Chapter 5 Data Encoding

1
Chapter 5Data Encoding
2
Review

• Information Numeric Data, characters, voice,
  pictures, codes or any massage that can be read
  by and has meaning to human and machine.

3
Review

• For transmission
• Information must be converted into binary first.
• ASCII table
• Unicode
• Information must be encoded into electromagnetic
  signals. (Analog or digital)

4
Review

• Digital Signal
• A digital signal is a sequence of discrete
  discontinuous voltage pulses.
• Each pulse is a signal element
• In its simplest form each signal element
  represents a binary 0 or 1.

5
Data Encoding

• Both analog and digital information can be
  encoded as either analog or digital. (Function of
  media and communication )
• Digital data, digital signal
• Digital data, analog signal
• Analog data, digital signal
• Analog data, analog signal

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Terminology (digital signal)

• Unipolar encoding If the signal elements all
  have the same algebraic signs, all positive or
  all negative, the signal is called unipolar.
• Polar encoding One logical state is represented
  by positive voltage and the other by the negative
  voltage level.

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Terminology (digital signal)

• Data rate The rate in bits per second that the
  data is transmitted. (R)
• Bit duration The amount of time for one bit
  transmission (1/R)
• Modulation rate The rate at which the signal
  level is changed. (baud rate, signal levels per
  second)

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Terminology

• Encoding scheme The mapping from data bits to
  signal elements
• Spectrum The spectrum of a signal is the range
  of frequencies that it contains.
• Absolute bandwidth The width of the spectrum
• Effective bandwidth The are of the bandwidth
  where most of the energy of the signal is
  concentrated.

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Terminology

• DC (direct current)component A component of a
  signal with the frequency of zero.
• Example
• S(t)1(4/?)sin(2 ? ft) .

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Evaluation of Various Encoding Techniques
(affecting factors)

• Signal spectrum
• Lack of high frequency components means less
  bandwidth required for transmission
• DC component It is desirable to have no DC
  component. (easier implementation)
• Clocking The beginning and end of each bit
  position must be determined.
• Providing separate clocking information.
• Implementation of some other ways of
  synchronization

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Evaluation of Various Encoding Techniques
(affecting factors)

• Error detection
• To detect errors more quickly, some error
  detection techniques must be built into signaling
  encoding methods.
• Signal interference and noise immunity
• Some signal encoding techniques provide better
  error rate (BER) than others
• Cost and complexity

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Data Encoding

• Digital data, analog signal
• A modem converts digital data to analog data
• Amplitude shift keying (ASK)
• Frequency shift keying (FSK)
• Phase shift keying (PSK)

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Data Encoding

• Analog data, Digital signals
• Pulse code modulation (PCM)
• Samples analog data periodically
• Quantizing (limiting the possible values to
  discrete set of values) the samples

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Data Encoding

• Digital data, digital signal
• Simplest form of digital encoding
• Two voltage level required
• It can be enhanced to improve performance.

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Digitalto-Digital Encoding Schemes

• Unipolar
• Uses only one level of voltage (almost obsolete)
• Polar
• Uses two level of voltage
• Bipolar
• Uses theree level of voltage

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Unipolar Encoding

• Presence and absence of a voltage level is used
  for two binary digits.
• The absence of voltage could represent zero.
• A constant positive voltage could represent 1.

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Unipolar

• Amplitude

0
1
0
0
0
Time
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Unipolar Encoding Issues

• Synchronization A major issue
• Example For a bit rate of 1000 bps, the
  receiving device must measure each bit for 0.005
  s.
• DC Component
• The average amplitude of a unipolar encoded
  signal is not zero.
• This creates a DC component ( a component with
  zero frequency).
• DC component can not travel through some media
  that can not handle DC component

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Polar Encoding

• Polar encoding uses tow voltage levels (positive
  and negative)

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Polar
NRZ
RZ
Biphase
Differential Manchester
NRZ-L
NRZ-I
Manchester
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Variation of Nonreturn to Zero (NRZ)

• NRZ-L, Nonreturn to Zero-level (polar)
• The level of the signal depends on the type of
  the bit it represents (a positive voltage usually
  represents bit 0 and negative voltage represents
  the bit 1 (or vice versa)
• The problem exist when receiver needs to
  interpret long streams of 1 or zero.
• Or NRZ-I (Nonreturn to Zero Invert on ones)

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Nonreturn to Zero-Level
Amplitude
1
1
1
0
1
0
0
0
Time
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Variation of Nonreturn to Zero (NRZ)

• NRZ-I (Nonreturn to Zero Invert on ones)
• An inversion of voltage level represents a 1 bit.
• The transition between a positive and negative
  voltage represents a 1 not the voltage level
  itself.
• A 0 is represented by no change
• Still a string of zeros is a problem.

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Nonreturn to Zero, invert on ones
Amplitude
1
1
0
1
0
1
0
0
0
Time
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Nonreturn to Zero-Level Nonreturn to Zero, invert
on ones
Amplitude
0
1
0
0
1
1
1
0
Time
0
1
0
0
0
1
1
1
0
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Return to Zero

• One solution to synchronization issue of NRZ-L
  and NRZ-I is using RZ (Return to Zero) encoding
  schemes.
• It uses three values positive, negative and
  zero.
• In RZ, the signal changes during each bit.
• A 1 bit is represented by positive-to zero and a
  0 bit by negative-to-zero.

