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CSCD 433/533Network ProgrammingWinter 2017
Lecture 3Physical Layer Line Coding
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Physical Layer Topics
• Motivation for studying this topic• Definitions of terms• Analog vs Digital• Characteristics of physical media• Wireless
• Reading: Tanenbaum, Chapter 2
2
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Motivation• Why study the physical layer?• Need to know basic data transmission concepts • Understand physical layer to understand media
influence on network performance
• Answer Questions such as:• What transmission speed is possible
with various media?• Where and how are errors
introduced?
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Physical Layer - Purpose• Transmit information across a distance• Source
Transmit bits from one point to another Encode bits onto a signal
• DestinationReceive signals, interpret or extract bits
What is a Signal?1. Mechanism used to carry information over time or distance2. Sign or gesture giving information3. Sequence of electrical or optical impulses or waves
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Signals
• Examples• Physical gesture, wave, hand signal• Morse code• Sound: vary tone, loudness or duration• Flags• Smoke• Electical voltages
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Transmission 1. Action of conveying electrical or optical signals from 1 point to 1 or more other points in space2. Process of sending information from 1 point to another
What do you need for a Transmission System ?• Medium for signal transfer• Method to transform signal to appropriate form• Way to transmit the signal• Way to remove, receive or detect the signal
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Transmission Not Perfect Along the way, signals are subject to less than
favorable conditions Distance affects the signal
Loss of signal strength with distance Recall what that is called?
Attenuation Noise affects the signal
Line noise obscures the signal Can make it impossible to send information
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Line Noise
Defined Unwanted electrical or electromagnetic energy
that degrades quality of signals and data
– External noise from appliances in area, from electrical transformers, from atmosphere (wired connections)
Communications engineers are constantly striving to develop better ways to deal with noise
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Bandwidth
What is bandwidth? More than one definition?Network bandwidth defined
• Bandwidth is same as data transfer rate, amount of data that can be carried from one point to another in a given time period
• Network bandwidth is usually expressed in bits per second (bps)
Signal Processing bandwidth defined• Bandwidth is range of frequencies carried by a
channel.• Difference between highest-frequency signal
component and the lowest-frequency signal component
• Bandwidth is measured in hertz (cycles per second)
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What is Attenuation?Attenuation is …
- Reduction of signal strength during transmission - Attenuation is gradual loss in intensity of any kind of flux through a medium
Ex. Reduction in signal strength from length of phone line Sunlight is attenuated by dark glasses, and X-rays are attenuated by lead.
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Attenuation Continued- Attenuation is measured in decibels - Decibel (dB) is used to measure sound, but also widely used in electronics, signals and communication Decibel (dB) measures relative strengths of two signals or a signal at two different points - The lower the Attenuation the stronger the received signal Note that decibel is negative if a signal is attenuated and positive if a signal
is amplified.
Attenuation can be measured by: dB = 10 log10 (P2 /P1) where P1 and P2 are the powers of a signal at points 1 and 2, respectively.
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ExamplesWhat happens if we lose power? p1 < p2dB = 10 log (p1/p2) p1=1 and p2=10dB = 10 log (0.10) dB = 10 × (–1) = –10 decibelsNegative sign tells us there is a loss of power, known as attenuation
What if there is a gain of power? p1 > p2 dB = 10 log (20/10) = 10 log (2) = 10 x .3 = 3.0 dBPositive sign means a gain of power,known as amplification
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Transmission Media
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Twisted Pair• Oldest transmission medium
• Historical use, Phone systems • Two insulated Copper wires
• Wires twisted together• Straight they would interfere
• To reduce electromagnetic induction between pairs of wires, two insulated copper wires are twisted around each other
• Twisted pair cabling – Several varieties• Category 5 – Two insulated wires – 4 pairs
• Encased in a protective plastic sheath• Category 7 – Higher quality yet• Has added shielding on individual twisted pairs• Helps reduce external interference and crosstalk
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Twisted Pair Bit Rates Twisted pairs can provide high
bit rates at short distances Asymmetric Digital Subscriber
Loop (ADSL) High-speed Internet Access Lower 3 kHz for voice Upper band for data
Much higher rates possible at shorter distances Strategy for telephone
companies is to bring fiber close to home & then twisted pair
Higher-speed access + video Still widely used in buildings and
telephone applications
Data rates of 24-gauge twisted pair
1000 feet, 300 m51.840 MbpsSTS-1
3000 feet, 0.9 km25.920 Mbps1/2 STS-1
