DATA COMMUNICATION
(ELA…)
1
SIGNAL ENCODING TECHNIQUES
ENCODING TECHNIQUES


Digital data
Analog data
• Digital Signal
• Analog Signal
• Digital Signal
• Analog Signal
2
ENCODING ONTO A DIGITAL
SIGNAL
3
MODULATION ONTO AN ANALOG
SIGNAL
4
DATA ENCODING CRITERIA
 An
increase in DR increases BER
 An increase in SNR decreases BER
 An increase in BW allows an increase in DR
5
DATA ENCODING CRITERIA (CONT.)

The other factor that improves performance is the
encoding scheme

The encoding scheme is simply the mapping from data
bits to signal elements
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DIGITAL DATA  DIGITAL
SIGNAL
DIGITAL DATA  DIGITAL SIGNAL
 Receiver


needs to know
Timing of bits
Signal levels
 Factors
affecting successful interpretation
of signals
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ENCODING SCHEMES
 Non-return
to Zero-Level (NRZ-L)
 Non-return to Zero Inverted (NRZI)
 Bipolar-Alternate Mark Inversion (AMI)
 Pseudoternary
 Manchester
 Differential Manchester
 Bipolar with 8-Zeros Substitution (B8ZS)
 High-Density Bipolar 3-zeros (HDB3)
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COMPARING ENCODING SCHEMES
 Signal



spectrum
With lack of high-frequency components, less
bandwidth required
With no DC component, AC coupling via
transformer possible
Concentrate power in the middle of the
bandwidth
 Clocking

Ease of determining beginning and end of each
bit position
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COMPARING ENCODING SCHEMES
 Error

detection
Can be built into signal encoding
 Signal
interference and noise
immunity

Performance in the presence of noise
 Cost


and complexity
The higher the signal rate to achieve a given
data rate, the greater the cost
Some codes require signal rate greater than
data rate
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NRZ-L
 Two
different voltages for 0 and 1 bits
 Voltage constant during bit interval
 No transition (i.e., no return to zero
voltage)
 Options:


Absence of voltage for zero, constant positive
voltage for one
More often, negative voltage for one value and
positive for the other
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NRZI
 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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DIFFERENTIAL ENCODING
In complex transmission layouts, it is easy to lose
sense of polarity
 Therefore

Data represented by changes (i.e., transitions) rather
than levels
 More reliable detection of transition rather than level

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NONRETURN TO ZERO (NRZ)
0
1
0
0
1
1
0
0
0
1
1
NRZ-L
NRZI
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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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BIPOLAR-AMI




Uses more than two levels
 “0”  represented by no line signal
 “1”  represented by positive or negative pulse,
pulses alternate in polarity
 No loss of sync if a long string of 1s (0s still a
problem)
No net DC component
 Because the “1” signals alternate in voltage from + to
Lower bandwidth
Easy error detection
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 Because pulses alternate in polarity
PSEUDOTERNARY

Uses more than two levels
 “1”  represented by absence of line signal
 “0”  represented by alternating positive and
negative levels
 No
advantage or disadvantage over
bipolar-AMI
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TRADE OFF FOR MULTILEVEL BINARY
 Not

Technically, a 3 signal level system




as efficient as NRZ
Log2 3 = 1.58 bits
However, each signal element only represents
one bit
Receiver must distinguish between three levels
(+A, -A, 0)
Requires ≈ 3dB more signal power for same
probability of bit error
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BIPOLAR-AMI & PSEUDOTERNARY
0
1
0
0
1
1
0
0
0
1
1
B-AMI
PT
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MANCHESTER
Transition in middle of each bit period
 Transition serves as clock AND data

Low-to-high represents “1”
 High-to-low represents “0”


Used in IEEE 802.3 (Ethernet LAN)
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DIFFERENTIAL MANCHESTER

Mid-bit transition is clocking only
Transition at start of a bit period represents “0”
 No transition at start of a bit period represents “1”

This is a differential encoding scheme
 Used in IEEE 802.5 (Token Ring LAN)

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BIPHASE (MANCHESTER AND DMANCHESTER)
0
1
0
0
1
1
0
0
0
1
1
Man
D-Man
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BIPHASE – PROS AND CONS
 Pros



Synchronization on mid bit transition (self
clocking)
No DC component
Error detection

Absence of expected transition
 Cons



At least one transition per bit time and possibly
two
Maximum modulation rate is twice NRZ
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Requires more bandwidth
TRANSMISSION RATES
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TRANSMISSION RATES
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MODULATION RATE
Tb  1 sec  R 
1
 1 Mbps
Tb
Tb  1 sec  R 
1
 1 Mbps
Tb
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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 replaced with
original
 Same length as original
 Design Goals





No DC component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
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BIPOLAR WITH 8 ZEROS SUBSTITUTION
(B8ZS)
 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
all zeros
detects and interprets as octet of
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HIGH DENSITY BIPOLAR 3 ZEROS
(HDB3)
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
DIGITAL DATA  ANALOG SIGNAL
 Public


300Hz to 3400Hz
Use modem (modulator-demodulator)
 Basic

Amplitude difference of carrier frequency
Frequency-shift keying (FSK)


Encoding Techniques
Amplitude-shift keying (ASK)


telephone system
Frequency difference near carrier frequency
Phase-shift keying (PSK)

