1. Multi Level, Multi Transition & Block Codes
Lecture 8
Multi Level, Multi Transition
&
Block Codes

2. m data is represented by sequence of n pulses
mBnL Scheme
A Multi level coding scheme is known as mBnL, where m is the length of the binary pattern. B means binary data, n is the length of the signal pattern and L is the number of levels in the signaling. A letter is often used in place of L: B (binary) for L = 2, T (ternary) for L = 3, and Q (quaternary) for L = 4. Note that the first two letters define the data pattern, and the second two define the signal pattern.
m data is represented by sequence of n pulses

3. 2B1Q
The two binary, one quaternary (2B1Q), uses data patterns of size 2 and encodes the 2-bit patterns as one signal element belonging to a four-level signal. In this type of encoding m = 2, n = 1, and L = 4 (quaternary). Figure shows an example of a 2B1Q signal.
The 2B1Q scheme is used in DSL (Digital Subscriber Line) technology to provide a high-speed connection to the Internet by using subscriber telephone lines.

4. 2 Binary e.g. 00,01,10,11 &
4 signals level e.g. +1, +3, -1, -3

5. 8B6T
A very interesting scheme is eight binary, six ternary (8B6T). This code is used with 100BASE-4T cable.
The idea is to encode a pattern of 8 bits as a pattern of six signal elements, where the signal has three levels (ternary).
Data element = 8 ; data patterns = 28 = 256 and
Signal element = 6 ; signal patterns = 36 = 729.
There are 729 – 256 = 473 redundant signal elements that provide synchronization and error detection.

6. Figure : Multilevel: 8B6T
8 bits data is converted into
6 signal elements using
3 levels (0, - , + ) of signal
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7. Figure shows an example of three data patterns encoded as three signal patterns.
The three possible signal levels are represented as -, 0, and +. The first 8-bit pattern is encoded as the signal pattern -0–0++ with weight 0; the second 8-bit pattern is encoded as with weight +1. The third 8-bit pattern should be encoded as with weight +1.
To create DC balance, the sender inverts the actual signal. The receiver can easily recognize that this is an inverted pattern because the weight is -1. The pattern is inverted before decoding.

8. 4D-PAM5
The last signaling scheme in this category is called four-dimensional five-level pulse amplitude modulation (4D-PAM5).
The 4D means that data is sent over four wires at the same time.
It uses five voltage levels, such as -2, -1, 0, 1 and 2. However, one level, level 0, is used only for forward error detection.
In other words, an 8-bit word is translated to a signal element of four different levels.
Gigabit LANs use this technology to send 1gbps over copper wires

9. Figure 4.12: Multilevel: 4D-PAMS scheme
Voltage 0 is not used here. It is used for forward error correction.
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10. Multi Transition : MLT-3
NRZ-I and differential Manchester are classified as differential encoding but use two transition rules to encode binary data (no inversion, inversion).
If we have a signal with more than two levels, we can design a differential encoding scheme with more than two transition rules.
MLT-3 technique uses more than two transition rules.
The multiline transmission, three-level (MLT-3) scheme uses three levels (+V, 0, and –V) and three transition rules to move between the levels.

11. If the next bit is 0, there is no transition.
If the next bit is 1 and the current level is not 0, the next level is 0.
If the next bit is 1 and the current level is 0, the next level is the opposite of the last nonzero level.
The signal rate for MLT-3 is one-fourth the bit rate. This makes MLT-3 a suitable choice when we need to send 100 Mbps on a copper wire.
FDDI over copper (CDDI) uses MLT-3 encoding instead, as does 100BASE-TX.

12. Figure 4.13: Multi-transition MLT-3 scheme
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13. Table: Summary of line coding schemes
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14. Block Coding
Block codes operate on a block of bits. Using a preset algorithm, we take a group of bits and add a coded part to make a larger block. This block is checked at the receiver. The receiver then makes a decision about the validity of the received sequence..
In general, block coding changes a block of m bits into a block of n bits, where n is larger than m. Block coding is referred to as an mB/nB encoding technique.
Block coding can give us this redundancy and improve the performance of line coding.
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15. Examples of block codes are Reed–Solomon codes, Hamming codes, Hadamard codes, Expander codes, Golay codes, and Reed–Muller codes. These examples also belong to the class of linear codes, and hence they are called linear block codes.

