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DATA COMMUNICATION(ELA…)
SIGNAL ENCODING TECHNIQUES1
ENCODING TECHNIQUES Digital data
Analog data
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• Digital Signal• Analog Signal• Digital Signal• Analog Signal
ENCODING ONTO A DIGITAL SIGNAL
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MODULATION ONTO AN ANALOG SIGNAL
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DATA ENCODING CRITERIA
An increase in DR increases BER An increase in SNR decreases BER An increase in BW allows an increase in
DR
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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 bitsSignal 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) 9
COMPARING ENCODING SCHEMESSignal 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
ClockingEase of determining beginning and end of
each bit position10
COMPARING ENCODING SCHEMESError detection
Can be built into signal encodingSignal interference and noise
immunityPerformance in the presence of noise
Cost and complexityThe higher the signal rate to achieve a
given data rate, the greater the costSome codes require signal rate greater
than data rate11
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 timeTransition (low-to-high or high-to-low)
denotes a binary 1No transition denotes binary 0
An example of differential encoding 13
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)
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0 1 0 0 1 1 0 0 0 1 1
NRZ-L
NRZI
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
Because pulses alternate in polarity 17
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 as efficient as NRZ
Technically, a 3 signal level system 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
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0 1 0 0 1 1 0 0 0 1 1
B-AMI
PT
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 D-MANCHESTER)
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0 1 0 0 1 1 0 0 0 1 1
Man
D-Man
BIPHASE – PROS AND CONS
ProsSynchronization on mid bit transition (self
clocking)No DC componentError detection
Absence of expected transition Cons
At least one transition per bit time and possibly two
Maximum modulation rate is twice NRZRequires more bandwidth 24
TRANSMISSION RATES
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TRANSMISSION RATES
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MODULATION RATE
Mbps1T1
Rsec1Tb
b Mbps1T1
Rsec1Tb
b
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SCRAMBLING
Use scrambling to replace sequences that would produce constant voltage
Filling sequence Must produce enough transitions to syncMust be recognized by receiver and replaced
with originalSame length as original
Design Goals No DC component No long sequences of zero level line signal No reduction in data rate Error detection capability 28
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 codeUnlikely to occur as a result of noise
Receiver detects and interprets as octet of all zeros
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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 telephone system
300Hz to 3400HzUse modem (modulator-demodulator)
Basic Encoding TechniquesAmplitude-shift keying (ASK)
Amplitude difference of carrier frequencyFrequency-shift keying (FSK)
Frequency difference near carrier frequencyPhase-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
where the carrier signal is A cos(2πfct)
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ts tfA c2cos
0
1binary 0binary
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
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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 receivedb) 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
where f1 and f2 are offset from carrier frequency fc by equal but opposite amounts
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ts tfA 12cos
tfA 22cos
1binary
0binary
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
Differential PSK
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ts tfA c2cos tfA c2cos 0binary
1binary
tfA c2cos tfA c2cos
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
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ts
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3cos 24cA f t
01
00
10
cos 24cA f t
3cos 24cA f t
cos 24cA f t
PHASE-SHIFT KEYING (PSK)
Multilevel PSKUsing multiple phase angles with each
angle having more than one amplitude, multiple signals elements can be achieved
D = modulation rate, baud R = data rate, bps M = number of different signal elements = 2L
L = number of bits per signal element49
MR
LRD
2log
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 51
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)
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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
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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 noiseApproximations 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 64kbps56
VOICE DIGITIZATION PROCESS
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SamplingCircuit
Quantizerand
Compander
PAM Signal PCM SignalSampling
Clock
AnalogVoiceSignal
DigitalVoiceSignal
PAM Pulse Analog ModulationPCM Pulse Code Modulation
PULSE CODE MODULATION
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EXAMPLES OF DIGITIZED SIGNALS
Digital VoicePCM 128 to 256 quantization levels (64
kbps)Adaptive Differential PCM (ADPCM) 32 kbps
Digital TV (5.5 MHz signal)11 x 106 samples/sec256 to 1024 quantization levelsData rates ≈ 100 Mbps
HDTV (35 MHz signal)70 x 106 samples/secData 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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