1. Lesson 05 NETS2150/2850 http://www.ug.cs.usyd.edu.au/~nets2150/
Signal Encoding
Lesson 05
NETS2150/2850
School of IT, The University of Sydney

2. 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

3. 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

4. Digital Data, Digital Signal (Digital to Digital)
Discrete, discontinuous voltage pulses
Each pulse is a signal element
Binary data encoded into signal elements

5. Interpreting Signals Need to know
Timing of bits - when they start and end
Signal levels
Factors affecting interpretation of signals
SNR
Data rate
Bandwidth

6. 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

7. 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

8. 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

9. 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

10. NRZ

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

12. NRZ pros and cons Pros Cons Used for digital magnetic recording
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

13. 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

14. Biphase Scheme1: Manchester
Transition in middle of each bit interval
Low to high represents one
High to low represents zero
Used by IEEE (Ethernet LAN)

15. 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 (Token Ring LAN)

17. Biphase Pros and Cons Cons Pros
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

18. 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

19. Bipolar-AMI Encoding

20. Trade Off for Multilevel Binary
Not as efficient as NRZ
Receiver must distinguish between three levels (+A, -A, 0)

21. 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)

22. Bipolar With 8 Zeros Substitution (B8ZS)
Based on bipolar-AMI
If octet of all zeros and last voltage pulse preceding was positive, encode as
If octet of all zeros and last voltage pulse preceding was negative, encode as
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

23. B8ZS

24. HDB3 High Density Bipolar 3 Zeros Based on bipolar-AMI
String of four zeros replaced with one or two pulses

25. HDB3 Substitution Rules
# of Bipolar Pulses (ones) since Last Substitution
Polarity of Preceding Pulse
Odd
Even -
000-
+00+ +
000+
-00-

26. B8ZS and HDB3
Change of polarity

27. Recap of Digital Signal Encoding Formats1
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
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

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

29. 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)

30. Modulation Techniques (digital data, analog signal)

31. 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

32. ASK

33. Relationship between baud rate and bandwidth in ASK

34. Example 1
Find the minimum bandwidth for an ASK signal transmitting at 2000 bps. The transmission mode is half-duplex.
In ASK, baud rate and bit rate are the same. The baud rate is therefore An ASK signal requires a minimum bandwidth equal to its baud rate. Therefore, the minimum bandwidth is 2000 Hz.
Solution

35. 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 But because the baud rate and the bit rate are also the same for ASK, the bit rate is 5000 bps.

36. 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 = / 2 = 5000 Hz
The carrier frequencies can be chosen at the middle of each band (see Fig. 5.5).
fc (forward) = /2 = 3500 Hz
fc (backward) = – 5000/2 = 8500 Hz

37. Solution to Example 3

38. 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

39. FSK

40. Multiple FSK More than two frequencies used More bandwidth efficient
More prone to error
Each signalling element represents more than one bit

41. FSK on Voice Grade Line

42. 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

43. PSK

44. Differential PSK

45. 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

46. 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.