MSIS 4523 Ch5.Analog Transmission

8/3/2019 MSIS 4523 Ch5.Analog Transmission

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Data Communications Systems Ch5: Analog Transmission

JinKyu Lee, Ph.D.

[email protected]

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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One amplitude encodes a 0 while another

amplitude encodes a 1 (a form of amplitude

modulation)



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Some systems use multiple amplitudes

Bit Rate: 8 Baud Rate: 4



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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)



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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)



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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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MSIS 4523 Ch5.Analog Transmission

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