Phase of carrier signal shifted
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AMPLITUDE-SHIFT KEYING (ASK)
One binary digit represented by presence of
carrier, at constant amplitude
 Other binary digit represented by absence of
carrier


 A cos2f ct 
s t   

0


binary 1
binary 0
where the carrier signal is A cos(2πfct)
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ASK CHARACTERISTICS
Susceptible to sudden gain changes
 Inefficient modulation technique
 On voice-grade lines, used up to 1200 bps
 Used to transmit digital data over optical fiber

35
ASK – PRINCIPLE OF OPERATION
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ASK – PRINCIPLE OF OPERATION
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ASK BANDWIDTH REQUIREMENTS
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ASK – EXAMPLE
Assuming ASK modulation is to be used, estimate the BW required of a
channel to transmit at the following bit rates: 300bps, 1200bps and
4800bps, assuming
a) the fo of the sequence 101010… is to be received
b) the fo and 3fo are to the received
Comment on your results in relation to the PSTN
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Source: Halsall, F., Data Communications, Computer Networks and Open Systems, (USA: Addison-Wiley, 1996), pg. 61
BINARY FREQUENCY-SHIFT
KEYING (BFSK)

Two binary digits represented by two different
frequencies near the carrier frequency

 A cos2f1t 
s t   

 A cos2f 2t 

binary 1
binary 0
where f1 and f2 are offset from carrier frequency fc by equal
but opposite amounts
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BFSK CHARACTERISTICS
Less susceptible to error than ASK
 On voice-grade lines, used up to 1200bps
 Used for high-frequency (3 to 30 MHz) radio
transmission
 Can be used at higher frequencies on LANs that
use coaxial cable

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BFSK – PRINCIPLE OF OPERATION
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BFSK – PRINCIPLE OF OPERATION
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BFSK BANDWIDTH REQUIREMENTS
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BFSK – EXAMPLE
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PHASE-SHIFT KEYING (PSK)

Two-level PSK (BPSK)

Uses two phases to represent binary digits

 A cos2f ct  binary 1
 A cos2f ct 
s t   


 A cos2f ct      A cos2f ct  binary 0

Differential PSK
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QUADRATURE PSK

More efficient use by each signal element
representing more than one bit
Uses shifts separated by multiples of /2 (90o)
 Each element represents two bits
 Can use 8 phase angles and have more than one
amplitude
 9600bps modems use 12 angles , four of which have
two amplitudes

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QUADRATURE PSK
 Quadrature

PSK (QPSK)
Each element represents more than one bit


A cos  2 f ct  
4

3 

A cos  2 f ct 

4


3 

A cos  2 f c t 

4 



A cos  2 f ct  
4



st   


11
01
00
10
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PHASE-SHIFT KEYING (PSK)
 Multilevel

PSK
Using multiple phase angles with each angle
having more than one amplitude, multiple
signals elements can be achieved
R
R
D 
L log 2 M
D = modulation rate, baud
 R = data rate, bps
 M = number of different signal elements = 2L
 L = number of bits per signal element

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PSK BANDWIDTH REQUIREMENTS
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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, the bit error
rates of PSK and QPSK are about 3dB
superior to ASK and FSK
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ANALOG DATA  DIGITAL
SIGNAL
ANALOG DATA  DIGITAL SIGNAL

Conversion of analog data into digital data

Digitization
Analog to digital conversion done using a CODEC
 Basic encoding techniques

Pulse code modulation (PCM)
 Delta modulation (DM)

53
ANALOG DATA  DIGITAL SIGNAL

Once analog data have been converted to digital
data, the digital data:
can be transmitted using NRZ-L
 can be encoded as a digital signal using a code other
than NRZ-L
 can be converted to an analog signal, using
previously discussed techniques

54
PULSE CODE MODULATION

Based on the sampling theorem


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
Each analog sample is assigned a binary code

Analog samples are referred to as pulse amplitude modulation
(PAM) samples
The digital signal consists of blocks of n bits, where
each n-bit number is the amplitude of a PCM pulse
 Voice data limited to below 4000Hz


Requires 8000 samples per second
55
PULSE CODE MODULATION
4
bit system gives 16 levels
 Quantized


Quantizing error or noise
Approximations mean it is impossible to
recover original signal 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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VOICE DIGITIZATION PROCESS
Sampling
Clock
Analog
Voice
Signal
Sampling
Circuit
PAM Signal
PCM Signal
Quantizer
and
Compander
PAM  Pulse Analog Modulation
PCM  Pulse Code Modulation
Digital
Voice
Signal
57
PULSE CODE MODULATION
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EXAMPLES OF DIGITIZED
SIGNALS
 Digital


PCM  128 to 256 quantization levels (64 kbps)
Adaptive Differential PCM (ADPCM)  32 kbps
 Digital




TV (5.5 MHz signal)
11 x 106 samples/sec
256 to 1024 quantization levels
Data rates ≈ 100 Mbps
 HDTV

Voice
(35 MHz signal)
70 x 106 samples/sec
Data rates of up to hundreds of Mbps
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BASIC ENCODING TECHNIQUES

Analog data to analog signal
Amplitude modulation (AM)
 Angle modulation

Frequency modulation (FM)
 Phase modulation (PM)


Spread Spectrum
Frequency Hopping
 Direct Sequence

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