1. After reading Chapters 2 and 3 in the Dumas textbook, give a technical explanation as to why AM radio stations are much more susceptible to noise and interference compared to FM radio stations.

2. Modern technology has given us satellite radio. With 100+ stations and availability across the country, it has many benefits. However, there are other social implications such as local businesses losing an advertising outlet. HD Radio is a local high quality alternative, but doesn’t have nearly the reach. Give your opinion on either technology. If you use either technology, give your impressions.

Chapter 2:

The Modern Signal Carriers: Electricity, Light, Media, and Impairments

Principles of Computer Networks and Communications

M. Barry Dumas and Morris Schwartz

Principles of Computer Networks and Communications

Chapter 2

Objectives

Describe properties of electricity and electrical media

Describe signal impairments in electrical transmission

Differentiate between guided and unguided media, and list types of each

Explain the role of light in communications, including sources, media, and transmission method

Principles of Computer Networks and Communications

Chapter 2

Overview

Electricity and electromagnetic waves:

Carry data as signals that propagate through a medium (physical path)

Consist of one or more types of

transmission media, that is:

Bounded—confined by cables

Unbounded—through air or space

Are connected by switching and other equipment

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media

Ampere (Amp)

Magnitude of electrical current

Named after André Marie Ampère (French)

Volt (v)

Measure of electrical potential or pressure

Named after Alessandro Giuseppe Volta (Italian)

Ohm (ohm)

Measure of resistance to current flow

Named after Georg Simon Ohm (Greek)

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media

“Electricity consists of a flow of electrons

called a current

whose magnitude is measured in amperes,

and strength (pressure) in volts.”

1 Volt =

Electrical pressure required

to move 1 AMP of current

through 1 OHM of resistance

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media

“Electricity consists of a flow of electrons

called a current

whose magnitude is measured in amperes,

and strength (pressure) in volts.”

Conduction—the process of electron flow

Conductors accommodate electrical flow:

Copper

Aluminum

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media

“Electricity consists of a flow of electrons

called a current

whose magnitude is measured in amperes,

and strength (pressure) in volts.”

Resistance—opposition to electron flow

Insulators resist electrical flow:

Rubber

Plastic

Air

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media

“Electricity consists of a flow of electrons

called a current

whose magnitude is measured in amperes,

and strength (pressure) in volts.”

Semi-conductors

Usually act as insulators but can also behave as conductors

Basis for computer chips

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Current

Current is the flow of electrons (electricity)

Alternating Current (AC)

Continuously changing direction and magnitude

at a regular rate

Provided by utility companies

Most relevant to communications

Direct Current (DC)

Current in batteries

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Current

Cycle:

Traces a sine wave pattern

One complete journey from 0

through positive and negative strength

and back to 0

Measured in Hz (cycles per second)

Evident in both electricity and light waves

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Radiation

How does a signal travel through air and space?

Radiation

Varying current through a wire produces magnetic and electrical forces

(electromagnetic waves)

Electromagnetic waves radiate from the wire and mimic the pattern of change of the current in the wire

Induction

Radiated waves from the original wire create a current flow in a

second wire that mimics the pattern of the current in the original wire,

creating another electromagnetic wave

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Radiation

Some notes about radiated energy:

Power of radiated energy depends on the

power of the current that creates it

The stronger the original energy, the greater the current

Power attenuates (drops off) as it travels

The farther the current goes in a wire, the weaker it gets

Radiated waves disperse (spread out) as they travel

Spreading the waves dilutes the wave power

Induced current is always weaker

than the current that induced it!

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Radiation

What purpose do we want our wire to serve?

Wire carrying signals within our own network

Objectives:

Conserve signal energy (minimize radiation)

Protect our signals from currents induced by other wires

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Radiation

What purpose do we want our wire to serve?

Wire transmitting as an antenna

Objective:

Radiate as much signal energy as possible

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Radiation

What purpose do we want our wire to serve?

Wire receiving as an antenna

Objective:

Absorb as much as the radiated signals as possible

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Pioneers

Michael Faraday (British physicist, chemist)

Built on earlier work of Hans Christian Oersted

Discovered phenomenon of electromagnetic induction

James Maxwell (Scottish mathematician, scientist)

Discovered propagation speed of an electromagnetic field

is equal to speed of light

Discovered light is a form of electromagnetic radiation

Heinrich Hertz (German physicist)

Discovered electricity could be propagated as

electromagnetic waves

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Waves

Wave:

“Regularly recurring pattern

that moves away

from the force that creates it”

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Waves

Period or, cycle

Time it takes a sine wave to trace one complete pattern

Periodic Waves are waves with period patterns

that repeat over time

T

Figure 2.1

T

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Waves

Period cycle (sec)

