10/11/40 page 1Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Data EncodingData Encoding

10/11/40 page 2Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Encoding schemeEncoding scheme

voiceTelephone

analog

digitalModem

analog

analogCODEC

digital

digital Digitaltransmitter

digital

Analog data, Analog signal

Digital data, Analog signal Digital data, Digital signal

Analog data, Digital signal

10/11/40 page 3Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Encoding and ModulationEncoding and Modulation

Encoder Decoder

Modulator Demodulator

digitaldigitalor or

analoganalog

digitaldigitalor or

analoganalog

digitaldigital

analoganalog

g(t)g(t)

m(t)m(t)

ffcc

s(f)s(f)

x(t)x(t)

tt

fffcfc

g(t)g(t)

m(t)m(t)

x(t)x(t)

s(t)s(t)

10/11/40 page 4Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

CONSYL

Why encoding?Why encoding?

Three factors determine successfulness of receiving signal S/N data rate bandwidth

More factor can be used to improve encoding scheme

10/11/40 page 5Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Encoding evaluation factorsEncoding evaluation factors

Signal spectrum

Clocking

Error detection

Signal interference& noise immunity

Cost and complexity

10/11/40 page 6Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Digital data, Digital signalDigital data, Digital signal

00 11 00 00 11 11 00 00 00 11 11NRZNRZ

NRZINRZI

Bipolar -AMIBipolar -AMI

PseudoternaryPseudoternary

ManchesterManchester

DifferentialDifferentialManchesterManchester

10/11/40 page 7Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Scrambling techniquesScrambling techniques

11 11 00 00 00 00 00 00 00 00 11 11 00 00 00 00 00 11 00

00 00 00

00 00 00

VV BB 00 VV BB

VV BB 00 00 VV BB 00 00 VV

Bit valueBit value

B8ZSB8ZS

Bipolar-AMIBipolar-AMI

HDB3HDB3

10/11/40 page 8Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Spectral densitySpectral density

-0.5

0

0.5

1

1.5

0 0.5 1 1.5

NRZ-L,NRZI

B8ZS,HDB3

AMI, Pseudoternary

Manchester, Differential Manchester

Mea

n sq

uare

vol

tage

per

uni

t ban

dwid

th

Normalized frequency (f/r)

10/11/40 page 9Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Digital data, Analog signalDigital data, Analog signal

Tranmitting digital data through PSTN

Modem is used to convert digital data to analog signal and vice versa

10/11/40 page 10Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Modultion techniquesModultion techniques

00 00 11 11 00 11 0000 00 11 00

ASKASK

FSKFSK

PSKPSK

10/11/40 page 11Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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ASKASK

VVdd(t)(t)

VVcc(t)(t)

VVASKASK(t)(t)

ffccffcc-f-f00ffcc-3f-3f00 ffcc+f+f00 ffcc+3f+3f00

SignalSignalpowerpower

FrequencyFrequency

frequency spectrumfrequency spectrum

10/11/40 page 12Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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FSKFSK

Carrier 2Carrier 2

DataDatasignalsignal

Carrier 1Carrier 1

vvdd(t)(t)

vv11(t)(t)

vv22(t)(t)

vvFSKFSK(t)(t)

ff11

SignalSignalpowerpower

FrequencyFrequency

frequency spectrumfrequency spectrum

ff22

10/11/40 page 14Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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PSKPSKDataDataSignalSignal

CarrierCarrier

PhasePhasecoherentcoherent

DifferentialDifferential

vvcc(t)(t)

vvcc(t)(t)

vvPSKPSK(t)(t)

vvPSKPSK(t)(t)

180=0180=0 0=10=1

phase diagramphase diagram

bit rate = signaling rate

10/11/40 page 15Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Multilevel modulation methodMultilevel modulation method

0011

10100000

00 +90+90 +18+1800

+27+2700

1111

bit rate = n x signaling rate

10/11/40 page 16Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Multilevel modulation methodMultilevel modulation method

+90?01+90?01

0?000?00

+270?11+270?11

+180?11+180?11

4-PSK phase diagram4-PSK phase diagram16-QAM phase diagram16-QAM phase diagram

11

10/11/40 page 17Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Analog data, Digital signalAnalog data, Digital signal

