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Chapter 5 Data Encoding

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Title: 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

6
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.

7
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)

8
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.

9
Terminology
  • DC (direct current)component A component of a
    signal with the frequency of zero.
  • Example
  • S(t)1(4/?)sin(2 ? ft) .

10
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

11
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

12
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)

13
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

14
Data Encoding
  • Digital data, digital signal
  • Simplest form of digital encoding
  • Two voltage level required
  • It can be enhanced to improve performance.

15
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

16
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.

17
Unipolar
  • Amplitude

0
1
0
0
0
Time
18
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

19
Polar Encoding
  • Polar encoding uses tow voltage levels (positive
    and negative)

20
Polar
NRZ
RZ
Biphase
Differential Manchester
NRZ-L
NRZ-I
Manchester
21
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)

22
Nonreturn to Zero-Level
Amplitude
1
1
1
0
1
0
0
0
Time
23
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.

24
Nonreturn to Zero, invert on ones
Amplitude
1
1
0
1
0
1
0
0
0
Time
25
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
26
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.

27
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
28
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

29
Polar
NRZ
RZ
Biphase
Differential Manchester
NRZ-L
NRZ-I
Manchester
30
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

31
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

32
Manchester Encoding
33
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

34
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.

35
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.
36
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

37
Multilevel Binary
  • Use more than two levels
  • Bipolar-AMI (Alternate mark inversion)
  • Pseudoternary (variation of Bipolar-AMI)

38
Bipolar Encoding
  • Uses there voltage levels
  • Positive, negative, and zero
  • Zero level represents binary 0
  • Ones are represented by alternating positive and
    negative voltages

39
Types of Bipolar Encoding
  • Bipolar Alternate Mark Inversion (AMI)
  • Bipolar 8-zero substitution (B8ZS)
  • High density bipolar 3 (HDB3)

40
Types of Bipolar Encoding
41
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

42
Bipolar Alternate mark inversion (AMI)
43
Bipolar-AMI and Pseudoternary
44
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

45
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.

46
Bipolar 8-zero substitution (B8ZS)
  • It works similar to BMI
  • Whenever 8 or more consecutive zeros occurs,
    signal level is forced to change.

47
Pseudoternary
  • One represented by absence of line signal
  • Zero represented by alternating positive and
    negative
  • No advantage or disadvantage over bipolar-AMI

48
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

49
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

50
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

51
HDB3
  • High Density Bipolar 3 Zeros
  • Based on bipolar-AMI
  • String of four zeros replaced with one or two
    pulses

52
B8ZS and HDB3
53
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)

54
Digital to Analog Encoding
55

56
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

57
Modulation Techniques (ASK)
Binary 1
Binary 0
58
ASK
59
Modulation Techniques(ASK)
60
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

61
Modulation Techniques (ASK)
Binary 1
Binary 0
f1 and f2 are offset from fc by equal but
opposite amount
62
FSK on Voice Grade Line
63
FSK
64
Modulation Techniques(FSK)
65
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

66
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.
67
PSK
68
PSK Constellation
69
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
71
4-PSK
72
4-PSK Constellation
73
8-QAM Signal
74
8-PSK Constellation
75
Have a great day . See you on Friday.
76
PSK Bandwidth
77
4-QAM and 8-QAM Constellation
78
Bandwidth for ASK
79
Bandwidth for FSK
80
16-QAM Constellation
81
Bit Rate and Baud Rate
82
Bit Rate and Baud Rate
83
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

85
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

86
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

87
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

88
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

90
Delta Modulation - example
91
Delta Modulation - Operation
92
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

93
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

94
Analog Modulation
95
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

96
Required Reading
  • Stallings chapter 5

97
Review
98
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)
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