Time it takes a sine wave to trace one complete pattern

Periodic waves are waves with period patterns that repeat over time

Frequency (Hz)

Number of times pattern repeats in 1 second

Inversely proportional to period

Wavelength

Distance a wave travels in one cycle

T

λ

f = 1 / T

Figure 2-2

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Waves

Wavelength calculation

λ =

νm

T

Wavelength

Velocity of light through the medium

One wave period

(seconds)

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Waves

Wavelength calculation in comms

λ =

Wavelength

Velocity of light through the medium

Wave frequency

(cycles/second)

νm

1

f

Principles of Computer Networks and Communications

Chapter 2

Properties of Electricity and Electrical Media—Waves

Speed of Light:

“In a vacuum,

all electromagnetic radiation

travels at the speed of light.”

In other media—that is, not in a vacuum— electromagnetic radiation

will travel slower

300,000 (km/sec)

186,000 (miles/sec)

Principles of Computer Networks and Communications

Chapter 2

Signal Impairments in Electrical Transmission

Noise

Unwelcome energy in the transmission media

Distortion

Unwanted changes in signal shapes

Caused by interactions between signals and media

Attenuation

Signal distortion from energy lost as the signal travels

Caused by resistance of the medium to electrical flow

Limiting factor of network cable length

Principles of Computer Networks and Communications

Chapter 2

Signal Impairments in Electrical Transmission [cont.]

Thermal Noise

Background noise, white noise, Gaussian noise, hiss

Unwanted energy in the transmission line

Cannot be eliminated!

Principles of Computer Networks and Communications

Chapter 2

Signal Impairments in Electrical Transmission [cont.]

Electromagnetic Interference (EMI)

Unwanted energy induced by radiation from an external source

Affects wireless signals

Crosstalk

Energy induced in a wire by signals from another wire

Impulse noise (“spikes”)

Unpredictable!

Large, sudden power surge

Usually very short duration

Principles of Computer Networks and Communications

Chapter 2

Signal Impairments in Electrical Transmission [cont.]

Delay distortion

From the way wires affect signal velocity

Frequency components of signals arrive at the receiver at different times

Limiting factor of network cable length

Intermodulation distortion

From non-linearity in a comms system

Harmonics (multiples of original signals) appear that were not present in the original signal, making it difficult to distinguish the original from the noise

Principles of Computer Networks and Communications

Chapter 2

Common Guided Electrical Media

Twisted pair

Most commonly used

1-wire carries signal; other wire carries ground

Wires are insulated and twisted

Number of twists per inch = twist rate

Coaxial

Two conductors are concentric (not twisted)

Wire conductor running down the center of the cable is surrounded by conducting braided metal or foil

protected by an outer jacket

Principles of Computer Networks and Communications

Chapter 2

Common Guided Electrical Media

Twisted pair—why twist?

Minimize prospect of parallel wires:

Induced currents are weakest where wires are not parallel

Twisting reduces crosstalk from external radiation

The greater the twist rate difference between pairs,

the less the intra-cable crosstalk

Principles of Computer Networks and Communications

Chapter 2

Common Guided Electrical Media

Types of twisted pair

UTP—Unshielded Twisted Pair

Most common

Widely used for telephone connections and

Ethernet LANs

STP—Shielded Twisted Pair

Added conductive shielding reduces external noise

Wire mesh or foil wrapped around twisted pair

Works in 2 directions:

Stops external EMI from distorting signals

Prevents internal EMI from distorting signals in other cables

Widely used in token ring LANs

Principles of Computer Networks and Communications

Chapter 2

Common Guided Electrical Media

Advantages of coaxial vs. twisted pair

Much greater capacity

Relatively immune from external interference

Disadvantages of coaxial vs. twisted pair

More expensive

Bulkier

Difficulty to modify

Difficult install around sharp bends

Principles of Computer Networks and Communications

Chapter 2

Common Guided Electrical Media

Backbone:

“A high-capacity common link to which

networks and communications devices

are attached. ”

A backbone must have significantly greater capacity than the networks it connects

Principles of Computer Networks and Communications

Chapter 2

Unguided Media and Antennas

All unguided media use antennas for transmission and receipt of signals

Anything that conducts electricity can be the transmitter or the recipient of induced radiation