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Return to Zero
It requires two signal changes to encode one
bit. (uses more bandwidth)
0
1
0
0
1
1
1
Time
These transitions can be used for synchronization
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NRZ pros and cons

• Pros
• Easy to engineer
• Make good use of bandwidth
• Cons
• dc component
• Lack of synchronization capability
• Used for magnetic recording
• Not often used for signal transmission

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Polar
NRZ
RZ
Biphase
Differential Manchester
NRZ-L
NRZ-I
Manchester
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Biphase Encoding

• The most popular encoding to deal with the
  synchronization problem.
• The signal changes at the middle of the bit
  interval and continues to the opposite pole (dose
  not return to zero).
• Types of biphase encoding
• Manchester
• Differential Manchester

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Biphase Encoding

• Manchester Encoding
• The inversion at the middle of each bit is used
  for both synchronization and bit representation
• i.e. Transition serves as clock and data
• Low to high represents one
• High to low represents zero
• Used by IEEE 802.3

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Manchester Encoding
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Differential Encoding

• Data represented by changes rather than levels
• More reliable detection of transition rather than
  level
• In complex transmission layouts it is easy to
  lose sense of polarity

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Biphase Encoding

• Differential Manchester
• Transition at the middle of bit interval is used
  for clocking only.
• Transition at the start of a bit period
  represents zero.
• No transition at start of a bit period represents
  one.
• Note this is a differential encoding scheme
• Used by IEEE 802.5.

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Differential Manchester Encoding
Presence of transition at the beginning of the
bit interval represents zero. Absence of
transition at the beginning of the bit interval
represents one.
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Biphase Pros and Cons

• Con
• At least one transition per bit time and possibly
  two
• Maximum modulation rate is twice NRZ
• Requires more bandwidth
• Pros
• Synchronization on mid bit transition (self
  clocking)
• No dc component
• Error detection
• Absence of expected transition

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Multilevel Binary

• Use more than two levels
• Bipolar-AMI (Alternate mark inversion)
• Pseudoternary (variation of Bipolar-AMI)

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Bipolar Encoding

• Uses there voltage levels
• Positive, negative, and zero
• Zero level represents binary 0
• Ones are represented by alternating positive and
  negative voltages

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Types of Bipolar Encoding

• Bipolar Alternate Mark Inversion (AMI)
• Bipolar 8-zero substitution (B8ZS)
• High density bipolar 3 (HDB3)

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Types of Bipolar Encoding
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Bipolar Alternate Mark Inversion (AMI)

• Mark comes from telegraphy (meaning 1)
• Zero voltage represents zero
• Binary 1s are represented by alternating
  positive and negative voltages

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Bipolar Alternate mark inversion (AMI)
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Bipolar-AMI and Pseudoternary
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Types of Bipolar Encoding

• Pros
• DC component is zero
• A long sequence of 1s is always synchronized.
• Lower bandwidth
• Easy error detection
• Cons
• No mechanism for synchronization of long string
  of zeros

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Variation of AMI

• Bipolar 8-zero substitution (B8ZS)
• (implemented in US)
• High Density bipolar 3 (HDB3)
• (implemented in Europe)
• In both methods the original pattern is modified
  in the case of multiple consecutive zeros.

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Bipolar 8-zero substitution (B8ZS)

• It works similar to BMI
• Whenever 8 or more consecutive zeros occurs,
  signal level is forced to change.

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Pseudoternary

• One represented by absence of line signal
• Zero represented by alternating positive and
  negative
• No advantage or disadvantage over bipolar-AMI

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Trade Off for Multilevel Binary

• Not as efficient as NRZ
• Each signal element only represents one bit
• In a 3 level system could represent log23 1.58
  bits
• Receiver must distinguish between three levels
  (A, -A, 0)
• Requires approx. 3dB more signal power for same
  probability of bit error

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Scrambling

• Use scrambling to replace sequences that would
  produce constant voltage
• Filling sequence
• Must produce enough transitions to sync
• Must be recognized by receiver and replace with
  original
• Same length as original
• No dc component
• No long sequences of zero level line signal
• No reduction in data rate
• Error detection capability

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B8ZS

• Bipolar With 8 Zeros Substitution
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse
  preceding was positive encode as 000-0-
• If octet of all zeros and last voltage pulse
  preceding was negative encode as 000-0-
• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all
  zeros

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HDB3

• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or two
  pulses

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B8ZS and HDB3
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Digital Data, Analog Signal

• Public telephone system
• 300Hz to 3400Hz
• Use modem (modulator-demodulator)
• Amplitude shift keying (ASK)
• Frequency shift keying (FSK)
• Phase shift keying (PK)

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Digital to Analog Encoding
55

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Amplitude Shift Keying

• Values represented by different amplitudes of
  carrier
• Usually, one amplitude is zero
• i.e. presence and absence of carrier is used
• Susceptible to sudden gain changes
• Inefficient
• Up to 1200bps on voice grade lines
• Used over optical fiber

