1. EEC4113 Data Communication & Multimedia System Chapter 2: Baseband Encoding by Muhazam Mustapha, September 2012

2. Learning Outcome
By the end of this chapter, students are expected to be able to explain link level baseband encoding for transmission

3. Chapter Content Polarity in baseband encoding Encoding techniques
NRZ-L, NRZI
Bipolar
Biphase
Modulation rate
Scrambling techniques

4. Polarity in Baseband Encoding
CO1, CO2

5. Baseband Encoding
Definition: encoding of the signal in the spectrum range from 0 Hz to the data rate frequency
Use: encoding of data for short distances, LAN, Ethernet
CO1

6. Polarity in Encoding Unipolar
All signals follow the values of the binary
Amplitude 1 1 1 1
Time
CO1, CO2

7. Polarity in Encoding Polar One signal sign follows one data binary
Amplitude 1 1 1 1
Time
CO1, CO2

8. Polarity in Encoding Bipolar 3 levels of signal: +ve, −ve, 0
Binary 0 is level 0; binary 1 alternates sign
Amplitude 1 1 1 1
Time
CO1, CO2

9. Encoding Techniques
CO1, CO2

10. Nonreturn to Zero-Level (NRZ-L)
Two different voltages for 0 and 1 bits
Negative voltage for 1
Positive voltage for 0
Amplitude 1 1 1 1
Time
CO1, CO2

11. Nonreturn to Zero-Inverted (NRZI)
Bit 1: Transition at the beginning of bit time
Bit 0: No transition
A kind of differential encoding – data is represented by transition rather than level
Amplitude
transitions 1 1 1 1
Time
CO1, CO2

12. Advantages of NRZ Coding
Easiest to engineer
Make efficient use of bandwidth
Most of the energy in NRZ-L & NRZ-I signals (80%) is between DC and half of the bit rate
e.g. If NRZ code is used to generate a signal with data rate of 9600 bps, then most of the energy in the signal is concentrated between DC & 4800 Hz
CO1, CO2

13. Spectral Density of Various Schemes
B8ZS, HDB3
AMI, pseudoternary
Manchester, differential Manchester
NRZ
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Normalized frequency (f/R)
CO1

14. Disadvantages of NRZ Coding
Presence of DC component (zero frequency)
Presents problems for a system that cannot pass low frequencies
e.g. Telephone line can’t pass frequencies below 300 Hz
CO1

15. Disadvantages of NRZ Coding
Also presents problems for a system that uses electrical coupling via transformer
There must be direct physical attachment of transmission component
Electrical (AC) coupling via transformer, which provides excellent electrical isolation that reduces interference, is not possible
e.g. A long distance link may use one or more transformers to isolate different parts of the line electrically
CO1

16. Disadvantages of NRZ Coding
Lack of synchronization capability
Consider long string of 1-s and 0-s for NRZ-L or long string of 0-s for NRZI
The output is a constant voltage over a long period of time
A drift between the timing of transmitter & receiver will result in loss of synchronization between both devices
CO1

17. Disadvantages of NRZ Coding
Due to these lacking, it is unattractive for signal transmission applications
Due to these shortcomings, it is only used in direct devices connection like in digital magnetic recording
CO1

18. Bipolar-AMI Alternate Mark Inversion
A kind of multilevel binary encoding
Binary 0: No line signal
Binary 1: +ve or –ve pulses alternately 1 1 1 1
Time
CO1, CO2

19. Advantages of Bipolar-AMI
No loss of synchronization if a long string of 1-s occur
Each 1 introduces a transition
Receiver can resynchronize on that transition
Long string of 0-s would still be a problem
No net DC component
1-s signals alternate in voltage
0-s is at zero volt
Hence 0 DC component
CO1

20. Pseudoternary Inversion of AMI A kind of multilevel binary encoding
Binary 1: No line signal
Binary 0: +ve or –ve pulses alternately 1 1 1 1
Time
CO1, CO2

21. Disadvantages of Multilevel Binary
Long string of 0-s (AMI) and 1-s (pseudoternary) still present a problem
Common technique: insert additional bits that force transition – called scrambling
Less efficient than NRZ
The receiver has to distinguish 3 levels
Requires ~3dB of power for the same BER as NRZ
BER is higher for the same SNR as NRZ
CO1

