# Chapter 5 Signal Encoding Techniques - University of Sydney

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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     1
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
– FSK Scheme
– PSK Scheme
2
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    Digital transmission
– Analogue data, digital signal
– Digital data, analog signal     Analog transmission
– Analogue data, analog signal

3
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

4
Interpreting Signals

   Need to know
– Timing of bits - when they start and end
– Signal levels
   Factors affecting interpretation of
signals
– SNR
– Data rate
– Bandwidth

5
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
6
Digital-to-Digital Encoding Schemes
 3 Broad Categories: Unipolar, Polar,
and Bipolar
Recording
-Manchester
LAN
-Differential Manchester
-Bipolar -AMI
-B8ZS                         WAN
-HDB3

7

 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

8

 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

10
Differential Encoding

 Polar
 Better encoding technique
 Data represented by changes rather
than levels
 More reliable detection of bit in noisy
channels rather than level

11
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
12
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

13
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)

14
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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16
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
17
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
18
Bipolar-AMI Encoding

19
Trade Off for Multilevel Binary

   Not as efficient as NRZ
– Receiver must distinguish between three
levels
(+A, -A, 0)

20
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)

21
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

23
HDB3

 High Density Bipolar 3 Zeros
 Based on bipolar-AMI
 String of four zeros replaced with one or
two pulses

24
HDB3 Substitution Rules
# of Bipolar Pulses (ones) since Last Substitution

Polarity of    Odd                           Even
Preceding
Pulse
-              000-                          +00+

+              000+                          -00-

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B8ZS and HDB3

Change of polarity   26
Recap of Digital Signal Encoding
Formats
0                             1
NRZL                          High level                    Low level

NRZI                          No transition at start of     transition
interval

Bipolar-AMI                   No line signal                +ve line signal
Manchester                    Transition from high to low   Transition from low to high
in the middle of interval     in the middle of interval
Diff Manchester               Tran at start of interval     No transition at start of
(always a Transition in the                                 interval
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

27
Digital Data, Analog Signal
 Some transmission media only transmit
analog signals.
 Public telephone system
– 300Hz to 3400Hz (voice frequency range)
– Use modem (modulator-demodulator)

28
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)

29
Modulation Techniques (digital data,
analog signal)

30
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                       31

32
Relationship between baud rate
and bandwidth in ASK

33
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.
34
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.

35
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             36
Solution to Example 3

37
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                                  38
FSK

39
Multiple FSK

 More than two frequencies used
 More bandwidth efficient
 More prone to error
 Each signalling element represents
more than one bit

40
FSK on Voice Grade Line

41
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
42
PSK

43
Differential PSK

44
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
45
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.

46

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