chapter 4 data encoding techniques
TRANSCRIPT
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Data and ComputerCommunications
DATA ENCODING
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Introduction
Either form of data can be encoded into
either form of signal
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Introduction
For DIGITAL SIGNALING, Data source g(t)which may be either digitalor analog, is encoded into a digital signal x(t).
The actual form of x(t) depends on the encoding technique, and is chosento optimize use of the transmission medium. For example, the encoding
may be chosen to either conserve bandwidth or to minimize errors.
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Introduction
The backbone for ANALOG SIGNALING is a continuous, constant-frequency signal known asthe CARRIER SIGNAL. The frequency of the carrier signal is chosen to be compatiblewith the transmission medium being used.Data may be transmitted using a carrier signal by modulation.
MODULATION is the process of encoding source data onto a carrier signal with frequency fc.All modulation techniques involve operation on one or more of the three fundamental
frequency-domain parameters:AmplitudeFrequencyPhase
The input signal m(t) may be analog or digital and is calledthe MODULATING SIGNAL or BASEBAND SIGNAL.The result of modulating the carrier signal is called theMODULATED SIGNAL.The location of the bandwidth on the spectrum is related tofcand is often centered onfc.Again, the actual form of the encoding is chosen to optimize
some characteristic of the transmission.
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Introduction
DIGITAL DATA DIGITAL SIGNALEquipment for encoding is less complex and less expensive compared to digital-to-analogmodulation equipment
ANALOG DATA DIGITAL SIGNALConversion of analog data to digital form permits the use of modern digital transmissionand switching equipment. Advantages of using the digital approach are summarized bythe following: Digital Technology-Data Integrity-Capacity Utilization-Security andPrivacy-Integration
DIGITAL DATA ANALOG SIGNALSome transmission media such as optical fiber and the unguided media will onlypropagate analog signal
ANALOG DATA ANALOG SIGNALAnalog data in electrical form can be transmitted as basebandsignals easily and cheaply by voice transmission over voice-gradelines.One common use of modulation is to shift the bandwidth of abaseband signal to another portion of the spectrum. In this way,multiple signals, each at a different position on the spectrum, canshare the same transmission medium; this is known as FREQUENCY
DIVISION MULTIPLEXING.
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Digital Data, DigitalSignal
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DIGITAL SIGNAL Sequence of discrete, discontinuous voltage pulses
Each pulse is a SIGNAL ELEMENT
Binary data are encoded into signal elements
Simplest case has one-to-one correspondence
Digital Signal
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Unipolar: signal elements all have the same algebraic sign(all + or all -)
Polar: one logic state is represented by a positive voltagelevel, and the other by a negative voltage level
Data Signaling Rate or Data Rate: rate in bits per second
that data is transmitted (R) Duration or length of a bit: amount of time it takes for the
transmitter to emit the bit (1/R)
Modulation/Signaling Rate: rate at which the signal level ischanged, this will depend on the nature of digital encodingand is expressed in BAUD (signal elements per second)
Mark and Space: refer to binary digit 1 and 0
Definition of Terms
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Need to Know Timing of bits - when they start
and end
Signal levels
Factors Affecting Signal
Interpretation SNR or Eb/No
Data Rate
Bandwidth
Encoding Scheme
Interpreting Signals
An increase in data rate
increases bit error rate (theprobability that a bit is receivedin error).
An increase in SNR decreasesbit error rate.
An increase in bandwidth allowsan increase in data rate.
