Data Communication & Computer Networks: Multi level, multi transition & block codes

Introduction to Data communication
Topic : Multi Level, Multi Transition &
Block Codes
Lecture #8
Dr Rajiv Srivastava
Director
Sagar Institute of Research & Technology (SIRT)
Sagar Group of Institutions, Bhopal
http://www.sirtbhopal.ac.in

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

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.

• 2 Binary e.g.
00,01,10,11 &
• 1 signal element
• 4 levels e.g. +1,
+3, -1, -3

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.

4.7
Figure : Multilevel: 8B6T
8 bits data is converted into
6 signal elements using
3 levels (0, - , + ) of signal

• 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 00010001 is
encoded as the signal pattern -0–0++ with weight 0;
the second 8-bit pattern 01010011 is encoded as
-+-++0 with weight +1. The third 8-bit pattern
01010000 should be encoded as +--+0+ 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.

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

4.10
Figure 4.12: Multilevel: 4D-PAMS scheme
Voltage 0 is not used here. It is used for forward error correction.

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.

–If the next bit is 0, there is no transition.
–If the next bit is 1 and the current level is 0, the
next level is the opposite of the last nonzero level.
–If the next bit is 1 and the current level is not 0,
the next level is 0.
• 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.

4.13
Figure: Multi-transition MLT-3 scheme
1. If the next bit is 0, there is no
transition.
2. If the next bit is 1 and the current
level is 0, the next level is the
opposite of the last nonzero level.
3. If the next bit is 1 and the current
level is not 0, the next level is 0.

Table: Summary of line coding schemes

4.15
Block Coding
• Line coding has problem of redundancy.
• 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.

• 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.

4.17
Figure: Block coding concept

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. 4B/5B maps groups of four bits onto
groups of 5 bits, with a minimum density of 1 bits in the
output. When NRZ-I-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.

• 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.

4.20
Figure: Using block coding 4B/5B with NRZ-I line coding scheme

Table: 4B/5B mapping codes
4.21
21

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?
Example
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

5B/6B
• A data-translation scheme used to precede signal encoding in
100BaseVG-AnyLAN copper networks. In 5B/6B, each group of five
bits is represented as a six-bit symbol, which is then associated with a
bit pattern, which is encoded using a standard method, often NRZI
(non-return to zero inverted).
• It has 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.
• 5B/6B 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

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).

• 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.

Figure : 8B/10B block encoding
Usage : Gigabit Ethernet (except for the twisted pair–based
1000Base-T) & USB 3.0

4.28
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.

4.29
Figure: AMI used with scrambling

4.30
Figure: Two cases of B8ZS scrambling technique

4.31
Figure: Different situations in HDB3 scrambling technique

Summary
• LAN codes –Manchester, differential
Manchester, mBnL, mB/nB
• WAN codes – B8ZS, HDB3

Thank You
Dr Rajiv Srivastava
Director
Sagar Institute of Research & Technology (SIRT)
Sagar Group of Institutions, Bhopal
http://www.sirtbhopal.ac.in

Data Communication & Computer Networks: Multi level, multi transition & block codes

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