data communication -signalencodingtechniques
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Dat a Com m unic a t ion
Chapt er 5
Signal Enc oding Tec hniques
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Enc oding Tec hniques Digital data, digital signal
Analog data, digital signal
Digital data, analog signal
Analog data, analog signal
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Signal Enc oding Tec hniques
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Digi t a l Dat a, Dig i t a l Signa l Digital signal
Discrete, discontinuous voltage pulses
Each pulse is a signal element
Binary data encoded into signal elements
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Term s (1) Unipolar
All signal elements have same sign
Polar
One logic state represented by positive voltage theother by negative voltage
Data rate
Rate of data transmission in bits per second
Duration or length of a bitTime taken for transmitter to emit the bit
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Term s (2) Modulation rate
Rate at which the signal level changes
Measured in baud = signal elements per second
Mark and Space
Binary 1 and Binary 0 respectively
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In t erpret ing Dig i t a l Signals Receiver Need to know
Timing of bits - when they start and end
Signal levels
Factors affecting successful interpreting of
signalsSignal to noise ratio (SNR).
Data rate
BandwidthEncoding scheme.
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Com par ison of Enc oding
Sc hem es (1) Signal Spectrum
Lack of high frequencies reduces required bandwidth
Lack of dc component allows ac coupling viatransformer, providing isolation, reduce interference.
Concentrate transmitted power in the middle of the
channel bandwidth to reduce distortion.
Clocking
Synchronizing transmitter and receiverExternal clock
Sync mechanism based on transmitted signal.
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Com par ison of Enc oding
Sc hem es (2) Error detection
Can be built into signal encoding scheme.
Signal interference and noise immunity
Some codes are better than others
Cost and complexityHigher signal rate (& thus data rate) lead to higher
costs
Some codes require signal rate greater than data rate
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Enc oding Sc hem es Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar AMI (Alternate mark inversion)
Pseudoternary
Manchester
Differential Manchester
B8ZS(bipolar with 8-zeros substitution) HDB3( high density bipolar-3 zeros)
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Nonre t urn t o 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 and
positive for zero.
This is NRZ-L
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Nonret urn t o Zero Inver t ed (NRZI) Nonreturn to zero inverted on ones
Constant voltage pulse for duration of bit
Data encoded as presence or absence of signaltransition at beginning of bit time
Transition (low to high or high to low) denotes abinary 1
No transition denotes binary 0
An example of differential encoding
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NRZ
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Dif ferent ia l Enc oding Data represented by changes rather than levels
More reliable detection of transition rather thanlevel
In complex transmission layouts it is easy to
lose sense of polarity
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NRZ pros and c ons Pros
Easy to engineer
Make good use of bandwidth (most of energyconcentrated in the middle of transmission BW).
Cons
dc component
Lack of synchronization capability
Used for magnetic recording (data storage). Not often used for signal transmission.
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Mul t i leve l B inary Use more than two voltage levels
Bipolar-AMI
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarityNo loss of sync if a long string of ones (zeros still a
problem)
No net dc componentLower bandwidth than NRZ.
Easy error detection
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Pseudoternary
One represented by absence of line signal
Zero represented by alternating positive andnegative
No advantage or disadvantage over bipolar-AMI
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Bipolar-AMI and Pseudot ernary
0 1 0 0 1 1 0 0 0 1 1
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Trade Of f for Mul t i level B inary
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 sameprobability of bit error
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Biphase
Manchester
Transition in middle of each bit duration
Transition serves as clock and data Low to high represents one
High to low represents zero
Used by IEEE 802.3
Differential Manchester
Midbit 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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Enc oding Sc hem es
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Manc hest er Enc oding
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Di f ferent ia l Manc hest er
Encoding
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Biphase Cons and Pros
Cons
At least one transition per bit time and possibly two
Maximum modulation rate is twice data rate.
Requires more bandwidth
ProsSynchronization on mid bit transition (self clocking)
No dc component
Error detection Absence of expected transition
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Modula t ion Rat e
2
R RD
L Log M = =
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Scrambl ing
Use scrambling to replace sequences that wouldproduce 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
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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 positiveencode as 000+-0-+
If octet of all zeros and last voltage pulse preceding was negativeencode 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
Polarity of preceding
pulse
substitution
+ 000+-0-+
- 000-+0+-
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HDB3
High Density Bipolar 3 Zeros
Based on bipolar-AMI
String of four zeros replaced with one or twopulses
Number of ones since last substitution
Polarity of preceding
pulse
odd even
- 000- +00+
+ 000+ -00-
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B8ZS and HDB3
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Digi t a l Dat a, Analog Signa l
Public telephone system
300Hz to 3400Hz
Use modem (modulator-demodulator)
Amplitude shift keying (ASK)
Frequency shift keying (FSK) Phase shift keying (PK)
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Modula t ion Tec hniques
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Am pl i t ude Sh i f t K ey ing
encode 0/1 by different carrier amplitudesusually have one amplitude zero
susceptible to sudden gain changes inefficient
used forup to 1200bps on voice grade lines
very high speeds over optical fiber
{ cos(2 ):binary 10 :binary 0( ) c A f t s t =
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Binary Frequenc y Shi f t K ey ing
most common is binary FSK (BFSK)
two binary values represented by two different
frequencies (near carrier) less susceptible to error than ASK
used for
up to 1200bps on voice grade lines high frequency radio
even higher frequency on LANs using co-ax
{ 12
cos(2 ):binary 1
cos(2 ):binary 0( )A f t
A f t s t
=
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FSK on Voic e Grade L ine
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Mult ip le FSK
More than two frequencies used
More bandwidth efficient than BFSK.