There are 3 electromagnetic radiation (EMR) groupings relevant to communications

Infrared light

Microwaves

Radio waves

Principles of Computer Networks and Communications

Chapter 2

EMR Frequency Bands for Communications

Category Frequency

(Hz) Wavelength

(m) Type

Visible 7.5 x 1014

4 x 1014 7 x 10-7 to 4 x 10-7 Line of sight

Microwave 3 x 1011

3 x 109 10-1 to 10-3 Line of sight

Infrared 4 x 1014

3 x 1011 10-1 to 7 x 10-7 Line of sight

Radio < 3 x 109 10-1 and greater Omni-directional
Principles of Computer Networks and Communications
Chapter 2
EMR Frequency Bands for Communications
Category Frequency
(Hz) Wavelength
(m) Type
Visible 7.5 x 1014
4 x 1014 7 x 10-7 to 4 x 10-7 Line of sight
Microwave 3 x 1011
3 x 109 10-1 to 10-3 Line of sight
Infrared 4 x 1014
3 x 1011 10-1 to 7 x 10-7 Line of sight
Radio < 3 x 109 10-1 and greater Omni-directional
The higher the frequency of the EMR
the more directional/more focused
Principles of Computer Networks and Communications
Chapter 2
EMR Frequency Bands for Communications
Category Frequency
(Hz) Wavelength
(m) Type
Visible 7.5 x 1014
4 x 1014 7 x 10-7 to 4 x 10-7 Line of sight
Microwave 3 x 1011
3 x 109 10-1 to 10-3 Line of sight
Infrared 4 x 1014
3 x 1011 10-1 to 7 x 10-7 Line of sight
Radio < 3 x 109 10-1 and greater Omni-directional
Lower frequency EMR is omni-directional,
propagating in all directions at once
Principles of Computer Networks and Communications
Chapter 2
Unguided Media and Antennas— Electromagnetic Radiation (EMR)
Ways to communicate without using line of sight:
Depending on the material, EMR can
Pass through [IR—TV remotes]
Be refracted [RF—cell phones]
Be diffracted [RF—cell phones]
Be reflected [RF—cell phones]
Principles of Computer Networks and Communications
Chapter 2
The Basic Nature of Light
Light behaves:
As a particle (quantum optics)
With motions like waves of energy (wave optics)
Light will change direction when it encounters another medium
When sunlight strikes the surface of a lake,
it becomes reflected or refracted (bent)
Principles of Computer Networks and Communications
Chapter 2
The Basic Nature of Light
What is diffraction?
“When an electromagnetic signal hits the edge of an object that is large compared to the signal wavelength, the signal propagates in many directions, with the edge as the apparent source.”
Light diffraction
Wave phenomena
Has direct application in communication by light
Separates a light beam into its component wavelengths
Each wavelength can carry information independently and simultaneously
Principles of Computer Networks and Communications
Chapter 2
Common Media for Use with Light
Optical fiber
Highly refined pure silica
Very low attenuation
(“half power point”
—point of travel
where a signal has lost half of its original power)
Principles of Computer Networks and Communications
Chapter 2
Common Media for Use with Light
Optical Fiber Cable
Core—signal carrying fiber runs through the cable
Cladding—surrounds the core; keeps light from reflecting
Coating—covers cladding; absorbs light escaping the core
Typically, hundreds and even thousands of fibers are bundled together
Figure 2.5
Figure 2.4
Principles of Computer Networks and Communications
Chapter 2
Light Sources for
Computer Communications
All communications systems have 3 related components:
Signal source
Medium to conduct the signals
Receiver to accept the information
For optical communication systems:
LEDs and Lasers
Light Detector
Optical Fiber
Principles of Computer Networks and Communications
Chapter 2
Lighting up the Core
Source light rays can enter the core from 3 ways:
(Ideal) light rays point straight through the core
Refraction angle is 90o+ at core/cladding interface
(Worst) refraction angle is < 90o; light is absorbed by coating
Types of optical fiber
Multimode
Step index—subject to absorption; suitable for short distances
Graded index—partial solution to zigzag cable problem
Single mode—long distance/high-speed comms
Principles of Computer Networks and Communications
Chapter 2
Signal Impairments in Light Transmission
Absorption
Impurities in the fiber during manufacturer
Shorter wavelengths have more absorption
Scattering
Small contaminants and density differences in the core
Bends
Macro-bending—can be seen; cable is too severely bent
Micro-bending—mishandling; kinks in the cable
Coupling
Splicing cables and attaching cable to connectors
Principles of Computer Networks and Communications
Chapter 3:
Signal Fundamentals
Principles of Computer Networks and Communications
M. Barry Dumas and Morris Schwartz
Principles of Computer Networks and Communications
Chapter 3
Objectives
Differentiate between signals and information
Describe characteristics, strengths, and weaknesses of analog and digital signals
Understand the relationship between signals and sine waves
Understand the role of noise in a system
Differentiate between signal amplification and signal regeneration
Describe the elements that are involved in and the methods for determining system bandwidth
Principles of Computer Networks and Communications
Chapter 3
Overview
There are 2 basic forms of information:
Analog
Produced by real world events (e.g., voice, music)
Can take on infinite values created from the event
Digital
Produced by computers
Only two values: 1 or 0
There are also 2 basic forms of signals (see above)
Principles of Computer Networks and Communications
Chapter 3
Overview [cont.]
Signals (analog or digital)
carry information (analog or digital)
Leaving 4 possibilities:
Analog information over analog signals
Analog information over digital signals
Digital information over analog signals
Digital information over digital signals
Principles of Computer Networks and Communications
Chapter 3
Analog Signals
Analog signals
Are continuous
Can take whatever shape or power needed to represent information
Can assume an infinite number of values
Cannot change shape instantaneously
Principles of Computer Networks and Communications
Chapter 3
Analog Signals
Analog signals
Are continuous
Cannot change shape instantaneously
ALL Signals are combinations
of simple sine waves.
Sine waves
Principles of Computer Networks and Communications
Chapter 3
Analog Signals—Sine Waves
3 characteristics of sine waves:
Amplitude
Frequency
Phase
Principles of Computer Networks and Communications
Chapter 3
Analog Signals—Sine Wave Equation
s(t) =
Amplitude
Wave location
at time (t)
Phase
A
sin (2 π f t + φ )
Wave frequency
(cycles/second)
Time
Principles of Computer Networks and Communications
Chapter 3
Analog Signals—Sine Waves
Characteristics: amplitude, frequency, phase
Two sine waves with
same frequency
same phase
different amplitudes
S2 amplitude > S1 amplitude