Two principle techniques used PCM DM

AnalogAnalogvoice signalvoice signal

Sampling clockSampling clock PAM signalPAM signal PCM signalPCM signal

SamplingCircuit

SamplingCircuit

Quantizerand compander

Quantizerand compander

DigitizedDigitizedvoice signalvoice signal

10/11/40 page 18Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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PCMPCM Sampling signal based on nyquist theorem

3.23.23.93.9

2.82.8 3.43.41.21.2

4.24.2

33 44 33 33

11

44

011011 100100 011011 011011 001001 100100

Original signalOriginal signal

PAM pulsePAM pulse

PCM pulsePCM pulse with quantized errorwith quantized error

011100011011001100011100011011001100 PCM outputPCM output

10/11/40 page 19Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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PCM Conversion processPCM Conversion process

BA CD

AA

BBCCDD

AnalogAnalogvoice signalvoice signal

Sampling clockSampling clock

PAM signalPAM signal PCM signalPCM signal

SamplingCircuit

SamplingCircuit

Quantizerand compander

Quantizerand compander

DigitizedDigitizedvoice signalvoice signal

10/11/40 page 20Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Nonlinear encodingNonlinear encoding

00112233445566778899101011111212131314141515

Strong signalStrong signal

Weak signalWeak signal

00

112233445566778899

101011111212131314141515

Quantizing levelQuantizing level

Without nonlinear encodingWithout nonlinear encoding With nonlinear encodingWith nonlinear encoding

10/11/40 page 21Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Companding processCompanding process Implement nonlinear encoding via

companding process Companding = Compressing Exp

anding

1111101001010000111110100101000011111010010100001111101001010000 1111101001010000111110100101000011111010010100001111101001010000

linear quantization intervallinear quantization interval

1111101001010000

1111101001010000

segmentsegmentcodescodes(+)(+)

segmentsegmentcodescodes(-)(-)

NetworkCompressorcircuit

Compressorcircuit

LinearADC

LinearADC

LinearDAC

LinearDAC

Expander circuit

Expander circuitvi

vi vo

vo

10/11/40 page 22Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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Analog Data, Analog signalAnalog Data, Analog signal

Use Modultion techniques Need a high frequency for effective tranmission Modulation permits frequency division multiplex

10/11/40 page 23Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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AnalogAnalog ModulationModulation

CarrierCarrier

Modulating sine-wave signalModulating sine-wave signal

Amplitude-modulated waveAmplitude-modulated wave

Frequency-modulated waveFrequency-modulated wave

Phase-modulated wavePhase-modulated wave

10/11/40 page 24Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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AM SpectrumAM Spectrum

case x t f t

s t na f t f t

s t f tn

f f tn

f f t

m

m c

ca

c ma

c m

( ) cos

( ) cos cos

( ) cos cos ( ) cos ( )

2

1 2 2

22

22

2

case x t f t

s t na f t f t

s t f tn

f f tn

f f t

m

m c

ca

c ma

c m

( ) cos

( ) cos cos

( ) cos cos ( ) cos ( )

2

1 2 2

22

22

2

carriercarrier lower sidebandlower sideband upper sidebandupper sideband

• each sideband contains the complete spectrum of s(t) !

M(f)M(f)

ff00 ffcc - B - B

Discrete carrier termDiscrete carrier term

Lower sidebandLower sideband Upper sidebandUpper sideband

ffcc + B + Bffcc

10/11/40 page 25Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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AM power savingAM power saving

SSB (Single ideband)

DSBSC (Double sideband supresses carrier)

VSB (Vestigial sideband)

10/11/40 page 26Department of Computer Engineering, Kasetsart University 204325 Introduction to Computer Communications and Networks

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FM and PMFM and PM

CarrierCarrier

Modulating sine-wave signalModulating sine-wave signal

Amplitude-modulated (DSBTC) waveAmplitude-modulated (DSBTC) wave

Frequency-modulated waveFrequency-modulated wave

Phase-modulated wavePhase-modulated wave