Digital TransmissionDigital Transmission

01204325: Data 01204325: Data Communication and Communication and Computer NetworksComputer Networks

Asst. Prof. Chaiporn Jaikaeo, Ph.D.Asst. Prof. Chaiporn Jaikaeo, Ph.D.chaiporn.j@ku.ac.thchaiporn.j@ku.ac.th

http://www.cpe.ku.ac.th/~cpjhttp://www.cpe.ku.ac.th/~cpjComputer Engineering DepartmentComputer Engineering Department

Kasetsart University, Bangkok, ThailandKasetsart University, Bangkok, ThailandAdapted from lecture slides by Behrouz A. Forouzan© The McGraw-Hill Companies, Inc. All rights reserved

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OutlineOutline Line codingLine coding Encoding considerationsEncoding considerations DC components in signalsDC components in signals SynchronizationSynchronization Various line coding methodsVarious line coding methods

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Line CodingLine Coding Process of converting binary data to Process of converting binary data to

digital signaldigital signal

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Signal Levels vs. Data Signal Levels vs. Data LevelsLevels

Number of signal levelsNumber of signal levels Number of different voltage levels Number of different voltage levels

allowed in a signalallowed in a signal Number of data levelsNumber of data levels

Number of voltage levels that actually Number of voltage levels that actually represent data valuesrepresent data values

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Signal vs. Data ElementsSignal vs. Data Elements

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Pulse Rate vs. Bit RatePulse Rate vs. Bit Rate

bb – number of bits per pulse – number of bits per pulse

LL – number of different signal elements – number of different signal elements

BitRate = PulseRate × b = PulseRate × log2LBitRate = PulseRate × b = PulseRate × log2L

t

00 11 01 10 01 10 11 00

-3

-1

+1

+3

One pulse(one signal element)

Bit rate Bits per secondPulse rate Baud (pulses or signals per second)

Bit rate Bits per secondPulse rate Baud (pulses or signals per second)

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Pulse Rate vs. Bit RatePulse Rate vs. Bit Rate Example: Example: In Manchester Encoding, if the bit rate In Manchester Encoding, if the bit rate

is is 10 Mbps, what is the pulse rate?10 Mbps, what is the pulse rate?

t

0 1 0 0 1 1 0 1

One bit One pulse(one signal element)

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Encoding ConsiderationsEncoding Considerations Signal spectrumSignal spectrum

Lack of DC componentsLack of DC components Lack of high frequency componentsLack of high frequency components

Clocking/synchronizationClocking/synchronization Error detectionError detection Noise immunityNoise immunity Cost and complexityCost and complexity

99

DC ComponentsDC Components DC components in signals are not DC components in signals are not

desirabledesirable Cannot pass thru certain devicesCannot pass thru certain devices Leave extra (useless) energy on the lineLeave extra (useless) energy on the line

t

0 1 0 0 1 1 0 1

t

0 1 0 0 1 1 0 1

Signal with DC component

Signal without DC component

1010

t

0 1 0 0 0 1 1 0 1 1

SynchronizationSynchronization To correctly decode a signal, receiver To correctly decode a signal, receiver

and sender must agree on bit and sender must agree on bit intervalinterval

t

0 1 0 0 1 1 0 1 Sender sends:01001101

Receiver sees:0100011011

1111

Providing SynchronizationProviding Synchronization Separate clock wireSeparate clock wire

Self-synchronizationSelf-synchronization

Sender Receiverdata

clock

t

0 1 0 0 1 1 0 1

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Line Coding MethodsLine Coding Methods UnipolarUnipolar

Uses only one voltage level (one side of time Uses only one voltage level (one side of time axis)axis)

PolarPolar Uses two voltage levels (negative and positive)Uses two voltage levels (negative and positive) E.g., NRZ, RZ, Manchester, Differential E.g., NRZ, RZ, Manchester, Differential

ManchesterManchester BipolarBipolar

Uses three voltage levels (+, 0, and –) for data Uses three voltage levels (+, 0, and –) for data bitsbits

MultilevelMultilevel

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UnipolarUnipolar Simplest form of digital encodingSimplest form of digital encoding

Rarely usedRarely used Only one polarity of voltage is usedOnly one polarity of voltage is used E.g., polarity assigned to 1E.g., polarity assigned to 1

t

0 1 0 0 1 1 0 0

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Polar EncodingPolar Encoding Two voltage levels (+,-) represent data Two voltage levels (+,-) represent data

bitsbits Most popular fourMost popular four

Nonreturn-to-Zero (NRZ)Nonreturn-to-Zero (NRZ) Return-to-Zero (RZ)Return-to-Zero (RZ) ManchesterManchester Differential ManchesterDifferential Manchester

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NRZ EncodingNRZ Encoding NNononrreturn to eturn to ZZeroero

NRZ-L (NRZ-Level):NRZ-L (NRZ-Level): Signal level depends on bit valueSignal level depends on bit value

NRZ-I (NRZ-Invert):NRZ-I (NRZ-Invert): Signal is inverted if 1 is Signal is inverted if 1 is encounteredencountered

t

0 1 0 0 1 1 1 0

t

0 1 0 0 1 1 1 0 N = Bit rateSave = Average signal rate

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RZ EncodingRZ Encoding RReturn to eturn to ZZeroero

Uses three voltage levels: +, - and 0, Uses three voltage levels: +, - and 0, but only + and - represent data bitsbut only + and - represent data bits

Half way thru each bit, signal returns to Half way thru each bit, signal returns to zerozero

t

0 1 0 0 1 1 0 0

?