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Modulation Techniques (ASK)
Binary 1
Binary 0
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ASK
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Modulation Techniques(ASK)
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Frequency Shift Keying

• Values represented by different frequencies (near
  carrier)
• Less susceptible to error than ASK
• Up to 1200bps on voice grade lines
• High frequency radio
• Even higher frequency on LANs using co-ax

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Modulation Techniques (ASK)
Binary 1
Binary 0
f1 and f2 are offset from fc by equal but
opposite amount
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FSK on Voice Grade Line
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FSK
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Modulation Techniques(FSK)
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Phase Shift Keying

• Phase of carrier signal is shifted to represent
  data
• Differential PSK
• Phase shifted relative to previous transmission
  rather than some reference signal

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Modulation Techniques (PSK)(Differential PSK)
Binary 1
Binary 0
The phase shift is is in reference to previous
bit transmitted Rather than to some constant
reference signal.
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PSK
68
PSK Constellation
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Quadrature PSK

• More efficient use by each signal element
  representing more than one bit
• e.g. shifts of ?/2 (90o)
• Each element represents two bits
• Can use 8 phase angles and have more than one
  amplitude
• 9600bps modem use 12 angles , four of which have
  two amplitudes

70
Modulation Techniques (PSK)(Differential QPSK)
Binary 11
Binary 10
Binary 00
Binary 01
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4-PSK
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4-PSK Constellation
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8-QAM Signal
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8-PSK Constellation
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Have a great day . See you on Friday.
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PSK Bandwidth
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4-QAM and 8-QAM Constellation
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Bandwidth for ASK
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Bandwidth for FSK
80
16-QAM Constellation
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Bit Rate and Baud Rate
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Bit Rate and Baud Rate
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Modulation Techniques(FSK)
84
Performance of Digital to Analog Modulation
Schemes

• Bandwidth
• ASK and PSK bandwidth directly related to bit
  rate
• FSK bandwidth related to data rate for lower
  frequencies, but to offset of modulated frequency
  from carrier at high frequencies
• (See Stallings for math)
• In the presence of noise, bit error rate of PSK
  and QPSK are about 3dB superior to ASK and FSK

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Analog Data, Digital Signal

• Digitization
• Conversion of analog data into digital data
• Digital data can then be transmitted using NRZ-L
• Digital data can then be transmitted using code
  other than NRZ-L
• Digital data can then be converted to analog
  signal
• Analog to digital conversion done using a codec
• Pulse code modulation
• Delta modulation

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Pulse Code Modulation(PCM) (1)

• If a signal is sampled at regular intervals at a
  rate higher than twice the highest signal
  frequency, the samples contain all the
  information of the original signal
• (Proof - Stallings appendix 4A)
• Voice data limited to below 4000Hz
• Require 8000 sample per second
• Analog samples (Pulse Amplitude Modulation, PAM)
• Each sample assigned digital value

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Pulse Code Modulation(PCM) (2)

• 4 bit system gives 16 levels
• Quantized
• Quantizing error or noise
• Approximations mean it is impossible to recover
  original exactly
• 8 bit sample gives 256 levels
• Quality comparable with analog transmission
• 8000 samples per second of 8 bits each gives
  64kbps

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Nonlinear Encoding

• Quantization levels not evenly spaced
• Reduces overall signal distortion
• Can also be done by companding

89
Delta Modulation

• Analog input is approximated by a staircase
  function
• Move up or down one level (?) at each sample
  interval
• Binary behavior
• Function moves up or down at each sample interval

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Delta Modulation - example
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Delta Modulation - Operation
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Delta Modulation - Performance

• Good voice reproduction
• PCM - 128 levels (7 bit)
• Voice bandwidth 4khz
• Should be 8000 x 7 56kbps for PCM
• Data compression can improve on this
• e.g. Interframe coding techniques for video

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Analog Data, Analog Signals

• Why modulate analog signals?
• Higher frequency can give more efficient
  transmission
• Permits frequency division multiplexing (chapter
  8)
• Types of modulation
• Amplitude
• Frequency
• Phase

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Analog Modulation
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Spread Spectrum

• Analog or digital data
• Analog signal
• Spread data over wide bandwidth
• Makes jamming and interception harder
• Frequency hoping
• Signal broadcast over seemingly random series of
  frequencies
• Direct Sequence
• Each bit is represented by multiple bits in
  transmitted signal
• Chipping code

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Required Reading

• Stallings chapter 5

97
Review
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Atmospheric and Extraterrestrial Noise

• Lightning It is a major source of noise, caused
  by the static discharge of thunderclouds.
• Several million volts
• Currents exceeding 20,000 amps.
• Solar Noise Ionized gases of the sun produces a
  wide range of frequencies that penetrate the
  Earths atmosphere.
• Cosmic Noise Radiation of noise by distant stars
  penetrating the Earths atmosphere.Long haul
  telecommunications service (1500 km support
  20,000 to 60,000 voice channels)
• An alternative to fiber optic and coaxial cable
• Short point-to-point links between buildings
  (closed-circuit TV or data link)