicate, by using Digital

Signals, with Very Good Accuracy!

An example of a Dedicated Channel where the

Entire Bandwidth of the Medium is Used as One

Single Channel is a LAN.

Almost every wired LAN today uses a Dedicated

Channel for Two stations Communicating with each

other.

Example – Dedicated Chanel – Entire Bandwidth Used - LAN

In a Bus Topology LAN with multipoint connections,

Only Two Stations Can Communicate with each

other at Each Moment in Time (timesharing); the

other stations need to refrain from sending data.

Dedicated Chanel – LAN – Bus Topology

In a Star Topology LAN the Entire Channel

Between Each Station & The Hub Is Used To

Communicate between these two entities.

Dedicated Chanel

Dedicated Chanel – LAN – Star Topology

Almost every Wired LAN uses a Dedicated Chanel

for Two Stations Communicating with each other.

Dedicated Chanel – LAN – Star Topology

If we wanted to send an Analog Signal that roughly

Simulate the Digital Signal We Would Have To

Consider The Worst Case

Maximum Number of Signal Changes

0 1 0 1 0 1 …

1 0 1 0 1 0 …

Positive Peak = 1

Negative Peak = 0

Digital Signal BitRate - N

B = N/2

112

A Digital Signal is a

Composite Signal with an

Infinite Bandwidth.

113

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

Bandwidth Requirement

114

Digital vs. Analog

115

The Bit Rate and the

Bandwidth are

Proportional to Each Other.

116

The analog bandwidthof a medium is

expressed in hertz.

The digital bandwidth,

in bits per second.

Note:

3.117

Rough Approximation of a Digital Signal

118

Transmission

Impairment

119

3 Major Impairment Types

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Attenuation

121

122

Suppose a signal travels through a transmission

medium and its power is reduced to one half. This

means that P2 = 0.5 P1. In this case, the attenuation

(loss of power) can be calculated as

Example 3.26

A loss of 3 dB (−3 dB) is equivalent to losing one-half

the power.

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Distortion

126

127

Distortion

128

Noise

129

Signal to Noise Ratio shows the Ratio of

Signal Power to Noise Power

Power often expressed in Watts

S/N = Signal Power/Noise Power

Just a Simple Ratio

130

Signal to Noise Ratio (SNR or S/N)Bandwidth Requirement

Signal to Noise RatiodB shows the Ratio of Signal

Power to Noise Power in Decibels

S/NdB = 10 log10 (signal power/noise power)

Example 1: Signal power = 1000 watts, noise

power = 20 mw

Example 2: Signal power = 100 w, noise power =

0.000002w 131

Signal to Noise RatiodB

(SNRdB or S/NdB)

The loss in a cable is usually defined in decibels

per kilometer (dB/km). If the signal at the beginning

of a cable with −0.3 dB/km has a power of 2 mW,

what is the power of the signal at 5 km?

Solution

The loss in the cable in decibels is 5 × (−0.3) = −1.5

dB. We can calculate the power as

132

Signal to Noise RatiodB

(SNRdB or S/NdB)

133

Two cases of SNR: a high SNR and a low SNR

134

Shannon

Capacity

135

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137

Media

Comparison

Criteria

138Consider The Future – CAT 6 or 7?

139