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Manchester EncodingManchester Encoding Uses an inversion at the middle of Uses an inversion at the middle of

each biteach bit For bit representationFor bit representation For synchronizationFor synchronization

t

0 1 0 0 1 1 0 1 = 0

= 1

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Differential Manchester Differential Manchester EncodingEncoding

The inversion on the middle of each The inversion on the middle of each bit is only for synchronizationbit is only for synchronization

Transition at the beginning of each Transition at the beginning of each bit tells the valuebit tells the value

t

0 1 0 0 1 1 0 1

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Bipolar EncodingBipolar Encoding Bipolar encoding uses three voltage levels: Bipolar encoding uses three voltage levels:

+, - and 0+, - and 0 Each of all three levels represents a bitEach of all three levels represents a bit E.g., E.g., Bipolar AMIBipolar AMI ( (AAlternate lternate MMark ark IInversion)nversion)

0V always represents binary 00V always represents binary 0 Binary 1s are represented by alternating + and Binary 1s are represented by alternating + and

--

t

0 1 0 0 1 1 0 1

2020

t

1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0

Bipolar AMI

B8ZS

BBnnZS SchemesZS Schemes BBnnZS – Bipolar ZS – Bipolar nn-zero substitution-zero substitution

Based on Bipolar AMIBased on Bipolar AMI nn consecutive zeros are substituted with some +/- levels consecutive zeros are substituted with some +/- levels

provides synchronization during long sequence of 0sprovides synchronization during long sequence of 0s E.g., B8ZSE.g., B8ZS

V B 0 V000 B

V – Bipolar violationB – Valid bipolar signal

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Other SchemesOther Schemes mmBBnLnL

mm data elements are substituted with data elements are substituted with nn signal elementssignal elements

E.g., 2B1Q (two binary, 1 quaternary)E.g., 2B1Q (two binary, 1 quaternary)

t

00 11 01 10 01 10 11 00

-3

-1

+1

+3

Bit sequence Voltage level

00 -301 -110 +311 +1

2222

Block CodingBlock Coding Improves the performance of line Improves the performance of line

codingcoding ProvidesProvides

SynchronizationSynchronization Error detectionError detection

Division SubstitutionLine

Coding :001011010001 :

…01011010001… :101100101101010 :

t

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DataData CodeCode DataData CodeCode

00000000 1111011110 1000 1000 1001010010

00010001 0100101001 10011001 1001110011

00100010 1010010100 10101010 1011010110

00110011 1010110101 10111011 1011110111

01000100 0101001010 11001100 1101011010

01010101 0101101011 11011101 1101111011

01100110 0111001110 11101110 1110011100

01110111 0111101111 11111111 1110111101

DataData CodeCodeQ (Quiet)Q (Quiet) 0000000000

I (Idle) I (Idle) 1111111111

H (Halt) H (Halt) 0010000100

J (start delimiter)J (start delimiter) 1100011000

K (start K (start delimiter)delimiter) 1000110001

T (end delimiter)T (end delimiter) 0110101101

S (Set)S (Set) 1100111001

R (Reset)R (Reset) 0011100111

4B/5B Encoding Table4B/5B Encoding Table

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Analog to Digital ConversionAnalog to Digital Conversion Pulse Amplitude Modulation (PAM)Pulse Amplitude Modulation (PAM)

Converts an analog signal into a series Converts an analog signal into a series of pulses by samplingof pulses by sampling

PAM

Analog signal PAM signal(Sampled analog data)

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Pulse Code Modulation Pulse Code Modulation (PCM)(PCM)

Converts an analog signal into a Converts an analog signal into a digital signaldigital signal PAMPAM QuantizationQuantization Binary encodingBinary encoding Line codingLine coding

2626

PCM: QuantizationPCM: Quantization

1 2 3 4 5 6 70Input

2

4

6

Output

Converts continuous values of data to a Converts continuous values of data to a finite number of discrete valuesfinite number of discrete values

2727

PCM: QuantizationPCM: Quantization

Quantization

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Quantization ErrorQuantization Error Assume sine-wave input and uniform Assume sine-wave input and uniform

quantizationquantization

Known as the 6 dB/bit approximationKnown as the 6 dB/bit approximation

See also: http://en.wikipedia.org/wiki/Quantization_error#Quantization_noise_model

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SolutionWe can calculate the number of bits as

Telephone companies usually assign 7 or 8 bits per sample.

Example: Quantization ErrorExample: Quantization Error A telephone subscriber line must A telephone subscriber line must

have an SNRdB above 40. What is have an SNRdB above 40. What is the minimum number of bits per the minimum number of bits per sample?sample?

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PCM: Binary EncodingPCM: Binary Encoding Maps discrete values to binary digitsMaps discrete values to binary digits

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PCM: The Whole ProcessPCM: The Whole Process

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Minimum Sampling RateMinimum Sampling Rate

Sampling rate must be greater thanSampling rate must be greater than twice the twice the highest frequencyhighest frequency

Sampling rate must be greater thanSampling rate must be greater than twice the twice the highest frequencyhighest frequency

Nyquist Theorem:Nyquist Theorem:

t

sampling interval

Ex. Find the maximum samplinginterval for recording human voice(freq. range 300Hz – 3000Hz)

3333

Nyquist’s Sampling Nyquist’s Sampling TheoremTheorem

See also: Wagon-wheel effect Demo: sampling.py

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Sampling and Bit RateSampling and Bit Rate Ex.Ex. Calculate the minimum bit rate for Calculate the minimum bit rate for

recoding human voice, if each sample recoding human voice, if each sample requires 60 levels of precisionrequires 60 levels of precision