Chapter 5: Signal Encoding
Techniques
Pars Mutaf
International Computer Institute
DATA AND COMPUTER COMMUNICATIONS – WILLIAM STALLINGS - 7th EDITION
Signal encoding
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Digital data, digital signal
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Less complex
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Less expensive
Analog data, digital signal
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Digital data, analog signal
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Sometimes, it is more advantageous to shift to digital domain
Some transmission media (unguided media, optical fiber) only propagate
analog signals
Analog data, analog signal
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Modulation to higher frequency allows
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Long distance transmission
Frequency Division Multiplexing
Digital data, digital signal
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Digital signal
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Discrete, discontinuous voltage pulses
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Each pulse is a signal element
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Binary data encoded into signal elements
Encoding schemes (or, line codes)
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Goal: Reliably transmitting data bits to the receiver despite
line noise and other transmission impairments, and as fast
as possible.
Resulting bandwidth of the signal (after encoding) should
not be too high so that it can be transmitted over low
quality links.
Or, we should be able to send data at a high rate.
Signal power should be concentrated in the middle of
bandwidth (sharp frequency spectrum)
Encoding schemes
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Good clock synchronization
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No DC component
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The maximum number of consecutive ones or zeros is
bounded to a reasonable number.
Most long-distance communication channels cannot
transport a DC component.
Lack of DC component allows AC coupling via
transformer, providing isolation and reducing interference
Error detection
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Error detection should be built in signal encoding
scheme, if possible.
Encoding schemes types
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Unipolar
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Polar
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One logic state represented by positive voltage the other
by negative voltage
Differential
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All signal elements have same sign
Data represented by signal level changes rather than
signal levels
Multilevel binary
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Use more than two voltage levels
Encoding schemes
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Nonreturn to Zero-Level (NRZ-L)
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Nonreturn to Zero Inverted (NRZI)
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Bipolar AMI (Alternate Mark Inversion)
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Pseudoternary
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Manchester
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Differential Manchester
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B8ZS
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HDB3
Nonreturn to Zero-Level (NRZ-L)
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Two different voltages for 0 and 1 bits
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Voltage constant during bit interval
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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
Nonreturn to Zero Inverted
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Nonreturn to zero inverted on ones
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Constant voltage pulse for duration of bit
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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
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No transition denotes binary 0
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Example of differential encoding
NRZ
NRZ pros and cons
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Pros
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Cons
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Easy to engineer
Make good use of bandwidth
Lack of synchronization capability
In case of long string of 1s or 0s (for NRZ-L) or long
string of 0s for NRZ-I, synchronization between
transmitter and receiver may be lost due to clock
drift.
DC component
Used for magnetic recording, RS-232 etc.
Bipolar AMI (Alternate Mark
Inversion)
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Multilevel
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Zero represented by no line signal
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One represented by positive or negative pulse
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One pulses alternate in polarity
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No loss of sync if a long string of ones (zeros still
a problem)
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Lower bandwidth
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Easy error detection
Pseudoternary
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One represented by absence of line signal
Zero represented by alternating positive and
negative
No advantage or disadvantage over bipolarAMI
0 = positive or negative level, alternating for
successive zeros, 1= no line signal.
Bipolar-AMI and Pseudoternary
0
1
0
0
1
1
0
0
0
1
1
Bipolar-AMI and Pseudoternary pros
and cons
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Pros
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Reduced DC component
Error detection
Cons
Receiver needs to distinguish three levels
(+A, -A, 0)
Requires more signal power for same probability of bit error
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Synchronization problem when long strings of 0s in the case
of Bipolar AMI and 1s with Pseudoternary
Bit error rate
Manchester (biphase)
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Manchester
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Transition in middle of each bit period
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Transition serves as clock and data
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Low to high represents one
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High to low represents zero
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Used by IEEE 802.3, the media access control (MAC) sublayer of
wired Ethernet.
Differential Manchester
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Midbit transition is clocking and error detection
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Transition at start of a bit period represents zero
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No transition at start of a bit period represents one
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Used by IEEE 802.5 (token ring).
Manchester
Manchester pros and cons
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Con
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At least one transition per bit time and possibly two
Maximum modulation rate is twice NRZ (explained in the
next slide)
Requires more bandwidth
Pros
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Synchronization on mid bit transition (self clocking)
Error detection (absence of expected transition)
No DC component
Modulation rate
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Data rate and modulation rate may differ
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The data rate or bit rate is 1/Tb where Tb is bit duration
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The modulation rate (also called baud rate) is the rate at which
signals are generated
For Manchester encoding the modulation rate is twice the data
rate (consumes more bandwidth)
Modulation rate
low->high : 1
Scrambling (B8ZS and HDB3)
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Manchester encoding solves the bit synchronization problem at
the cost of higher modulation rate
Manchester encoding techniques are mostly used in LANs
which have high bandwidth
Since high signaling rate is required relative to data rate, these
techniques are not suitable for long distance transmission
Scrambling techniques improve this rate by replacing long
sequences of zeros by a fixed pattern that is unlikely to be
caused by noise or other transmission impairments
B8ZS
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Bipolar With 8 Zeros Substitution
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Based on bipolar-AMI
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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+-
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Causes two violations of AMI code
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Unlikely to occur as a result of noise
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Receiver detects and interprets as octet of all zeros
HDB3
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High Density Bipolar 3 zeros
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Based on bipolar-AMI
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Fourth zero replaced by a code violation
B8ZS and HDB3
+0+: invalid
0-: invalid
0-:invalid
+-+:invalid
B8ZS and HDB3
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Cons
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Complexity
No solution for 7 consecutive 0s
Pros
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Synchronization
Error detection
No DC component
Comparison
Comparison
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Most of the energy in NRZ and NRZI signals is between DC
and half the bit rate, but main limitations are: lack of
synchronization and presence of DC component
Bipolar AMI and pseudoternary signals have less bandwidth
(this is good) than NRZ, and there is no net DC component
Wider bandwidth with Manchester, but best synchronization
and error detection
B8ZS and HDB3 have a sharp spectrum and suitable for high
data rate transmission, also good synchronization, no DC
component and error detection.
