Data communications 4 1

4.1
Chapter 4
Digital Transmission
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

4.2
4-1 DIGITAL-TO-DIGITAL CO
In this section, we see how we can represent digital
data by using digital signals. The conversion involves
three techniques: line coding, block coding, and
scrambling. Line coding is always needed; block
coding and scrambling may or may not be needed.
 Line Coding
 Line Coding Schemes
 Block Coding
 Scrambling
Topics discussed in this section:

4.3
Line Coding
 Converting a string of 1‟s and 0‟s
(digital data) into a sequence of signals
that denote the 1‟s and 0‟s.
 For example a high voltage level (+V)
could represent a “1” and a low voltage
level (0 or -V) could represent a “0”.

4.4
Figure 4.1 Line coding and decoding

4.5
Mapping Data symbols onto
Signal levels
 A data symbol (or element) can consist of a
number of data bits:
 1 , 0 or
 11, 10, 01, ……
 A data symbol can be coded into a single
signal element or multiple signal elements
 1 -> +V, 0 -> -V
 1 -> +V and -V, 0 -> -V and +V
 The ratio „r‟ is the number of data elements
carried by a signal element.

4.6
Figure 4.2 Signal element versus data element

4.7
Relationship between data
rate and signal rate
 The data rate defines the number of bits sent
per sec - bps. It is often referred to the bit
rate.
 The signal rate is the number of signal
elements sent in a second and is measured in
bauds. It is also referred to as the modulation
rate.
 Goal is to increase the data rate whilst
reducing the baud rate.

4.8
Data rate and Baud rate
 The baud or signal rate can be
expressed as:
S = c x N x 1/r bauds
where N is data rate
c is the case factor (worst, best & avg.)
r is the ratio between data element &
signal element

4.9
A signal is carrying data in which one data element is
encoded as one signal element ( r = 1). If the bit rate is
100 kbps, what is the average value of the baud rate if c is
between 0 and 1?
Solution
We assume that the average
value of c is 1/2 . The baud
rate is then
Example 4.1

4.10
Although the actual bandwidth of a
digital signal is infinite, the effective
bandwidth is finite.
Note

4.11
The maximum data rate of a channel (see Chapter 3) is
Nmax = 2 × B × log2 L (defined by the Nyquist formula).
Does this agree with the previous formula for Nmax?
Solution
A signal with L levels
actually can carry log2L bits
per level. If each level
corresponds to one signal
element and we assume the
average case (c = 1/2), then
we have
Example 4.2

4.12
Considerations for choosing a good
signal element referred to as line
encoding
 Baseline wandering - a receiver will evaluate
the average power of the received signal
(called the baseline) and use that to determine
the value of the incoming data elements. If
the incoming signal does not vary over a long
period of time, the baseline will drift and thus
cause errors in detection of incoming data
elements.
 A good line encoding scheme will prevent long
runs of fixed amplitude.

4.13
Line encoding characteristics
 DC components - when the voltage
level remains constant for long periods
of time, there is an increase in the low
frequencies of the signal. Most channels
are bandpass and may not support the
low frequencies.
 This will require the removal of the dc
component of a transmitted signal.

4.14
 Self synchronization - the clocks at the
sender and the receiver must have the
same bit interval.
 If the receiver clock is faster or slower it
will misinterpret the incoming bit
stream.

4.15
Figure 4.3 Effect of lack of synchronization

4.16
In a digital transmission, the receiver clock is 0.1 percent
faster than the sender clock. How many extra bits per
second does the receiver receive if the data rate is
1 kbps? How many if the data rate is 1 Mbps?
Solution
At 1 kbps, the receiver
receives 1001 bps instead of
1000 bps.
Example 4.3
At 1 Mbps, the receiver receives 1,001,000 bps instead of
1,000,000 bps.

4.17
 Built-in Error detection - errors occur
during transmission due to line
impairments.
 Some codes are constructed such that
when an error occurs it can be
detected. For example: a particular
signal transition is not part of the code.
When it occurs, the receiver will know
that a symbol error has occurred.

4.18
 Immunity to Noise and Interference -
there are line encoding techniques that
make the transmitted signal “immune”
to noise and interference.
 This means that the signal cannot be
corrupted, it is stronger than error
detection.

4.19
 Complexity - the more robust and
resilient the code, the more complex it
is to implement and the price is often
paid in baud rate or required
bandwidth.

4.20
Figure 4.4 Line coding schemes

4.21
Unipolar
 All signal levels are on one side of the time
axis - either above or below
 NRZ - Non Return to Zero scheme is an
example of this code. The signal level does
not return to zero during a symbol
transmission.
 Scheme is prone to baseline wandering and
DC components. It has no synchronization or
any error detection. It is simple but costly in
power consumption.

4.22
Figure 4.5 Unipolar NRZ scheme

4.23
Polar - NRZ
 The voltages are on both sides of the time
axis.
 Polar NRZ scheme can be implemented with
two voltages. E.g. +V for 1 and -V for 0.
 There are two versions:
 NRZ - Level (NRZ-L) - positive voltage for one
symbol and negative for the other
 NRZ - Inversion (NRZ-I) - the change or lack of
change in polarity determines the value of a
symbol. E.g. a “1” symbol inverts the polarity a “0”
does not.