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

Characteristics: amplitude, frequency, phase

Two sine waves with

different frequencies

same phase

same amplitudes

S1 frequency > S2 frequency

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

Characteristics: amplitude, frequency, phase

Two sine waves with

same frequencies

different phases

same amplitudes

Same wave shifted to right

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

Consolidated: amplitude, frequency, phase

Principles of Computer Networks and Communications

Chapter 3

Analog Signals as Digital Data

Ways to represent digital data (1s or 0s) using analog signals:

Vary peak amplitudes (A1 , A2)

Vary frequencies (f1 , f2)

Vary phases (φ1 , φ2)

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Advantages

Faithful copy of original analog signal

Conceptually represent real world events

Can travel far without shape distortion from the medium

Easy to create, handle

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Disadvantage

Noise!

For computer communications,

susceptibility to damage from noise

outweighs all other advantages

for analog signals.

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Noise

Adds to a signal

Changes original signal shape

Must be separated out

to recover original signal

Cannot be known accurately

(because shapes/strengths are random)

Reconstructing a noise-deformed analog signal exactly

is an impossible task!

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Characteristics

Digital signals are discrete

Voltage is limited to small set of values

Signal values change instantaneously

No time elapses between amplitude changes

Good approximations of real-world events

Figure 3.3 – Some Digital Signal Shapes

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Advantages

Can be restored to original shape

even when corrupted by noise

Natural and intuitive for representing computer information

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Advantages

Can be restored to original shape

even when corrupted by noise

Natural and intuitive for representing computer information [1s or 0s]

Fig 3.4

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Disadvantages

Digital signals never exactly represent real-world (analog) data

Digital signals cannot travel as far through a medium without being distorted

[Former Disadvantage] Technology to handle digital signals is more complex

No longer an issue because costs have come down!

Digital signals are standard in

computer communications

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

Why amplify and regenerate?

Attenuation

Form of distortion

Signal energy is lost as signal travels through the medium (i.e., original signal shape is deformed)

All signals suffer some attenuation

as they travel

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

Where to amplify and regenerate?

Signals are intercepted at points where they are

still accurately recognizable. Here, they are strengthened, and sent on.

How many interception points? Depends on:

Type of signal

Media characteristics

Distance

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

Noise and other distortions change original signal shape and affect every signal

Characteristics of an amplifier

Signal enters and exits with the same shape

Signal is increased in strength (including noise) and sent

Characteristics of a regenerator

Discerns original signal shape

Re-creates original signal (without noise) and sends

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

s(t) =

Equation for a sine wave

A

sin (2 π f t + φ)

s(t) =

Equation for a sine wave after amplification by a factor of 10

A

sin (2 π f t + φ)

+ 10 other distortions

10

+ 10 noise

Amplified components

Principles of Computer Networks and Communications

Chapter 3

Signal Analysis

Signals that carry information must travel:

Over thousands of miles of media

Through a variety of equipment

For a communications system to be useful:

The media and equipment

that interact with the signals

must not change the signals

beyond proper recognition

Principles of Computer Networks and Communications

Chapter 3

Signal Analysis

Beam’s spectrum

When a beam of light is separated into its component colors

Signal spectrum

When a signal (analog or digital) is separated into its elementary signals

sine waves

Principles of Computer Networks and Communications

Chapter 3

Signal Analysis

2 Methods to determine the spectrum of a signal:

Mathematical analysis—using Fourier’s technique to mathematically describe a signal

Spectrum analyzer—graphic display of a “live” signal showing the sine waves that make up its spectrum

Principles of Computer Networks and Communications

Chapter 3

Historical Note—Newton and Sunlight

Isaac Newton (1642–1727) observed: Streaming sunlight through a prism created a rainbow of colors

Colors through a second prism could not be further decomposed; these were called primary colors.