22. Disadvantages of Multilevel Binary
AMI, pseudoternary, ASK, FSK
BER
NRZ, biphase, PSK, QPSK
3 dB
SNR (dB)
CO1

23. Manchester A kind of biphase encoding
Transition in the middle of each bit period
Binary 1: Low to High transition
Binary 0: High to Low transition 1 1 1 1
Time
CO1, CO2

24. Differential Manchester
A kind of biphase & differential encoding
Binary 0: Transition at start of bit period
Binary 1: No transition at start of bit period 1 1 1 1
Time
CO1, CO2

25. Advantages of Biphase Encoding
Synchronization
Biphase codes are self-clocking codes
Predictable transition during each bit period
Receiver can synchronize on that transition
No DC component
Error detection
Absence of expected transition can be used to detect errors
Due to these advantages it is popular for LAN connection
CO1

26. Disadvantages of Biphase Encoding
Requires double the bandwidth of non-biphase encoding
Requires more signaling power
Due to these disadvantages it is not popular in long distance connection
CO1

27. Modulation Rate
CO1, CO2

28. Data Rate Also known as BIT Rate
Definition: The rate at which data (or bits) are communicated per second
Unit: bit per second (bps)
Example: 1000 bps means 1000 bit is transmitted and received in 1 second
CO1, CO2

29. Modulation Rate Also known as BAUD rate or SYMBOL rate
Definition: The MAXIMUM no. symbol at which the signal in communication channel can have per second
Unit: baud per second
Example: Consider an NRZ optical signaling between red & green. If the system has to produce the colors at max 2400 times per second then it is 2400 baud per second
CO1, CO2

30. Baud vs Bit Rate 1 signal symbol may represent more that 1 bits
Hence this gives room for more than 1 bps in each baud rate
bps = baud × no. bit per baud
Example: Consider an NRZ optical signaling between green (00), red (01), yellow (10) and blue (11). If the system has to produce the colors at max 2400 times per second then it is 2400 baud per second. Since there are 2 bits per symbol, then it is 4800 bps.
CO1, CO2

31. Baud vs Bit Rate In general: D = R / L = R / log2M
D = Modulation rate, baud
R = Data rate, bps
L = Number of bits per symbol or signal element
M = Number of different symbols used = 2L
CO2

32. Baud vs Bit Rate NRZL Time Time CO2 Data rate = 1 Mbps
Modulation rate = 1 Mbaud
NRZL
Time
1 bit/μs
1 sym/μs
1 bit/μs
2 sym/μs
Data rate = 1 Mbps
Modulation rate = 2 Mbaud
Manchester
Time
CO2

33. Scrambling Techniques
CO1, CO2

34. Scrambling Techniques
Multilevel binary with scrambling techniques
Commonly used in long-distance transmission
Sequences that would result in a constant voltage level would be replaced with a new filling sequence
The filling sequence would provide enough transitions for the receiver’s clock to maintain synchronization
CO1

35. Scrambling Techniques
The filling sequence must be recognized by the receiver & to be replaced with the original data sequence
The filling sequence is the same length as the original sequence, hence there is no data rate penalty
CO1

36. Scrambling Techniques
Design goals:
No DC component
No long sequence of zero-level line signal
No reduction in data rate
Error detection capability
Two scrambling techniques commonly used in long-distance transmission
B8ZS
HDB3
CO1

37. B8ZS Bipolar with 8-Zeros Substitution Based on Bipolar-AMI encoding
Long string of 0-s may result in loss synchronization
Replaces strings of eight 0-s with:
If the last voltage pulse preceding this 8 0-s was +ve, then they
are encoded with − 0 − +
If the last voltage pulse preceding this 8 0-s was -ve, then they
are encoded with − −
CO2

38. B8ZS 1 1 1 1 1
Bipolar-AMI
B8ZS
This technique forces 2 code violations, which is unlikely to occur due to noise, and the parity is also maintained
CO2

39. HDB3 High-Density Bipolar 3-Zeros Based on Bipolar-AMI encoding
Replaces strings of four 0-s with:
No. bipolar pulses since last substitution
Polarity of Preceding Pulse
Odd
Even −
0 0 0 − +
− 0 0 −
CO2

40. Consider pulses count at this point is odd
HDB3 1 1 1 1 1
Bipolar-AMI
HDB3
Consider pulses count at this point is odd
CO2