Transcript
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Data Communications Systems Ch5: Analog Transmission
JinKyu Lee, Ph.D.
Jinkyu.lee@okstate.edu
Include the course code (MSIS4523) in every email subject!!
Layer1
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A Categorization of Transmission Techniques
Digital
Transmission
Analog
Transmission
Data Transmission
Line
Coding
Digital Source
Conversion
Analog Source
Conversion
Block
CodingPCM DM
ASK
Digital Source
Conversion
Analog Source
Conversion
FSK
AM FM
PSKQAM
PM
Ch.5
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Topics
Analog-to-Analog Conversion Modulation (data signal vs. carrier signal)
Modulation Schemes (AM, FM)
Digital-to-Analog Conversion Modulation Schemes (ASK, FSK, PSK, QAM)
Using Telecom NWs for Digital Data
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Analog Data to Analog Signal Conversion
Analog to Analog ModulationAmplitude Modulation (AM)Frequency Modulation (FM)
Phase Modulation (PM)
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Basic Amplitude modulation
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AM Radio Band Allocation
The bandwidth of a mono audio signal is usually5 KHz. An AM station needs a bandwidth of twice the
original signal (10 KHz). The FCC specifies the bandwidth
of a station and a guard band to be 10 KHz.
The bandwidth of a mono audio signal is usually5 KHz. An AM station needs a bandwidth of twice the
original signal (10 KHz). The FCC specifies the bandwidth
of a station and a guard band to be 10 KHz.
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Basic Frequency Modulation
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FM Radio Band Allocation
The bandwidth of a stereo audio signal is usually15 KHz. A FM station needs a bandwidth of 10 times of
the original signal (150 KHz). The FCC specifies the
bandwidth to be 200 KHz (0.2 MHz)
The bandwidth of a stereo audio signal is usually15 KHz. A FM station needs a bandwidth of 10 times of
the original signal (150 KHz). The FCC specifies the
bandwidth to be 200 KHz (0.2 MHz)
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Digital Data to Analog Signal Conversion
Digital to Analog ModulationAmplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Quadrature Amplitude Modulation (QAM)
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Amplitude Shift Keying (ASK)
In radio transmission, known as amplitude
modulation (AM)
The amplitude (or height) of the sine wave varies
to transmit the ones and zeros
Major disadvantage Telephone lines are very susceptible to variations in
transmission quality that affect amplitude
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Amplitude Shift Keying (ASK)
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One amplitude encodes a 0 while another
amplitude encodes a 1 (a form of amplitude
modulation)
Amplitude Shift Keying (ASK)
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Some systems use multiple amplitudes
Bit Rate: 8 Baud Rate: 4
Amplitude Shift Keying (ASK)
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Frequency Shift Keying (FSK)
In radio transmission, known as frequency
modulation (FM)
Frequency of the carrier wave varies in
accordance with the signal to be sent
Signal transmitted at constant amplitudeMore resistant to noise than ASK
Less attractive because it requires more analog
bandwidth than ASK
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One frequency encodes a 0 while another frequency
encodes a 1 (a form of frequency modulation)
Frequency Shift Keying (FSK)
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Frequency Shift Keying (FSK)
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Phase Shift Keying (PSK)
Also known as phase modulation (PM)
Frequency and amplitude of the carrier signal
are kept constant
The carrier signal is shifted in phase according
to the input data stream Each phase can have a constant value, or value
can be based on whether or not phase changes
(differential keying)
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One phase change encodes a 0 while another phasechange encodes a 1 (a form of phase modulation)
Phase Shift Keying (PSK)
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Phase Shift Keying (PSK)
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Four different phase angles are used 1 baud = 2bps
Quadrature Phase Shift Keying
0 90 180 270 360
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The 4-PSK method
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The 8-PSK Characteristics
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Complex Modulations
In practice, the maximum number of bits that can be sentwith any one modulation technique is about five bits The solution is to combine modulation techniques
Combining modulation techniques allows us to transmitmultiple bit values per signal change (baud) Typically, combine amplitude and phase
Increases information-carrying capacity of a channel withoutincreasing bandwidth
Increased combinations also leads to increased likelihood
of errors
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12 different phases are combined with 2 differentamplitudes
Since only 4 phase angles have 2 different amplitudes,
there are a total of 16 combinations.With 16 signal combinations, each baud equals 4 bits
of information (24 = 16)
Commonly used in contemporary modemsA modem using 2400 baud signal achieves a data transfer
rate of 9600bps (4x2400)
Quadrature Amplitude Modulation (QAM)
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Quadrature Amplitude Modulation(cont.)
Constellations
Diagram
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Quadrature amplitude modulation is a combination of ASK
and PSK so that a maximum contrast between each signal
unit (bit, dibit, tribit, and so on) is achieved
Quadrature amplitude modulation is a combination of ASK
and PSK so that a maximum contrast between each signal
unit (bit, dibit, tribit, and so on) is achieved
The 4-QAM and 8-QAM Constellations
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Time Domain Plot for an 8-QAM Signal
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Modem stands formodulator/demodulator
Modem stands formodulator/demodulator
Using Telephone Networks for Data Comm.
Modulation & Demodulation
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Modems
An acronym formodulator-demodulator
Uses a constant-frequency signal known as a
carrier signal
Converts a series of binary voltage pulses into
an analog signal by modulating the carrier signal The receiving modem translates the analog
signal back into digital data
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Telephone Line Bandwidth
A telephone line has a bandwidth of almost
2400 Hz for data transmission
A telephone line has a bandwidth of almost
2400 Hz for data transmission
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Codec vs. Modem
Codec is for coding analog data into digital form
(0s and 1s) and decoding it back The digital data coded by Codec are samples of analog
waves
Modem is for modulating digital data into analog
form (changing the shape of a wave) and
demodulating it back
The analog symbols carry digital data
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How 56kbps Modems Work
56K modems take the basic concepts of PCM and turn them backwards
They are designed to recognize an 8-bit digital signal 8000 times per second
The signal is generated directly by the phone company voice CODEC, driven in
turn by the ISP's equipment at their central site
It is impractical to use all 256 discrete CODEC codes
The output voltage levels near zero are too closely spaced to accurately represent
data on a noisy loop
The V.90 encoder uses subsets of the 256 codes to eliminate output signals most
susceptible to noise
The most robust 128 levels are used for 56 Kbps (Log2128=7bit * 8000=56K), 92
levels to send 52 Kbps, 64 levels to send 48 Kbps and so on
Using fewer levels provides more robust operation, but at a lower data rate
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Traditional Modem Operation
Codec
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