Colors could be recombined into white light

by passing through an inverted prism

Newton concluded that white light was actually composed of all the colors blended together

Principles of Computer Networks and Communications

Chapter 3

Historical Note—Fourier and Decomposition of Signals

Jean Baptist Fourier (1768–1830) proved:

Heat flows were a form of signal flows

Any signal could be constructed by a combination of sinusoids

Fourier series for periodic signals

Fourier transform for aperiodic signals

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

To see how a signal evolves over time,

use a 2-dimensional time domain view

Fig 3.1

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

To see sine wave components that create a signal’s spectrum, use a 2-dimensional frequency domain view

Fig 3.8

Frequency components

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

What is bandwidth?

For a signal, bandwidth is the significant

range of frequencies in the spectrum

For a system, bandwidth is the usable range of frequencies in the spectrum

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Relationship between network (system) capability and signal requirement

Bm bandwidth of signal to be carried

If Bm ≤ BS

BS bandwidth of network system

Network can carry the signals

If Bm > BS

Network can not carry the signals

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Significant range of frequencies in a signal’s spectrum:

Bm = fh – fl

Bandwidth of signal

we need to carry

Highest significant

frequency in spectrum

Lowest significant

frequency in spectrum

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Significant range of frequencies in a signal’s spectrum:

Fig 3.9

Not

significant

Not

significant

significant

Bandwidth tells us the range but does not tell us where the spectrum is

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Initial power level for all signals

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Actual power level for all signals

after attenuation

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Signals above

half power level

after attenuation

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Signal bandwidth

after attenuation

20 – 5 = 15 KHz

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Attenuation

Not uniform for all frequencies

Frequencies at ends of spectrum attenuate more quickly than frequencies in the middle

Higher frequencies attenuate more quickly than lower frequencies

Attenuation for different frequencies is a characteristic of the wire

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Wire bandwidth—“half power rule”

“To be called usable, the power of the frequency received should be at least one half

of the power sent.”

Bandwidth calculation

Difference between the highest and lowest frequencies received…

Whose powers are at least half of that sent

Bm = fh – fl

Principles of Computer Networks and Communications

Chapter 3:

Signal Fundamentals

Principles of Computer Networks and Communications

M. Barry Dumas and Morris Schwartz

Principles of Computer Networks and Communications

Chapter 3

Objectives

Differentiate between signals and information

Describe characteristics, strengths, and weaknesses of analog and digital signals

Understand the relationship between signals and sine waves

Understand the role of noise in a system

Differentiate between signal amplification and signal regeneration

Describe the elements that are involved in and the methods for determining system bandwidth

Principles of Computer Networks and Communications

Chapter 3

Overview

There are 2 basic forms of information:

Analog

Produced by real world events (e.g., voice, music)

Can take on infinite values created from the event

Digital

Produced by computers

Only two values: 1 or 0

There are also 2 basic forms of signals (see above)

Principles of Computer Networks and Communications

Chapter 3

Overview [cont.]

Signals (analog or digital)

carry information (analog or digital)

Leaving 4 possibilities:

Analog information over analog signals

Analog information over digital signals

Digital information over analog signals

Digital information over digital signals

Principles of Computer Networks and Communications

Chapter 3

Analog Signals

Analog signals

Are continuous

Can take whatever shape or power needed to represent information

Can assume an infinite number of values

Cannot change shape instantaneously

Principles of Computer Networks and Communications

Chapter 3

Analog Signals

Analog signals

Are continuous

Cannot change shape instantaneously

ALL Signals are combinations

of simple sine waves.

