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Digital TransmissionDigital Transmission
01204325: Data 01204325: Data Communication and Communication and Computer NetworksComputer Networks
Asst. Prof. Chaiporn Jaikaeo, Ph.D.Asst. Prof. Chaiporn Jaikaeo, [email protected]@ku.ac.th
http://www.cpe.ku.ac.th/~cpjhttp://www.cpe.ku.ac.th/~cpjComputer Engineering DepartmentComputer Engineering Department
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
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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
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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
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
Converts an analog signal into a Converts an analog signal into a digital signaldigital signal PAMPAM QuantizationQuantization Binary encodingBinary encoding Line codingLine coding
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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
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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)