Signal Encoding

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Signal Encoding

• Lesson 05
• NETS2150/2850
• http//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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NRZ
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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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Bipolar-AMI Encoding
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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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B8ZS
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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 of Bipolar Pulses (ones) since Last Substitution of Bipolar Pulses (ones) since Last Substitution
Polarity of Preceding Pulse Odd Even
- 000- 00
000 -00-
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B8ZS and HDB3
Change of polarity
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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 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 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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Modulation Techniques (digital data, analog
signal)
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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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ASK
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Relationship between baud rate and bandwidth in
ASK
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Find the minimum bandwidth for an ASK signal
transmitting at 2000 bps. The transmission mode
is half-duplex.
Example 1

• 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.

Solution
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Example 2
Given a bandwidth of 5000 Hz for an ASK signal,
what are the baud rate and bit rate?
Solution
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 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.
Solution
For full-duplex ASK, the bandwidth for each
direction is BW 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 Hz fc (backward) 11000
5000/2 8500 Hz
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Solution to Example 3
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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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FSK
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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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FSK on Voice Grade Line
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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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PSK
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Differential PSK
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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

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Summary

• Various encoding schemes
• Some used in LANs
• Others more suitable in WAN with scrambling
• Read Stallings Section 5.1
• Next Data link layer functions.