Sine waves

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

3 characteristics of sine waves:

Amplitude

Frequency

Phase

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Wave Equation

s(t) =

Amplitude

Wave location

at time (t)

Phase

A

sin (2 π f t + φ )

Wave frequency

(cycles/second)

Time

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

Characteristics: amplitude, frequency, phase

Two sine waves with

same frequency

same phase

different amplitudes

S2 amplitude > S1 amplitude

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

Characteristics: amplitude, frequency, phase

Two sine waves with

different frequencies

same phase

same amplitudes

S1 frequency > S2 frequency

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

Characteristics: amplitude, frequency, phase

Two sine waves with

same frequencies

different phases

same amplitudes

Same wave shifted to right

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

Consolidated: amplitude, frequency, phase

Principles of Computer Networks and Communications

Chapter 3

Analog Signals as Digital Data

Ways to represent digital data (1s or 0s) using analog signals:

Vary peak amplitudes (A1 , A2)

Vary frequencies (f1 , f2)

Vary phases (φ1 , φ2)

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Advantages

Faithful copy of original analog signal

Conceptually represent real world events

Can travel far without shape distortion from the medium

Easy to create, handle

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Disadvantage

Noise!

For computer communications,

susceptibility to damage from noise

outweighs all other advantages

for analog signals.

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Noise

Adds to a signal

Changes original signal shape

Must be separated out

to recover original signal

Cannot be known accurately

(because shapes/strengths are random)

Reconstructing a noise-deformed analog signal exactly

is an impossible task!

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Characteristics

Digital signals are discrete

Voltage is limited to small set of values

Signal values change instantaneously

No time elapses between amplitude changes

Good approximations of real-world events

Figure 3.3 – Some Digital Signal Shapes

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Advantages

Can be restored to original shape

even when corrupted by noise

Natural and intuitive for representing computer information

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Advantages

Can be restored to original shape

even when corrupted by noise

Natural and intuitive for representing computer information [1s or 0s]

Fig 3.4

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Disadvantages

Digital signals never exactly represent real-world (analog) data

Digital signals cannot travel as far through a medium without being distorted

[Former Disadvantage] Technology to handle digital signals is more complex

No longer an issue because costs have come down!

Digital signals are standard in

computer communications

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

Why amplify and regenerate?

Attenuation

Form of distortion

Signal energy is lost as signal travels through the medium (i.e., original signal shape is deformed)

All signals suffer some attenuation

as they travel

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

Where to amplify and regenerate?

Signals are intercepted at points where they are

still accurately recognizable. Here, they are strengthened, and sent on.

How many interception points? Depends on:

Type of signal

Media characteristics

Distance

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

Noise and other distortions change original signal shape and affect every signal

Characteristics of an amplifier

Signal enters and exits with the same shape

Signal is increased in strength (including noise) and sent

Characteristics of a regenerator

Discerns original signal shape

Re-creates original signal (without noise) and sends

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

s(t) =

Equation for a sine wave

A

sin (2 π f t + φ)

s(t) =

Equation for a sine wave after amplification by a factor of 10

A

sin (2 π f t + φ)

+ 10 other distortions

10

+ 10 noise

Amplified components

Principles of Computer Networks and Communications

Chapter 3

Signal Analysis

Signals that carry information must travel:

Over thousands of miles of media

Through a variety of equipment

For a communications system to be useful:

The media and equipment

that interact with the signals

must not change the signals

beyond proper recognition

Principles of Computer Networks and Communications

Chapter 3

Signal Analysis

Beam’s spectrum

When a beam of light is separated into its component colors

Signal spectrum

When a signal (analog or digital) is separated into its elementary signals

sine waves

Principles of Computer Networks and Communications

Chapter 3

Signal Analysis

2 Methods to determine the spectrum of a signal:

Mathematical analysis—using Fourier’s technique to mathematically describe a signal

Spectrum analyzer—graphic display of a “live” signal showing the sine waves that make up its spectrum

Principles of Computer Networks and Communications

Chapter 3

Historical Note—Newton and Sunlight

Isaac Newton (1642–1727) observed: Streaming sunlight through a prism created a rainbow of colors

Colors through a second prism could not be further decomposed; these were called primary colors.

Colors could be recombined into white light

by passing through an inverted prism

Newton concluded that white light was actually composed of all the colors blended together

Principles of Computer Networks and Communications

Chapter 3

Historical Note—Fourier and Decomposition of Signals

Jean Baptist Fourier (1768–1830) proved:

Heat flows were a form of signal flows

Any signal could be constructed by a combination of sinusoids

Fourier series for periodic signals

Fourier transform for aperiodic signals

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

To see how a signal evolves over time,

use a 2-dimensional time domain view

Fig 3.1

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

To see sine wave components that create a signal’s spectrum, use a 2-dimensional frequency domain view

Fig 3.8

Frequency components

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

What is bandwidth?