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Signal Spectrum Lack of HF component reduces W requirement
Lack of DC is desirable because ac-coupling via transformer ispossible thus providing excellent electrical isolation and reductionof interference
Good signal design should concentrate the transmitted power in the
middle of the transmission W Clocking
Expensive approach is to provide a separate clock-lead to synch theTX and RX and the alternative is to provide some synch mechanismthat is based on the encoding scheme of the transmitted signal
Ways of Evaluating EncodingSchemes
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Error Detection Responsibility of the DATA LINK CONTROL
Useful to have some error-detection capability built into thephysical signaling-encoding scheme permitting errors to be detectedmore quickly
Signal Interference and Noise Immunity Certain codes exhibit superior performance in the presence of noise
This ability is usually expressed in terms of a BIT ERROR RATE
Cost and Complexity The higher the signaling rate to achieve a given data rate, the
greater the cost
Some codes require a signaling rate that is greater than the actualdata rate
Ways of Evaluating EncodingSchemes
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Digital Signal Encoding Formats
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NRZ
NRZ-L and NRZI Advantage:
Easy to engineer
Makes efficient use of W
Disadvantage:
Presence of DC component
Lack of synchronization Differential Encoding
NRZI and Differential Manchester
The signal is decoded by comparing the polarity of adjacent signal elements ratherthan determining the absolute value of a signal element
Advantage:
more reliable to detect a transition in the presence of noise than to compare a
value to a threshold Disadvantage:
Presence of DC component
Lack of synchronization
Digital Signal Encoding Formats
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Multilevel Binary
Bipolar-AMI and Pseudoternary Advantage:
No loss of synchronization on long strings of 1s (B-AMI) or 0s (Pseudoternary)
No net DC component
Less W compared to NRZ
Pulse alternation provides some means of error detection
Disadvantage: Receiver needs to distinguish between 3 levels instead of 2
Needs higher signal power for the same level of bit error rate
Biphase
Manchester and Differential Manchester
Midbit transition
Advantage:
No net DC component
Synchronization
Error detection (absence of midbit transition)
Disadvantage:
W is considerably greater since modulation rate is 2x the data rate
Digital Signal Encoding Formats
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Modulation/Signaling Rate vsData Rate
Data rate (expressed in bits per second) Modulation rate (expressed in baud).
The data rate, or bit rate, is l/tB, where tB = bitduration.
The modulation rate is the rate at which signal elementsare generated.
D = modulation rate, baud
R = data rate, bps
N = number of bits per signalingelement
M = number of discrete signal
D = R / N = R /log2M
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Process of replacing sequences that would produce a long string ofconstant voltage that could result to loss of synchronization
These filling sequences must: produce enough transitions to sync
be recognized by receiver & replaced with original
be same length as original
Design Goals: no dc component
no long sequences of zero level line signal
no reduction in data rate
give error detection capability
Scrambling
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B8ZS and HDB3 B8ZS (bipolar with 8-zeros substitution)
Common in North America Based in Bipolar-AMI (drawback is on long strings of 0)
This technique forces two code violations (signal patterns not allowed in AMI) of theAM1 code, an event unlikely to be caused by noise or other transmission impairment.
The receiver recognizes the pattern and interprets the octet as consisting of allzeros.
HDB3 (high-density bipolar-3 zeros)
Common in Europe and Japan
Based in Bipolar-AMI (drawback is on long strings of 0)
In this case, the scheme replaces strings of four zeros with sequences containing oneor two pulses.
In each case, the fourth zero is replaced with a code violation.
In addition, a rule is needed to ensure that successive violations are of alternate
polarity so that no dc component is introduced. Thus, if the last violation was positive, this violation must be negative, and vice versa.
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B8ZS and HDB3
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Seatwork (20min)
Encode the following bit stream using (+) pulse = 5V and (-) pulse =-5V NRZ-L NRZI last pulse prior the bit stream is (+)
Bipolar-AMI
start with (+) pulse Pseudoternary start with (+) pulse Manchester Differential Manchester last pulse prior the bit stream is
low-to-high
1 0 1 0 0 0 1 1 1 0 0 1
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Seatwork (20min)
Give the B8ZS equivalent of the ff. bit stream. Start with the (+)pulse. Label the violation.
1110000000011101101000 Give the HDB3 equivalent of the ff. bit stream. Start with the (+)
pulse Label the violation and stuffing bit.
1010000011000011000000
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Seatwork - Answer
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Seatwork - Answer
Give the B8ZS equivalent of the ff. bit stream. Start with the (+) pulse.Label the violation.
1110000000011101101000+-+000+-0-+-+-0+-0+000Red violation
Give the HDB3 equivalent of the ff. bit stream. Start with the (+) pulseLabel the violation and stuffing bit.