4500 feet, 1.4 km12.960 Mbps1/4 STS-1
12,000 feet, 3.7 km6.312 MbpsDS2
18,000 feet, 5.5 km1.544 MbpsT-1
DistanceData RateStandard
STS - Synchronous Transport Signal
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Coaxial Cable• Better shielding and greater bandwidth than
unshielded twisted pairs• Can handle longer distance at higher speeds• Coaxial cable is stiff copper wire surrounded
by insulation• Encased in conductor – woven mesh and finally a
plastic sheath• Cable has bandwidth up to a few GHz• Has been replaced by fiber optics in Telco systems
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Coaxial Cable Construction of cable allows High interference immunity Higher bandwidth than twisted pair Hundreds of MHz
Who uses this? Cable TV distribution Long distance telephone
transmission Original Ethernet LAN mediumis
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Fiber Optics• Fiber consists of a light, transmission medium and
detector• Transmission medium is thin fiber of glass
• Detector generates a pulse when it detects a light• So, attach a light at one end, detector at other end• Accepts electrical signals, converts and transmits light
pulses and converts back to signals at receiving end
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Fiber Optics• Consists of core of glass, very thin• Surrounded by glass cladding to keep all light in the core• Surrounded by plastic jacket
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Optical Fiber
Light sources (lasers, LEDs) generate pulses of light that are transmitted on optical fiber Very long distances (>1000 km) Very high speeds (> 40 Gbps/wavelength) Nearly error-free
Huge influence on network architecture Dominates long distance transmission Distance less of a cost factor in communications Plentiful bandwidth for new services
Optical fiber
Optical source
ModulatorElectricalsignal
Receiver Electricalsignal
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Core
Cladding JacketLight
θ c
Geometry of optical fiber
Total Internal Reflection in optical fiber
Transmission in Optical Fiber
Very fine glass cylindrical core surrounded by concentric layer of glass (cladding)
Core has higher index of refraction than cladding Light rays incident at less than critical angle θc is completely reflected
back into the core
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Optical Fiber PropertiesAdvantages Very low attenuation Noise immunity Extremely high bandwidth Security: Very difficult to
tap without breaking No corrosion More compact & lighter
than copper wire
Disadvantages New types of optical signal
impairments & dispersion Difficult to splice Mechanical vibration
becomes signal noise
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Radios• Radios work by Frequency
• Frequencies are easy to generate• Can travel long distances• Penetrate buildings • Widely used for communications, waves are
omnidirectional• Low frequencies pass through obstacles well, but
power falls off sharply with distance from source
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104 106 107 108 109 1010 1011 1012
Frequency (Hz)
Wavelength (meters)
103 102 101 1 10-1 10-2 10-3
105
Satellite and terrestrial microwave
AM radio
FM radio and TV
LF MF HF VHF UHF SHF EHF104
Cellularand PCS
Wireless cable
Radio Spectrum
Omni-directional applicationsVHF – Very High FreuencyUHF – Ultra High FrequencySHF – Super High FrequencyEHF – Extremely High Frequency
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More Complete Spectrum
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Radio Transmission
(a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.
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Radio Spectrum ExamplesCellular Phone Allocated spectrum 1st generation:
800, 900 MHz Initially analog voice
2nd generation: 1800-1900 MHz Digital voice, messaging
Wireless LAN Unlicenced ISM spectrum
Industrial, Scientific, Medical 902-928 MHz, 2.400-2.4835
GHz, 5.725-5.850 GHz IEEE 802.11 LAN standard
11-54 Mbps
Point-to-Multipoint Systems Directional antennas at
microwave frequencies High-speed digital
communications between sites High-speed Internet Access
Radio backbone links for rural areas
Satellite Communications Geostationary satellite @ 36000
km above equator Relays microwave signals from
uplink frequency to downlink frequency
Long distance telephone Satellite TV broadcast
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Compare Wireless to Wired Media
Wireless Media Signal energy propagates in
space Interference possible, so
spectrum regulated Limited bandwidth Simple infrastructure:
antennas & transmitters No physical connection
between network & user Users can move
Wired Media Signal energy contained &
guided within medium Spectrum can be directed
separate media (wires or cables), more scalable
Extremely high bandwidth Complex infrastructure: Physical ducts, conduits,
poles, right-of-way Users at not mobile
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Microwave Transmission• Above 100 MHz, waves travel in nearly straight
lines• Uses transmitting and receiving antennas
• Before fiber optics, for decades microwaves formed heart of long-distance telephone transmission system
• MCI – Built system with microwave communications – stands for Microwave Communication Incorporated
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Infrared Transmission• Unguided infrared waves
• Used for short range communication• Remote controls for TV, VCR and Stereos• Cheap, easy to build but has a major drawback• What is it?