For a signal, bandwidth is the significant

range of frequencies in the spectrum

For a system, bandwidth is the usable range of frequencies in the spectrum

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Relationship between network (system) capability and signal requirement

Bm bandwidth of signal to be carried

If Bm ≤ BS

BS bandwidth of network system

Network can carry the signals

If Bm > BS

Network can not carry the signals

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Significant range of frequencies in a signal’s spectrum:

Bm = fh – fl

Bandwidth of signal

we need to carry

Highest significant

frequency in spectrum

Lowest significant

frequency in spectrum

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Significant range of frequencies in a signal’s spectrum:

Fig 3.9

Not

significant

Not

significant

significant

Bandwidth tells us the range but does not tell us where the spectrum is

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Initial power level for all signals

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Actual power level for all signals

after attenuation

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Signals above

half power level

after attenuation

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Signal bandwidth

after attenuation

20 – 5 = 15 KHz

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Attenuation

Not uniform for all frequencies

Frequencies at ends of spectrum attenuate more quickly than frequencies in the middle

Higher frequencies attenuate more quickly than lower frequencies

Attenuation for different frequencies is a characteristic of the wire

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Wire bandwidth—“half power rule”

“To be called usable, the power of the frequency received should be at least one half

of the power sent.”

Bandwidth calculation

Difference between the highest and lowest frequencies received…

Whose powers are at least half of that sent

Bm = fh – fl

Principles of Computer Networks and Communications

Chapter 3:

Signal Fundamentals

Principles of Computer Networks and Communications

M. Barry Dumas and Morris Schwartz

Principles of Computer Networks and Communications

Chapter 3

Objectives

Differentiate between signals and information

Describe characteristics, strengths, and weaknesses of analog and digital signals

Understand the relationship between signals and sine waves

Understand the role of noise in a system

Differentiate between signal amplification and signal regeneration

Describe the elements that are involved in and the methods for determining system bandwidth

Principles of Computer Networks and Communications

Chapter 3

Overview

There are 2 basic forms of information:

Analog

Produced by real world events (e.g., voice, music)

Can take on infinite values created from the event

Digital

Produced by computers

Only two values: 1 or 0

There are also 2 basic forms of signals (see above)

Principles of Computer Networks and Communications

Chapter 3

Overview [cont.]

Signals (analog or digital)

carry information (analog or digital)

Leaving 4 possibilities:

Analog information over analog signals

Analog information over digital signals

Digital information over analog signals

Digital information over digital signals

Principles of Computer Networks and Communications

Chapter 3

Analog Signals

Analog signals

Are continuous

Can take whatever shape or power needed to represent information

Can assume an infinite number of values

Cannot change shape instantaneously

Principles of Computer Networks and Communications

Chapter 3

Analog Signals

Analog signals

Are continuous

Cannot change shape instantaneously

ALL Signals are combinations

of simple sine waves.

Sine waves

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

3 characteristics of sine waves:

Amplitude

Frequency

Phase

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Wave Equation

s(t) =

Amplitude

Wave location

at time (t)

Phase

A

sin (2 π f t + φ )

Wave frequency

(cycles/second)

Time

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

Characteristics: amplitude, frequency, phase

Two sine waves with

same frequency

same phase

different amplitudes

S2 amplitude > S1 amplitude

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

Characteristics: amplitude, frequency, phase

Two sine waves with

different frequencies

same phase

same amplitudes

S1 frequency > S2 frequency

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

Characteristics: amplitude, frequency, phase

Two sine waves with

same frequencies

different phases

same amplitudes

Same wave shifted to right

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Sine Waves

Consolidated: amplitude, frequency, phase

Principles of Computer Networks and Communications

Chapter 3

Analog Signals as Digital Data

Ways to represent digital data (1s or 0s) using analog signals:

Vary peak amplitudes (A1 , A2)

Vary frequencies (f1 , f2)

Vary phases (φ1 , φ2)

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Advantages

Faithful copy of original analog signal

Conceptually represent real world events

Can travel far without shape distortion from the medium

Easy to create, handle

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Disadvantage

Noise!

For computer communications,

susceptibility to damage from noise

outweighs all other advantages

for analog signals.

Principles of Computer Networks and Communications

Chapter 3

Analog Signals—Noise

Adds to a signal

Changes original signal shape

Must be separated out

to recover original signal

Cannot be known accurately

(because shapes/strengths are random)

Reconstructing a noise-deformed analog signal exactly

is an impossible task!

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Characteristics

Digital signals are discrete

Voltage is limited to small set of values

Signal values change instantaneously

No time elapses between amplitude changes

Good approximations of real-world events

Figure 3.3 – Some Digital Signal Shapes

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Advantages

Can be restored to original shape

even when corrupted by noise

Natural and intuitive for representing computer information

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Advantages

Can be restored to original shape

even when corrupted by noise

Natural and intuitive for representing computer information [1s or 0s]

Fig 3.4

Principles of Computer Networks and Communications

Chapter 3

Digital Signals—Disadvantages

Digital signals never exactly represent real-world (analog) data

Digital signals cannot travel as far through a medium without being distorted

[Former Disadvantage] Technology to handle digital signals is more complex

No longer an issue because costs have come down!

