ALGORITHM

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```					  Chapter 5
Data Encoding
Review

Information: Numeric Data,
characters, voice, pictures,
codes or any massage that can
be read by and has meaning to
human and machine.
Review
• For transmission:
– Information must be converted into binary
first.
• ASCII table
• Unicode
– Information must be encoded into
electromagnetic signals. (Analog or digital)
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.
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
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.
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)
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.
Terminology
• DC (direct current)component: A
component of a signal with the frequency of
zero.
• Example
– S(t)=1+(4/)sin(2  ft) + ….
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
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
Data Encoding
Digital data, analog signal

• A modem converts digital data to analog
data
– Frequency –shift keying (FSK)
– Phase –shift keying (PSK)
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
Data Encoding
Digital data, digital signal

• Simplest form of digital encoding

– Two voltage level required
– It can be enhanced to improve performance.
Digital–to-Digital Encoding
Schemes
• Unipolar
– Uses only one level of voltage
(almost obsolete)
• Polar
– Uses two level of voltage
• Bipolar
– Uses theree level of voltage
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.
Unipolar

Amplitude

0   1     0    0   0
Time
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
Polar Encoding

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

NRZ             RZ             Biphase

NRZ-L     NRZ-I                         Differential
Manchester
Manchester
(NRZ)

• 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.

Amplitude

0        1   0   0   1    1    1      0
Time
(NRZ)

• 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.

Amplitude

0        1   0   0   0   1   1   1          0

Time
Amplitude

0        1       0       0   1     1   1       0
Time

0        1   0       0   0   1     1       1       0
• One solution to synchronization
issue of NRZ-L and NRZ-I is using RZ
– 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.
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
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
Polar

NRZ             RZ             Biphase

NRZ-L     NRZ-I                         Differential
Manchester
Manchester
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

• Types of biphase encoding:
– Manchester

– Differential Manchester
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
Manchester Encoding

0      1   0   0     1   1   1   0

Zero               One
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
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.
Differential Manchester
Encoding

0   1   0   0   1   1   1   0

Presence of transition at the beginning of the bit
interval represents zero.
Absence of transition at the beginning of the bit
interval represents one.
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
Multilevel Binary
Use more than two levels

• Bipolar-AMI (Alternate mark inversion)

• Pseudoternary (variation of Bipolar-
AMI)
Bipolar Encoding
• Uses there voltage levels
– Positive, negative, and zero

• Zero level represents binary 0

• One’s are represented by alternating
positive and negative voltages
Types of Bipolar Encoding
• Bipolar Alternate Mark Inversion
(AMI)

– Bipolar 8-zero substitution (B8ZS)

– High density bipolar 3 (HDB3)
Types of Bipolar Encoding

Bipolar

AMI     B8ZS HDB3
Bipolar Alternate Mark
Inversion (AMI)

• Mark comes from telegraphy (meaning 1)
• Zero voltage represents zero
• Binary 1’s are represented by alternating
positive and negative voltages
Bipolar
Alternate mark inversion (AMI)

0 1 0 0 1 1 10
Bipolar-AMI and Pseudoternary
Types of Bipolar Encoding
• Pros:
– DC component is zero
– A long sequence of 1’s is always
synchronized.
– Lower bandwidth
– Easy error detection
• Cons
– No mechanism for synchronization of long string
of zeros
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.
Bipolar 8-zero substitution
(B8ZS)

• It works similar to BMI
• Whenever 8 or more consecutive
zeros occurs, signal level is forced to
change.
Pseudoternary
• One represented by absence of line
signal
• Zero represented by alternating positive
and negative
bipolar-AMI
• 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
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
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
HDB3
• High Density Bipolar 3 Zeros
• Based on bipolar-AMI
• String of four zeros replaced with one or
two pulses
B8ZS and HDB3
Digital Data, Analog Signal
• Public telephone system
– 300Hz to 3400Hz
– Use modem (modulator-demodulator)
• Frequency shift keying (FSK)
• Phase shift keying (PK)
Digital to Analog Encoding
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

s(t )  A cos(2   f       t)    Binary 1
c

s(t )  0                      Binary 0
Frequency Shift Keying
• Values represented by different
frequencies (near carrier)
• Less susceptible to error than ASK
• Up to 1200bps on voice grade lines
• Even higher frequency on LANs using
co-ax

s(t )  A cos(2                f       t)        Binary 1
1

s(t )  A cos(2                f         t)   Binary 0
2
f1 and f2 are offset from fc by equal but opposite amount
FSK
Modulation Techniques(FSK)
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
Modulation Techniques (PSK)
(Differential PSK)

s(t )  A cos(2               f       t  )        Binary 1
c

s(t )  A cos(2                  f       t)   Binary 0
c
The phase shift is is in reference to previous bit transmitted
Rather than to some constant reference signal.
PSK
PSK
Constellation
• 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
Modulation Techniques (PSK)
(Differential QPSK)

s(t )  A cos(2 f t   / 4)     Binary 11
c

s(t )  A cos(2 f t  3 / 4)    Binary 10
c

s(t )  A cos(2 f t  5 / 4)   Binary 00
c

s(t )  A cos(2 f t  7 / 4)    Binary 01
c
4-PSK
4-PSK
Constellation
8-QAM Signal
8-PSK
Constellation
Have a great day .
See you on Friday.
PSK
Bandwidth
4-QAM and 8-QAM
Constellation
Bandwidth for FSK
16-QAM
Constellation
Bit Rate and Baud Rate
Bit Rate and Baud Rate
Modulation Techniques(FSK)
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 FSK
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
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
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
Nonlinear Encoding
• Quantization levels not evenly spaced
• Reduces overall signal distortion
• Can also be done by companding
Delta Modulation
staircase function
• Move up or down one level () at each
sample interval
• Binary behavior
– Function moves up or down at each
sample interval
Delta Modulation - example
Delta Modulation - Operation
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
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
Analog
Modulation
•   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
• Stallings chapter 5
Review
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
Earth’s atmosphere.
–Cosmic Noise Radiation of noise by distant stars
penetrating the Earth’s atmosphere.Long haul
telecommunications service (1500 km support
20,000 to 60,000 voice channels)
•An alternative to fiber optic and coaxial cable