• Can't pass through solid walls
• Advantage – No interference in other rooms• Don't need a government license
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Politics • National and International agreements
• FCC regulates spectrum for United States• AM/FM radio, TV and mobile phones• They regulate some frequencies of the spectrum• Unregulated frequencies
• ISM – Industrial, Scientific and Medical unlicensed bands
• Garage door openers, cordless phones, radio controlled toys and wireless mice
• FCC mandates all devices limit power in this unlicensed band
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Politics • In the US,
• 900 Hz was used for early versions of 802.11 • It was crowded• Baby monitors, garage door openers, cordless
phones• So, 802.11 moved to different frequency bands• 2.4 GHz band is available in most countries for 802.11
b/g/n and Bluetooth• 5 GHz is partly used for 802.11a/n
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Digital vs Analog
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Analog and Digital Both data and signals that represent them can
take either analog or digital form. Digital signal has discrete values, not continuous
Example of Digital data or signalExample of Digital data or signal? 0's and 1's stored in computer as a number
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Analog and Digital
Both data and signals that represent them can take either analog or digital form.
Analog has continuous values, not discrete
Example of Analog SignalWhat might be an example of an Analog signal?Human voice. Analog wave is created in the air
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Analog vs. Digital SignalsDigital Signal
1. Limited to finite number of values 2. Has meaning only at discrete points in
time
Examples: Text, bits, integers
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Analog vs. Digital SignalsAnalog Signal
1. Signal that is an analog of the quantity being represented
2. Continuous range of values3. Also continuous in time, always valued
Examples: Sound, vision, music, original TV signal
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Analog vs. Digital
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Analog Signals
• An analog signal is continuous has infinite number of values in a range
• Primary shortcoming of analog signals is difficulty to separate noise from original waveform
• An example is a sine wave which can be specified by three characteristics:
ω (t) = Α sin (2 π f t + p) A: amplitude or height
f : frequency p : phase
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Sine Waves Characteristics
Amplitude, height (intensity) of waveFrequency, number of waves that pass in a single second and is measured in Hertz (cycles/second) (wavelength, the length of the wave from crest to crest, is related to frequency)Phase is a third characteristicDescribes point in wave’s cycle at which a wave begins and is measured in degrees
How is the wavelength related to frequency?
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A Carrier Wave
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Sine Wave
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Analog Long-Distance Communications
• Used repeaters to boost signal - attempts to restore analog signal to its original form
• Restoration is imperfect• Distortion not completely eliminated• Noise & interference only partially removed
• Signal quality decreases with increased repeaters• Communications is distance-limited• Still used in analog cable TV systems• Analogy: Copy a song using a cassette recorder
Source DestinationRepeater
Transmission segment
Repeater. . .
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Digital Long-Distance Communications
• Regenerator recovers original data sequence and retransmits on next segment
• Can design so error probability is very small• Each regeneration is like the first time!• Analogy: Copy an MP3 file• Communications possible over very long distances• Other Advantages of Digital systems vs. analog
systems• Less power, longer distances, lower system cost
Source DestinationRegenerator
Transmission segment
Regenerator. . .
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Analog vs. Digital TransmissionAnalog transmission: all details must be reproduced
accuratelySent
Sent
Received
Received
DistortionAttenuation
Digital transmission: only discrete levels need to be reproduced
DistortionAttenuation
Receiver: Was original pulse
positive or negative?
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Digital Binary Signal
For a given communications medium• How do we increase transmission speed?• How do we achieve reliable communications?• Are there limits to speed and reliability?
+A
-A0 T 2T 3T 4T 5T 6T
1 1 1 10 0
Bit rate = 1 bit / second
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signal noise signal + noise
signal noise signal + noise
HighSNR
LowSNR
SNR (dB) = 10 log10 (Ave Signal Power/ Ave Noise Power)SNR – Signal to Noise Ratio, measures signal strength with addition of noise
virtually error-free
error-prone
Channel Noise affects Reliability
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Summary
• Looked physical medium and began to look at analog vs digital
• Continue this ….
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Read Chapter 2 - Textbook• Also, review basic Wireshark Lab
– I will put it on the Lab page
No Class Monday – Holiday.
There will be a lab on Tuesday related to Wireshark.
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1
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Transmission Not Perfect Along the way, signals are subject to less than
favorable conditions Distance affects the signal
Loss of signal strength with distance Recall what that is called?
Attenuation Noise affects the signal
Line noise obscures the signal Can make it impossible to send information
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Line Noise
Defined Unwanted electrical or electromagnetic energy
that degrades quality of signals and data
– External noise from appliances in area, from electrical transformers, from atmosphere (wired connections)
Communications engineers are constantly striving to develop better ways to deal with noise
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Bandwidth
What is bandwidth? More than one definition?Network bandwidth defined
• Bandwidth is same as data transfer rate, amount of data that can be carried from one point to another in a given time period
• Network bandwidth is usually expressed in bits per second (bps)
Signal Processing bandwidth defined• Bandwidth is range of frequencies carried by a
channel.• Difference between highest-frequency signal
component and the lowest-frequency signal component
• Bandwidth is measured in hertz (cycles per second)
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What is Attenuation?Attenuation is …
- Reduction of signal strength during transmission - Attenuation is gradual loss in intensity of any kind of flux through a medium
Ex. Reduction in signal strength from length of phone line Sunlight is attenuated by dark glasses, and X-rays are attenuated by lead.
