Download Chapter 5 Signal Encoding Techniques

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
Magnetic
-Nonreturn to Zero-Level (NRZ-L)
Recording
-Nonreturn to Zero Inverted (NRZI)
-Manchester
LAN
-Differential Manchester
-Bipolar -AMI
-B8ZS
WAN
-HDB3
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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
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
Tran at start of interval
No transition at start of
interval
(always a Transition in the
middle 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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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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Example 1
Find the minimum bandwidth for an ASK signal
transmitting at 2000 bps. The transmission mode is
half-duplex.
Solution
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.
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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
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to ASK and FSK
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.

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