ccnet lec 06 data encoding

8/6/2019 Ccnet Lec 06 Data Encoding

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Lecture

06Data Communication & Networks

Data Encoding Techniques

Muhammad Yousaf



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Some Useful Terms:

Signal element:Individual bits converted into signal (voltage pulse)

Unipolar signal:All signal elements having same polarity

All positives or all negatives

Polar signal:Some elements having +ve polarity & some having -ve

Data rate:Bits per second

Modulation rate (Baud rate):Signal elements per second

Differential Encoding:Rather than sending actual data values, sending the changes inconsecutive data values



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Encoding Techniques:

Digital Transmission

Analog Transmission

Digital Data

Analog Data



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Digital-to-Digital Encoding:

Mapping of data bits onto the voltage levelsMany schemes for encoding digital data into digitalsignal:

NRZ-L

NRZ-I

Bipolar-AMI

Pseudoternary

ManchesterDifferential Manchester



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NRZ-I:

Nonreturn to zero-invert on oneData is represented as transition on start of bit

Binary 0 = no transition

Binary 1 = transition (low to high / high to low)

Voltage remains constant for the duration of bitlength

It is an example of differential encoding schemes



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NRZ:



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Bipolar-AMI & Pseudoternary:

Bipolar-Alternate Mark Inversion:Use more than two levels (+A, -A, 0)

Binary 0 = zero voltage level

Binary 1 = alternatively +A & -A

Pseudoternary:Just opposite to Bipolar-AMI

Binary 0 = alternatively +A & -A

Binary 1 = zero voltage level



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Bipolar-AMI & Pseudoternary:



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Manchester & Differential Manchester:

Widely used in LANsOne bit is represented with two signal elements

Voltage doesn’t remain constant for the duration of bit length

Transition is must in the middle-of-bit

Manchester:Binary 0 = transition from high to low level

Binary 1 = transition from low to high level

Used by IEEE 802.3 (Ethernet)

Differential Manchester:Binary 0 = transition at start-of-bit

Binary 1 = no transition at start-of-bit

Used by IEEE 802.5 (Token ring)




Manchester & Differential Manchester:




Modulation Rate:

Manchester, Differential Manchester:

2 Signal Elements per bit Modulation Rate = 2 x Bit Rate

NRZ, NRZ-I:

1 Signal Elements per bit Modulation Rate = Bit Rate

Number of signal elements per unit time




Digital-to-Analog:

Modem is used for this conversionModulation/Demodulation

Have a re-look at the simple sinusoid:

S(t) = A Cos(2 π f t + φ)

t is independent variable

A, f, φ can be varied

Amplitude shift keying (ASK)

Frequency shift keying (FSK)Phase shift keying (PSK)

Quadrature PSK (QPSK)




ASK, FSK, PSK:




Amplitude Shift Keying:

Information is mapped into the signal with changesin the amplitude levels

Two binary values are represented by two differentamplitude levels e.g. (A1, A2)

Binary 0 = A1 Cos(2 π f t + φ)

Binary 1 = A2 Cos(2 π f t + φ)

If one of the amplitudes is zero:

Binary 0 = Amplitude 0-voltsBinary 1 = Amplitude 10-volts

This is also called On-Off Keying




Frequency Shift Keying:

Information is mapped into the signal with changesin the frequency

Two binary values are represented by two differentfrequencies e.g. (f 1 =10KHz, f 2 =12KHz )

Binary 0 = A Cos(2 π f 1 t + φ)

Binary 1 = A Cos(2 π f 2 t + φ)




Phase Shift Keying:

Information is mapped into the signal with changesin the Phase

Two binary values are represented by two differentphases e.g. (φ1=0, φ2 = π )

Binary 0 = A Cos(2 π f t + φ1 )





Quadrature PSK:

More efficient form of PSK

One signal element can represent information of more thanone bits

e.g. using four phase values can represent 2-bits

(φ1=0o, φ

2=90o, φ

3=180o, φ

4=270o)





Some implementations use 8 phase angles and also twoamplitude levels

So each signal element can represent information of four bits




Analog-to-Analog:

Why modulate analog signals?For transmission shift towards the higher frequencies

Higher bandwidth is available

Antenna size is inversely proportional to the frequency

Transmission in higher frequency requires smallerantenna

Provides basis for Frequency Division Multiplexing




Modulation:

•X(t)•User information signal•Analog / Digital signal•Low frequency signal•Baseband signal

•Modulating signal•F(t) = A cos(2 π fc t)•Carrier signal•Analog signal•Higher frequency signal

•m(t)=X(t)*A cos(2 π fc t)•Transmitted signal•Analog signal•Higher frequency signal•Passband signal

•Modulated signal




Types of modulation:

m(t)=X(t)*A cos(2 π f c t + φ )

Amplitude modulation (AM)

Angle modulation

Frequency modulation (FM)

Phase modulation (PM)




Amplitude Modulation:




Frequency Modulation:




Phase Modulation:




Analog-to-Digital:

DigitizationConvert analog data into digital data

Then use digital encoding to form digital signal

Pulse code modulation (PCM)

Delta modulation (DM)




Pulse Code Modulation:

Consists of two steps:Sampling

Discretize along horizontal axis

Sampling Theorem:

Number of samples per second should be greater than or equal to twicethe highest frequency component of the analog signal

fs >= 2 * fmax

Quantization

Discretize along vertical axis




Sampling:

Time (sec)

f max = 4000 Hz

Sampling rate (f s) = 8000 samples/sec

Sampling period (Ts) = 1/8000 sec

1/8000 second




Quantization: 1bit/sample

+2.5 v = 1

-2.5 v = 0

Mapping to bits: 1 1 1 1 1 1 0 0 0 0 1 1 1




Quantization: 2bits/sample

+3.75 v = 11

+1.25 v = 10

-1.25 v = 01

-3.75 v = 00

Mapping to bits: 11 11 10 11 11 11 01 01 01 01 10 10 11




PCM: cont…

Greater the number of samples, better will be thequality

Telephony quality = 8000 samples/sec

CD quality = 44100 samples/sec

Greater the number of quantization levels, betterwill be the quality

128 quantization levels = 7-bits/sample = 56Kbps

256 quantization levels = 8-bits/sample = 64Kbps

Better quality demands more storage, processingpower, data rate, bandwidth




Delta Modulation:

Analog input is approximated by a staircasefunction

Move up or down one level () at each sampleinterval

Binary behaviorFunction moves up or down at each sample interval

Requires 1-bit/sample

Data rate for DM = 8KbpsData rate for 128-PCM = 56Kbps




Delta Modulation: cont…



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