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Attenuation Continued- Attenuation is measured in decibels - Decibel (dB) is used to measure sound, but also widely used in electronics, signals and communication Decibel (dB) measures relative strengths of two signals or a signal at two different points - The lower the Attenuation the stronger the received signal Note that decibel is negative if a signal is attenuated and positive if a signal
is amplified.
Attenuation can be measured by: dB = 10 log10 (P2 /P1) where P1 and P2 are the powers of a signal at points 1 and 2, respectively.
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ExamplesWhat happens if we lose power? p1 < p2dB = 10 log (p1/p2) p1=1 and p2=10dB = 10 log (0.10) dB = 10 × (–1) = –10 decibelsNegative sign tells us there is a loss of power, known as attenuation
What if there is a gain of power? p1 > p2 dB = 10 log (20/10) = 10 log (2) = 10 x .3 = 3.0 dBPositive sign means a gain of power,known as amplification
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Transmission Media
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Twisted Pair• Oldest transmission medium
• Historical use, Phone systems • Two insulated Copper wires
• Wires twisted together• Straight they would interfere
• To reduce electromagnetic induction between pairs of wires, two insulated copper wires are twisted around each other
• Twisted pair cabling – Several varieties• Category 5 – Two insulated wires – 4 pairs
• Encased in a protective plastic sheath• Category 7 – Higher quality yet• Has added shielding on individual twisted pairs• Helps reduce external interference and crosstalk
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Coaxial Cable• Better shielding and greater bandwidth than
unshielded twisted pairs• Can handle longer distance at higher speeds• Coaxial cable is stiff copper wire surrounded
by insulation• Encased in conductor – woven mesh and finally a
plastic sheath• Cable has bandwidth up to a few GHz• Has been replaced by fiber optics in Telco systems
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Fiber Optics• Fiber consists of a light, transmission medium and
detector• Transmission medium is thin fiber of glass
• Detector generates a pulse when it detects a light• So, attach a light at one end, detector at other end• Accepts electrical signals, converts and transmits light
pulses and converts back to signals at receiving end
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Fiber Optics• Consists of core of glass, very thin• Surrounded by glass cladding to keep all light in the core• Surrounded by plastic jacket
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Radios• Radios work by Frequency
• Frequencies are easy to generate• Can travel long distances• Penetrate buildings • Widely used for communications, waves are
omnidirectional• Low frequencies pass through obstacles well, but
power falls off sharply with distance from source
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More Complete Spectrum
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Radio Transmission
(a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth.
(b) In the HF band, they bounce off the ionosphere.
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Microwave Transmission• Above 100 MHz, waves travel in nearly straight
lines• Uses transmitting and receiving antennas
• Before fiber optics, for decades microwaves formed heart of long-distance telephone transmission system
• MCI – Built system with microwave communications – stands for Microwave Communication Incorporated
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Infrared Transmission• Unguided infrared waves
• Used for short range communication• Remote controls for TV, VCR and Stereos• Cheap, easy to build but has a major drawback• What is it?
• Can't pass through solid walls
• Advantage – No interference in other rooms• Don't need a government license
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Politics • National and International agreements
• FCC regulates spectrum for United States• AM/FM radio, TV and mobile phones• They regulate some frequencies of the spectrum• Unregulated frequencies
• ISM – Industrial, Scientific and Medical unlicensed bands
• Garage door openers, cordless phones, radio controlled toys and wireless mice
• FCC mandates all devices limit power in this unlicensed band
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Politics • In the US,
• 900 Hz was used for early versions of 802.11 • It was crowded• Baby monitors, garage door openers, cordless
phones• So, 802.11 moved to different frequency bands• 2.4 GHz band is available in most countries for 802.11
b/g/n and Bluetooth• 5 GHz is partly used for 802.11a/n
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Digital vs Analog
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Analog and Digital Both data and signals that represent them can
take either analog or digital form. Digital signal has discrete values, not continuous
Example of Digital data or signalExample of Digital data or signal? 0's and 1's stored in computer as a number
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Analog and Digital
Both data and signals that represent them can take either analog or digital form.
Analog has continuous values, not discrete
Example of Analog SignalWhat might be an example of an Analog signal?Human voice. Analog wave is created in the air
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Sine Waves Characteristics
Amplitude, height (intensity) of waveFrequency, number of waves that pass in a single second and is measured in Hertz (cycles/second) (wavelength, the length of the wave from crest to crest, is related to frequency)Phase is a third characteristicDescribes point in wave’s cycle at which a wave begins and is measured in degrees
How is the wavelength related to frequency?
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A Carrier Wave
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Sine Wave
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