chapter 5cseku.ac.bd/faculty/~kazi/files/cn/chapter5.pdf · no advantage or disadvantage over...
TRANSCRIPT
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Chapter 5
Signal Encoding Techniques
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Reading Materials
Data and Computer Communications,
William Stallings
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Encoding Techniques
Digital data→Digital signal
Analog data→Digital signal
Digital data→Analog signal
Analog data→Analog signal
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Digital Data → Digital Signal
Digital signal
– Discrete, discontinuous voltage pulses
– Each pulse is a signal element
– Binary data encoded into signal elements
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Terms
Unipolar
– All signal elements have same sign
Polar
– One logic state represented by positive voltage
the other by negative voltage
Data rate
– Rate of data transmission in bits per second
Duration or length of a bit
– Time taken for transmitter to emit the bit
Modulation rate
– Rate at which the signal level changes
– Measured in baud = signal elements per second
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Encoding Schemes
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Nonreturn to Zero-Level (NRZ-L)
Two different voltages for 0 and 1 bits
Voltage constant during bit interval
– no transition i.e. no return to zero voltage
e.g. absence of voltage for zero, constant positive
voltage for one
More often, negative voltage for one value and
positive for the other
This is NRZ-L
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Nonreturn to Zero Inverted
Nonreturn to zero inverted on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of signal
transition at beginning of bit time
Transition (low to high or high to low) denotes a binary 1
No transition denotes binary 0
An example of differential encoding
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Multilevel Binary
Use more than two levels
Bipolar-AMI (alternate mark inversion)
– zero represented by no line signal
– one represented by positive or negative pulse
– one pulses alternate in polarity– no loss of sync if a long string of ones (zeros still a
problem)
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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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Biphase
Manchester
– Transition in middle of each bit period
– Transition serves as clock and data
– Low to high represents one
– High to low represents zero
– Used by IEEE 802.3
Differential Manchester
– Mid-bit transition is clocking only
– Transition at 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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Manchester Encoding
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Scrambling
Use scrambling to replace sequences that would
produce constant voltage
Filling sequence
– Must produce enough transitions to synchronize
– 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
Amplitude Shift Keying (ASK)
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
Quadrature Amplitude Modulation (QAM)
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Modulation Techniques
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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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Binary Frequency Shift Keying
Most common form is binary FSK (BFSK)
Two binary values represented by two different
frequencies (near carrier)
Less susceptible to error than ASK
Up to 1200bps on voice grade lines
Even higher frequency on LANs using coaxial cable
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Full-Duplex FSK
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Phase Shift Keying
Phase of carrier signal is shifted to represent
data
Binary PSK
– Two phases represent two binary digits
Differential PSK
– Phase shifted relative to previous transmission
rather than some reference signal
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Differential PSK
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Quadrature PSK
More efficient use by each signal element
representing more than one bit
– e.g. shifts of π/2 (90o )
– each element represents two bits
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PSK Constellation
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Quadrature Amplitude Modulation
QAM used on fast modems and asymmetric
digital subscriber line (ADSL)
Combination of ASK and PSK
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Time Domain for an 8-QAM Signal
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Analog Data → Digital Signal
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Analog Signal to PCM Digital Code
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Nyquist Theorem
The sampling rate must be at least 2 times
the highest frequency.
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PCM Example
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☺
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Problems
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Draw the digital encoded signals of the following bit stream using
Manchester, Differential Manchester, and HDB3 encoding schemes.
1 0 0 0 0 1 1 0 0 0 0 0 0 0 0 1 1 1 0 1
Modulate the above digital bit stream to analog signal using BFSK, BPSK and
differential BPSK modulation techniques.
Draw 8-QAM signal of the following bit stream.
111 010 100 001 101 010 100 011 101 010 110 000
At least how many PAM samples are required to quantize the above signal? If
each of the PAM sample is quantized to any of 16 different levels, what would
be the PCM code of the signal?
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Courtesy
Professor Jiying Zhao, University of Ottawa