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1
Signal Encoding
Lesson 05
NETS2150/2850http://www.ug.cs.usyd.edu.au/~nets2150/
School of IT, The University of Sydney
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Lecture Outline Encoding schemes for digital data to
transmit in digital transmission systems– NRZ schemes– Manchester schemes in LANs– AMI schemes
• With scrambling for WANs use
Encoding schemes for digital data to transmit in analog transmission systems– ASK Scheme– FSK Scheme– PSK Scheme
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Various Encoding Techniques
Encoding is the conversion of streams of bits into a signal (digital or analog).
Categories of Encoding techniques:– Digital data, digital signal– Analogue data, digital signal– Digital data, analog signal– Analogue data, analog signal
Digital transmission
Analog transmission
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Digital Data, Digital Signal(Digital to Digital)
Digital signal– Discrete, discontinuous voltage pulses– Each pulse is a signal element– Binary data encoded into signal elements
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Interpreting Signals
Need to know– Timing of bits - when they start and end– Signal levels
Factors affecting interpretation of signals– SNR– Data rate– Bandwidth
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Comparison of Encoding Schemes
Error detection– Can be built into signal encoding
Cost and complexity– Higher signal rate (& thus data rate) lead to higher
costs Clocking
– Synchronizing transmitter and receiver Signal spectrum
– Bandwidth requirement– Presence of dc component
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Digital-to-Digital Encoding Schemes 3 Broad Categories: Unipolar, Polar,
and Bipolar
-Nonreturn to Zero-Level (NRZ-L)
-Nonreturn to Zero Inverted (NRZI)
-Manchester
-Differential Manchester
-Bipolar -AMI
-B8ZS
-HDB3
Magnetic Recording
LAN
WAN
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Nonreturn to Zero-Level (NRZ-L)
Polar Encoding Two different voltages for 0 and 1 bits Voltage constant during bit interval
– no transition i.e. no return to zero voltage Negative voltage for one value and
positive for the other
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Nonreturn to Zero Inverted (NRZI)
Polar Transition (low to high or high to low)
denotes a binary 1 No transition denotes binary 0 This is an example of differential
encoding
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Differential Encoding
Polar Better encoding technique Data represented by changes rather
than levels More reliable detection of bit in noisy
channels rather than level
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NRZ pros and cons
Pros– Easy to engineer– Make good use of bandwidth
Cons– Lack of synchronisation capability– Presence of a dc component
Used for digital magnetic recording Not often used for signal transmission
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Biphase Schemes Polar- signal elements have opposite voltage
level (-ve and +ve)
Overcomes the limitations on NRZ codes
Two biphase techniques are commonly used:– Manchester– Differential Manchester
Heavily used in LAN applications
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Biphase Scheme1: Manchester
Transition in middle of each bit interval
Low to high represents one High to low represents zero Used by IEEE 802.3 (Ethernet LAN)
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Biphase Scheme 2: Differential Manchester
Midbit transition is clocking only Transition at start of a bit interval represents
zero No transition at start of a bit interval
represents one Note: this is a differential encoding scheme Used by IEEE 802.5 (Token Ring LAN)
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Biphase Pros and Cons
Cons– At least one transition per bit time and possibly
two– Maximum baud rate is twice NRZ– Requires more bandwidth
Pros– Synchronization on mid bit transition (self clocking)– Error detection
• Absence of expected transition
– No dc component
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Multilevel Binary (Bipolar)
Use more than two voltage levels Bipolar-AMI (Alternate Mark Inversion)
– zero represented by no line signal– one represented by positive or negative pulse– ‘one’ pulses alternate in polarity– No loss of sync if a long string of ones (zeros still a
problem)– Lower bandwidth– Easy error detection
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Trade Off for Multilevel Binary
Not as efficient as NRZ– Receiver must distinguish between three
levels (+A, -A, 0)
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Scrambling Technique Used to replace sequences that would produce
constant voltage Produce “filling” sequence that:
– Must produce enough transitions to sync– Must be recognized by receiver and replace with original– Same length as original
Avoid long sequences of zero level line signal No reduction in data rate Error detection capability Two commonly used techniques are: B8ZS, and
HDB3 Used for long distance transmission (WAN)
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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 - intentional
– Unlikely to occur as a result of noise Receiver detects and interprets as octet of all
zeros HDB3 – similar but based on 4 zeros
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HDB3
High Density Bipolar 3 Zeros Based on bipolar-AMI String of four zeros replaced with one or
two pulses
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HDB3 Substitution Rules
# of Bipolar Pulses (ones) since Last Substitution
Polarity of Preceding Pulse
Odd Even
- 000- +00+
+ 000+ -00-
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Recap of Digital Signal Encoding Formats
0 1
NRZL High level Low level
NRZI No transition at start of interval
transition
Bipolar-AMI No line signal +ve line signal
Manchester Transition from high to low in the middle of interval
Transition from low to high in the middle of interval
Diff Manchester (always a Transition in the middle of interval)
Tran at start of interval No transition at start of interval
HDB3 Same as bipolar-AMI, except that any string of four zeros is replaced by a string with one code violation
B8ZS Same as bipolar-AMI, except that any string of eight zeros are replaced by a string of two code violations
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Digital Data, Analog Signal Some transmission media only transmit
analog signals. Public telephone system
– 300Hz to 3400Hz (voice frequency range)– Use modem (modulator-demodulator)
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Digital to Analog modulation techniques:Modulation involves operation on signal
characteristics: frequency, phase, amplitude.
Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PSK)
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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
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In ASK, baud rate and bit rate are the same. The baud rate is therefore 2000. An ASK signal requires a minimum bandwidth equal to its baud rate. Therefore, the minimum bandwidth is 2000 Hz.
Find the minimum bandwidth for an ASK signal transmitting at 2000 bps. The transmission mode is half-duplex.
SolutionSolution
Example 1Example 1
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Example 2Example 2
Given a bandwidth of 5000 Hz for an ASK signal, what are the baud rate and bit rate?
SolutionSolution
In ASK the baud rate is the same as the bandwidth, which means the baud rate is 5000. But because the baud rate and the bit rate are also the same for ASK, the bit rate is 5000 bps.
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Example 3Example 3
Given a bandwidth of 10,000 Hz (1000 to 11,000 Hz), draw the full-duplex ASK diagram of the system. Find the carriers and the bandwidths in each direction. Assume there is no gap between the bands in the two directions.
SolutionSolution
For full-duplex ASK, the bandwidth for each direction isBW = 10000 / 2 = 5000 Hz
The carrier frequencies can be chosen at the middle of each band (see Fig. 5.5).
fc (forward) = 1000 + 5000/2 = 3500 Hzfc (backward) = 11000 – 5000/2 = 8500 Hz
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Binary Frequency Shift Keying Most common form is binary FSK (BFSK) Two binary values represented by two
different frequencies (near carrier) Less susceptible to error than ASK Up to 1200bps on voice grade lines High frequency radio Even higher frequency on LANs using co-
ax
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Multiple FSK
More than two frequencies used More bandwidth efficient More prone to error Each signalling element represents
more than one bit
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Phase Shift Keying
Phase of carrier signal is shifted to represent data
Binary PSK– Two phases represent two binary digits
Differential PSK– Phase shifted relative to previous
transmission rather than some reference signal
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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, bit error rate of PSK
and QPSK are about 3dB superior to ASK and FSK