Digital data, analog signal
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Public telephone system
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300Hz to 3400Hz
Use modem (modulator-demodulator)
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Amplitude shift keying (ASK)
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Frequency shift keying (FSK)
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Phase shift keying (PSK)
Modulation Techniques
Amplitude Shift Keying (1)
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Values represented by different amplitudes of
carrier
Usually, one amplitude is zero
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i.e. presence and absence of carrier is used
Inefficient and susceptible to noise (a 1 may be
changed to 0, and 0 to 1)
Up to 1200bps on voice grade lines
Used over optical fiber (one signal element is
light pulse, the other is absence of light).
Amplitude Shift Keying (2)
Acos (2πfct)
if 1
0
if 0
s(t) =
Binary Frequency Shift Keying (1)
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Most common form is binary FSK (BFSK)
Two binary values represented by two different
frequencies (near carrier)
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Less susceptible to error than ASK
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Up to 1200bps on voice grade lines
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High frequency radio
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Even higher frequency on LANs using co-ax
Binary Frequency Shift Keying (2)
Acos (2πf1t)
if 1
Acos (2πf2t)
if 0
s(t) =
Multiple FSK
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More than two frequencies used
Each signalling element represents more than
one bit
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More bandwidth efficient
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More prone to error
FSK on Voice Grade Line
Voice grade line will pass frequencies between 300 and 3400 hz.
Phase Shift Keying
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Phase of carrier signal is shifted to represent
data
Differential PSK: Phase shifted relative to
previous transmission rather than some
reference signal
Acos (2πfct + Φ)
if 1
s(t) =
Acos (2πfct)
if 0
Differential PSK
1 represented by a phase shift
Quadrature PSK (1)
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More efficient use by each signal element
representing more than one bit
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Shifts of π/2 (90o)
Each element represents two bits
Can use 8 phase angles and have more than one
amplitude
9600bps modem uses 12 angles, four of which have two
amplitudes
Quadrature PSK (2)
s(t) =
Acos (2πfct + π/4)
Acos (2πfct)+ 3π/4)
Acos (2πfct + 5π/4)
Acos (2πfct + 7π/4)
if 11
if 10
if 00
if 01
Bandwidth
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For ASK and PSK the bandwidth is given as BT = (1 + r) R,
where R is the bit rate and r is a constant between 0 and 1.
For FSK, the bandwidth is given as BT = 2 ΔF + (1 + r) R, where
ΔF = f2 – fc = fc – f1
For multilevel PSK, bandwidth can be given as
BT = ((1 + r)/ log2 M)R, where M is the number of different signal
elements.
For multilevel FSK, we have, BT = ((1 + r)M/ log2M)R
Analog data, digital signal
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Conversion of analog data into digital data
Digital data can then be transmitted using
digital signal encoding techniques (e.g. NRZ-L,
B8ZS, Bipolar AMI, etc.)
Digital data can then be converted to analog
signal
Analog to digital conversion done using a codec
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Pulse code modulation
Delta modulation
Why go digital?
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Repeaters can be used instead of amplifiers
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Data compression and encryption is possible
Digitizing analog data
Pulse Code Modulation (PCM)
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Nyquist Theorem:
Let signal bandwidth B
– Let sampling rate f
– If f>2B, then the samples will contain all the information
of the original signal
– f is called the Nyquist rate.
Voice data limited to below B=4000Hz, requiring f=8000 sample
per second
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Each sample assigned digital value and sent over the
transmission medium
PCM example
Pulse Code Modulation (PCM)
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Voice data limited to below 4000Hz, requiring
8000 samples per second
8 bit sample gives 256 levels (Compact Disc
uses 16 bits)
8000 samples per second of 8 bits each gives
64kbps
Data compression can improve this
Delta modulation
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Analog input is approximated by a staircase
function
Move up or down one level (δ) at each sample
interval
Binary behavior
Function moves up or down at each sample
interval
Delta modulation example
Delta modulation
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Used when quality is not of primary importance.
The transmitted data is reduced to a 1-bit data
stream
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1 means increase
0 means decrease
To achieve high signal-to-noise ratio, delta
modulation must use oversampling
techniques, that is, the analog signal is
sampled at a rate several times higher than the
Nyquist rate.
Analog data, analog signals
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Why modulate analog signals?
Higher frequency can give more efficient
transmission
Permits frequency division multiplexing
(Chapter 8)
Types of modulation
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Amplitude
Frequency
Phase
Analog modulation
Amplitude modulation
s t =[1na x t ]cos 2πf c t
where,
DC component
removed by cos
2πfct is the carrier,
x(t) is the input signal, normalized to unity amplitude
na is the modulating index (na <1, otherwise signal envelop will cross
the time axis and information will be lost)
m(t)=nax(t)
Amplitude modulation