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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