More susceptible to error Each signal element represents more than one bit.
2
( ) cos 2 , 1
(2 1 )
the difference frequancy
L=# of bits per signal element.
log
i i
i c d
d
s t A f t i M
f f i M f
f
M L
= = +
=
=
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Phase Shi f t K eying
phase of carrier signal is shifted to represent data
binary PSK(BPSK)
two phases represent two binary digits
differential PSK(DPSK)
phase shifted relative to previous transmission ratherthan some reference signal
{ cos(2 ):binary 1cos(2 ):binary 0( ) c c A f t
A f t s t
=
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Dif fe rent ia l PSK (DPSK )
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Quadrat ure PSK (QPSK )
get more efficient use of BW if each signalelement represents more than one bit
eg. shifts of/2 (90o)each element represents two bits
split input data stream in two & modulate onto carrier
& phase shifted carrier can use 8 phase angles & more than one
amplitude
9600bps modem uses 12 angles, four of which havetwo amplitudes
Offset QPSK (orthogonal OQPSK)
Delay in Q stream
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QPSK and OQPSK Modu la t ors
1 1( ) ( ) cos 2 ( )sin 22 2
c cs t I t f t Q t f t =
E l f QPSK d OQPSK
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Ex am ples of QPSK and OQPSK
Waveforms
P f f Di i l A l
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Per form anc e o f Dig i t a l t o Ana log
Modu lat ion Sc hem es
bandwidth
ASK/PSK bandwidth directly relates to bit rate
multilevel PSK gives significant improvements
in presence of noise:
bit error rate of PSK and QPSK are about 3dBsuperior to ASK and FSK
for MFSK & MPSK have tradeoff between bandwidthefficiency and error performance
Q adrat re Am pl i t de
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Quadrat ure Am pl i t ude
Modula t ion
QAM used on asymmetric digital subscriber line(ADSL) and some wireless
Combination of ASK and PSK
Logical extension of QPSK
Send two different signals simultaneously onsame carrier frequency
Use two copies of carrier, one shifted 90
Each carrier is ASK modulated
Two independent signals over same medium
Demodulate and combine for original binary output
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QAM Modulat or
1 2( ) ( ) cos 2 ( )sin 2c cs t d t f t d t f t = +
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QAM Leve ls
Two level ASK
Each of two streams in one of two states
Four state system
Essentially QPSK
Four level ASK
Combined stream in one of 16 states
64 and 256 state systems have been
implemented Improved data rate for given bandwidth
Increased potential error rate
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Analog Dat a, Dig i t a l Signa l
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 otherthan NRZ-L
Digital data can then be converted to analog signal
Analog to digital conversion done using a codec
Pulse code modulation
Delta modulation
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Digi t izing Ana log Dat a
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Pulse Code Modu la t ion(PCM) (1)
sampling theorem:If a signal is sampled at regular intervals at a rate
higher than twice the highest signal frequency, thesamples contain all information in original signal
Voice data limited to below 4000Hz
Require 8000 sample per second Analog samples (Pulse Amplitude Modulation,
PAM)
Each sample assigned digital value
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Pulse Code Modu la t ion(PCM) (2)
4 bit system gives 16 levels
Quantized
Quantizing error or noise
Approximations mean it is impossible to recoveroriginal exactly
8 bit sample gives 256 levels
Quality comparable with analog transmission
8000 samples per second of 8 bits each gives64kbps
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PCM Ex am ple
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PCM Bloc k Diagram
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Nonl inear Enc oding
Quantization levels not evenly spaced
Reduces overall signal distortion
Can also be done by companding
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Effec t of Non-Linear Coding
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Typic a l Com panding Func t ions
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Del t a Modula t ion
Analog input is approximated by a staircasefunction
Move up or down one level () at each sampleinterval
Binary behavior
Function moves up or down at each sample interval
hence can encode each sample as single bit
1 for up or 0 for down
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Del t a Modula t ion Ex am ple
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Del t a Modulat ion Operat ion
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PCM verses Del t a Modulat ion
DM has simplicity compared to PCM but has worse SNR
issue of bandwidth usedeg. for good voice reproduction with PCM
want 128 levels (7 bit) & voice bandwidth 4khz need 8000 x 7 = 56kbps
data compression can improve on this still growing demand for digital signals
use of repeaters, TDM, efficient switching
PCM preferred to DM for analog signals
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Analog Dat a, Analog Signa ls
Why modulate analog signals?
Higher frequency can give more efficient transmission
Permits frequency division multiplexing (chapter 8)
Types of modulation
Amplitude
Frequency
Phase
Analog
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Analog
Modula t ion
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Am pl i t ude m odula t ion (AM) The AM signal is
The transmitted power
Transmission bandwidth
DSBSC
SSB
( ) [1 ( )]cos 2 , where ( ) ( )a c as t n x t f t m t n x t = + =
: modulation indexif 1, then ( ) can be recoverd
from the envolpe [1 ( )] of ( )
a
a
a
nn m t
n x t s t
+
2
12
at c
np p
= +
2 , where is bandwidth of ( )T B B B m t =
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Angle m odulat ion(2)
Modulation index of FM and PM is
Carsons rule
f
B
=
2( 1) 2( )T B B f B= + = +