1010000011000011000000+0-000-0+-+00+-+-00-00Red
violationOrange stuffing bit
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Digital Data, Analog
Signal
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Main use is transmitting data through the publictelephone system
has freq range of 300Hz to 3400Hz
use modem (modulator-demodulator)
Modulation involves operation on one or more ofthe 3 characteristics of the carrier signal. In allthese cases the resulting signal occupies the Wcentered on the carrier frequency
Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK)
Phase Shift Keying (PK)
Digital Data, Analog Signal
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Bandwidth ASK/PSK bandwidth directly relates to
bit rate Multilevel PSK gives significantimprovements
In presence of noise: Bit error rate of PSK are about 3dB
superior to ASK and FSK
Performance of Digital toAnalog Modulation Schemes
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Analog Data, Digital
Signal
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DIGITIZATION is conversion of analog data intodigital data which can then:
1. Be transmitted using NRZ-L. In this case, we have gone directlyfrom analog data to a digital signal.
2. Be encoded as a digital signal using a code other than NRZ-L.Thus, an extra step is required.
3. Be converted into an analog signal, using one of the modulationtechniques discussed previously.
Analog Data, Digital Signal
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Digitizing Analog Data
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Based on the sampling theorem:
If a signal is sampled at regular intervals at a rate higherthan twice the highest signal frequency, the samples containall information in original signal
eg. 4000Hz voice data, requires 8000 sample per sec (NOTE:These are analog samples)
Samples are represented as narrow pulses whose amplitude isproportional to the value of the original signal PULSEAMPLITUDE MODULATION (PAM)
PAM data are quantized to produced PCM DATA.
The amplitude of each PAM pulse is approximated by an n-bit
integer.
Pulse Code Modulation (PCM)
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Pulse Code Modulation (PCM)
In the example, n = 3.
Thus, 8 = 23 levels are availablefor approximating the PAM
pulses.
k (10 )
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Seatwork (10min)
Give the quantized code and the PCM equivalent of the given signal.
PAM value is given.
k A
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Seatwork - Answer
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QUANTIZING ERROR The error caused by quantizing the PAM pulse
The original signal is now only approximated and cannot berecovered exactly.
The signal-to-noise ratio for quantizing noise can be expressed
as:
Pulse Code Modulation (PCM)
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Non-Linear Coding Quantization levels are not equally spaced.
The problem with equal spacing is that the mean absolute errorfor each sample is the same, regardless of signal level. Consequently, lower amplitude values are relatively more
distorted. By using a greater number of quantizing steps for signals of low
amplitude, and a smaller number of quantizing steps for signalsof large amplitude, a marked reduction in overall signal distortionis achieve.
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Companding The same effect can be achieved by using uniform quantizing but
companding (compressing-expanding) the input analog signal.
COMPANDING is a process that compresses the intensity range of asignal by imparting more gain to weak signals than to strong signals oninput.
At output, the reverse operation is performed. Note that the effect onthe input side is to compress the sample so that the higher values are
reduced with respect to the lower values. ]Thus, with a fixed number of quantizing levels, more levels are available
for lower-level signals. On the output side, the compander expands thesamples so the compressed values are restored to their original values.
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Companding
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Popular alternative to PCM
Analog input is approximated by a staircase function that
moves up or down by one quantizing level() at eachsampling interval
Behavior is binary At each sampling time, the function only moves up or
down at a constant level
Thus, the output can be encoded as a single binary digitfor each sample
1 for up or 0 for down
Delta Modulation
Two Important ParametersSize of the step ()
Sampling size
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Delta Modulation Example
The transition (up or down) that occurs at eachsampling interval is chosen so that the staircasefunction tracks the original analog waveform asclosely as possible.For transmission, the following occurs: At eachsampling time, the analog input is compared to themost recent value of the approximating staircasefunction.
If the value of the sampled waveform exceedsthat of the staircase function, a 1 is generated;otherwise, a 0 is generated.The output of the DM process is therefore a binarysequence that can be used at the receiver toreconstruct the staircase function.The staircase function can then be smoothed by sometype of integration process or by passing it through a
low-pass filter to produce an analog approximation of
the analog input signal.
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DM has simplicity compared to PCM buthas worse SNR
PCM preferred to DM for digitizing
analog signals that represent digitaldata
PCM verses Delta Modulation
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Analog Data, Analog
Signal
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Modulation has been defined as the process of combining an inputsignal m(t)and a carrier at frequency f, to produce a signal s(t)whose bandwidth is (usually) centered on f,.
For digital data, the motivation for modulation should be clear: Whenonly analog transmission facilities are available, modulation is
required to convert the digital data to analog form. For analog data, the principal reasons for modulation are as follows:
A higher frequency may be needed for effective transmission.For unguided transmission, it is virtually impossible to transmitbaseband signals. The required antennas would be manykilometers in diameter.
Modulation permits frequency-division multiplexing
Analog Data, Analog Signals
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AmplitudeModulation
FrequencyModulation
PhaseModulation
Analog
ModulationTechniques
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END