16. Figure: Block coding concept
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17. 4B/5B Block Coding
The four binary/five binary (4B/5B) coding scheme was designed to be used in combination with NRZ-I.
Since long sequence of 0s can make the receiver clock lose synchronization. 4B5B maps groups of four bits onto groups of 5 bits, with a minimum density of 1 bits in the output. When NRZI-encoded, the 1 bits provide necessary clock transitions for the receiver.
For example, a run of 4 bits such as 0000 contains no transitions and that causes clocking problems for the receiver. 4B/5B solves this problem by assigning each block of 4 consecutive bits an equivalent word of 5 bits.
The 4B/5B scheme achieves this goal. The block-coded stream does not have more than three consecutive 0s, as we will see later.

18. At the receiver, the NRZ-I encoded digital signal is first decoded into a stream of bits and then decoded to remove the redundancy. Figure shows the idea.
In 4B/5B, the 5-bit output that replaces the 4-bit input has no more than one leading zero (left bit) and no more than two trailing zeros (right bits). So when different groups are combined to make a new sequence, there are never more than three consecutive 0s.
The 4B5B output is NRZI-encoded.
4B5B was popularized by Fiber distributed data interface (FDDI) in the mid-1980s, and was later adopted by 100BASE-TX standard defined by IEEE 802.3u in 1995.

19. Figure: Using block coding 4B/5B with NRZ-I line coding scheme
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20. Table: 4B/5B mapping codes
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21. Example
We need to send data at a 1-Mbps rate. What is the minimum required bandwidth, using a combination of 4B/5B and NRZ-I or Manchester coding?
Solution
First 4B/5B block coding increases the bit rate to 4.25 Mbps. The minimum bandwidth using NRZ-I is N/2 or 625 kHz.
The Manchester scheme needs a minimum bandwidth of 1 MHz.
The first choice needs a lower bandwidth, but has a DC component problem; the second choice needs a higher bandwidth, but does not have a DC component problem.

22. Figure: Substitution in 4B/5B block coding
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23. 5B/6B
Same idea as 4B/5B but you can have DC balance (3 zero bits and 3 one bits in each group of 6) to prevent polarisation. 5B/6B Encoding is the process of encoding the scrambled 5-bit data patterns into predetermined 6-bit symbols. This creates a balanced data pattern, containing equal numbers of 0's and 1's, to provide guaranteed clock transitions synchronization for receiver circuitry, as well as an even power value on the line.
5B6B encoding also provides an added error-checking capability. Invalid symbols and invalid data patterns, such as more than three 0's or three 1's in a row, are easily detected

24. 8B/10B Block Coding
The eight binary / ten binary (8B/10B) encoding is similar to 4B/5B encoding except that a group of 8 bits of data is now substituted by a 10-bit code. It provides greater error detection capability than 4B/5B.
The 8B/10B block coding is actually a combination of 5B/6B & 3B/4B encoding, as shown in Figure.
The five most significant bits of a 10-bit block are fed into the 5B/6B encoder; the three least significant bits are fed into a 3B/4B encoder. The split is done to simplify the mapping table. To prevent a long run of consecutive 0s or 1s, the code uses a disparity controller which keeps track of excess 0s over 1s (or 1s over 0s).

25. The coding has 210 – 28 = 768 redundant groups that can be used for disparity checking and error detection. In general, the technique is superior to 4B/5B because of better built-in-error-checking capability and better synchronization.

26. Figure 4.17: 8B/10B block encoding
Usage : Gigabit Ethernet (except for the twisted pair–based 1000Base-T) & USB 3.0
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27. Scrambling
The line and block coding is modified to include scrambling, as shown in Figure.
Note that scrambling, as opposed to block coding, is done at the same time as encoding. The system needs to insert the required pulses based on the defined scrambling rules.
Two common scrambling techniques are B8ZS and HDB3.
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28. Figure: AMI used with scrambling
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29. Figure: Two cases of B8ZS scrambling technique
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30. Figure: Different situations in HDB3 scrambling technique
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31. Summary • LAN codes –Manchester, differential Manchester, mBnL, mB/nB
• WAN codes – B8ZS, HDB3