Digital signals are standard in

computer communications

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

Why amplify and regenerate?

Attenuation

Form of distortion

Signal energy is lost as signal travels through the medium (i.e., original signal shape is deformed)

All signals suffer some attenuation

as they travel

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

Where to amplify and regenerate?

Signals are intercepted at points where they are

still accurately recognizable. Here, they are strengthened, and sent on.

How many interception points? Depends on:

Type of signal

Media characteristics

Distance

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

Noise and other distortions change original signal shape and affect every signal

Characteristics of an amplifier

Signal enters and exits with the same shape

Signal is increased in strength (including noise) and sent

Characteristics of a regenerator

Discerns original signal shape

Re-creates original signal (without noise) and sends

Principles of Computer Networks and Communications

Chapter 3

Signal Amplification and Regeneration

s(t) =

Equation for a sine wave

A

sin (2 π f t + φ)

s(t) =

Equation for a sine wave after amplification by a factor of 10

A

sin (2 π f t + φ)

+ 10 other distortions

10

+ 10 noise

Amplified components

Principles of Computer Networks and Communications

Chapter 3

Signal Analysis

Signals that carry information must travel:

Over thousands of miles of media

Through a variety of equipment

For a communications system to be useful:

The media and equipment

that interact with the signals

must not change the signals

beyond proper recognition

Principles of Computer Networks and Communications

Chapter 3

Signal Analysis

Beam’s spectrum

When a beam of light is separated into its component colors

Signal spectrum

When a signal (analog or digital) is separated into its elementary signals

sine waves

Principles of Computer Networks and Communications

Chapter 3

Signal Analysis

2 Methods to determine the spectrum of a signal:

Mathematical analysis—using Fourier’s technique to mathematically describe a signal

Spectrum analyzer—graphic display of a “live” signal showing the sine waves that make up its spectrum

Principles of Computer Networks and Communications

Chapter 3

Historical Note—Newton and Sunlight

Isaac Newton (1642–1727) observed: Streaming sunlight through a prism created a rainbow of colors

Colors through a second prism could not be further decomposed; these were called primary colors.

Colors could be recombined into white light

by passing through an inverted prism

Newton concluded that white light was actually composed of all the colors blended together

Principles of Computer Networks and Communications

Chapter 3

Historical Note—Fourier and Decomposition of Signals

Jean Baptist Fourier (1768–1830) proved:

Heat flows were a form of signal flows

Any signal could be constructed by a combination of sinusoids

Fourier series for periodic signals

Fourier transform for aperiodic signals

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

To see how a signal evolves over time,

use a 2-dimensional time domain view

Fig 3.1

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

To see sine wave components that create a signal’s spectrum, use a 2-dimensional frequency domain view

Fig 3.8

Frequency components

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

What is bandwidth?

For a signal, bandwidth is the significant

range of frequencies in the spectrum

For a system, bandwidth is the usable range of frequencies in the spectrum

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Relationship between network (system) capability and signal requirement

Bm bandwidth of signal to be carried

If Bm ≤ BS

BS bandwidth of network system

Network can carry the signals

If Bm > BS

Network can not carry the signals

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Significant range of frequencies in a signal’s spectrum:

Bm = fh – fl

Bandwidth of signal

we need to carry

Highest significant

frequency in spectrum

Lowest significant

frequency in spectrum

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Significant range of frequencies in a signal’s spectrum:

Fig 3.9

Not

significant

Not

significant

significant

Bandwidth tells us the range but does not tell us where the spectrum is

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Initial power level for all signals

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Actual power level for all signals

after attenuation

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Signals above

half power level

after attenuation

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

“For our signal to pass through a communications system successfully, all the frequencies in its spectrum must be able to pass successfully.”

Fig 3.10

Signal bandwidth

after attenuation

20 – 5 = 15 KHz

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Attenuation

Not uniform for all frequencies

Frequencies at ends of spectrum attenuate more quickly than frequencies in the middle

Higher frequencies attenuate more quickly than lower frequencies

Attenuation for different frequencies is a characteristic of the wire

Principles of Computer Networks and Communications

Chapter 3

Bandwidth

Wire bandwidth—“half power rule”

“To be called usable, the power of the frequency received should be at least one half

of the power sent.”

Bandwidth calculation

Difference between the highest and lowest frequencies received…

Whose powers are at least half of that sent

Bm = fh – fl

Principles of Computer Networks and Communications

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