4.24
Figure 4.6 Polar NRZ-L and NRZ-I schemes

4.25
In NRZ-L the level of the voltage
determines the value of the bit.
In NRZ-I the inversion
or the lack of inversion
determines the value of the bit.
Note

4.26
NRZ-L and NRZ-I both have an average
signal rate of N/2 Bd.
Note

4.27
NRZ-L and NRZ-I both have a DC
component problem and baseline
wandering, it is worse for NRZ-L. Both
have no self synchronization &no error
detection. Both are relatively simple to
implement.
Note

4.28
A system is using NRZ-I to transfer 1-Mbps data. What
are the average signal rate and minimum bandwidth?
Solution
The average signal rate is S=
c x N x R = 1/2 x N x 1 = 500
kbaud. The minimum bandwidth
for this average baud rate is
Bmin = S = 500 kHz.
Note c = 1/2 for the avg.
Example 4.4

4.29
Polar - RZ
 The Return to Zero (RZ) scheme uses three
voltage values. +, 0, -.
 Each symbol has a transition in the middle.
Either from high to zero or from low to zero.
 This scheme has more signal transitions (two
per symbol) and therefore requires a wider
bandwidth.
 No DC components or baseline wandering.
 Self synchronization - transition indicates
symbol value.
 More complex as it uses three voltage level.
It has no error detection capability.

4.30
Figure 4.7 Polar RZ scheme

4.31
Polar - Biphase: Manchester and
Differential Manchester
 Manchester coding consists of combining the
NRZ-L and RZ schemes.
 Every symbol has a level transition in the middle:
from high to low or low to high. Uses only two
voltage levels.
 Differential Manchester coding consists of
combining the NRZ-I and RZ schemes.
 Every symbol has a level transition in the middle.
But the level at the beginning of the symbol is
determined by the symbol value. One symbol
causes a level change the other does not.

4.32
Figure 4.8 Polar biphase: Manchester and differential Manchester schemes

4.33
In Manchester and differential
Manchester encoding, the transition
at the middle of the bit is used for
synchronization.
Note

4.34
The minimum bandwidth of Manchester
and differential Manchester is 2 times
that of NRZ. The is no DC component
and no baseline wandering. None of
these codes has error detection.
Note

4.35
Bipolar - AMI and Pseudoternary
 Code uses 3 voltage levels: +, 0, -, to
represent the symbols (note not transitions to
zero as in RZ).
 Voltage level for one symbol is at “0” and the
other alternates between + & -.
 Bipolar Alternate Mark Inversion (AMI) - the
“0” symbol is represented by zero voltage
and the “1” symbol alternates between +V
and -V.
 Pseudoternary is the reverse of AMI.

4.36
Figure 4.9 Bipolar schemes: AMI and pseudoternary

4.37
Bipolar characteristics
 It is a better alternative to NRZ.
 Has no DC component or baseline
wandering.
 Has no self synchronization because
long runs of “0”s results in no signal
transitions.
 No error detection.

4.38
Multilevel Schemes
 In these schemes we increase the number of
data bits per symbol thereby increasing the
bit rate.
 Since we are dealing with binary data we only
have 2 types of data element a 1 or a 0.
 We can combine the 2 data elements into a
pattern of “m” elements to create “2m”
symbols.
 If we have L signal levels, we can use “n”
signal elements to create Ln signal elements.

4.39
Code characteristics
 Now we have 2m symbols and Ln signals.
 If 2m > Ln then we cannot represent the data
elements, we don‟t have enough signals.
 If 2m = Ln then we have an exact mapping of
one symbol on one signal.
 If 2m < Ln then we have more signals than
symbols and we can choose the signals that
are more distinct to represent the symbols
and therefore have better noise immunity and
error detection as some signals are not valid.

4.40
Representing Multilevel Codes
 We use the notation mBnL, where m is
the length of the binary pattern, B
represents binary data, n represents the
length of the signal pattern and L the
number of levels.
 L = B binary, L = T for 3 ternary, L = Q
for 4 quaternary.

4.41
In mBnL schemes, a pattern of m data
elements is encoded as a pattern of n
signal elements in which 2m ≤ Ln.
Note

2B1Q (2 Binary 1 Quaternary)
 data patterns of size 2 bits are encoded as
one signal element belonging to a four-level
signal
 data is sent two times faster than with NRZ-L
 receiver has to discern 4 different thresholds
 the scheme was intended to be used by the
ISDN DSL and SDSL applications.
4.42

4.43
Figure 4.10 Multilevel: 2B1Q scheme

4.44
Multilevel using multiple channels
 In some cases, we split the signal transmission
up and distribute it over several links.
 The separate segments are transmitted
simultaneously. This reduces the signalling
rate per link -> lower bandwidth.
 This requires all bits for a code to be stored.
 xD: means that we use „x‟ links
 YYYz: We use „z‟ levels of modulation where
YYY represents the type of modulation (e.g.
pulse ampl. mod. PAM).
 Codes are represented as: xD-YYYz

Multilevel: 8B6T scheme
 8 Binary 6 Ternary
 Encodes 8 bits as a pattern of 6 signal
elements, where the signal has three (ternary).
 Each signal pattern has a weight of 0 or +1 DC
values.
 To make the whole stream Dc-balanced, the
sender keeps track of the weight.
 If two groups of weight 1 are encountered one
after another, the first one is sent as is, while
the next one is totally inverted to give a weight
of -1.
4.45

4.46
Figure 4.11 Multilevel: 8B6T scheme

Multilevel: 4D-PAM5 scheme
 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.
 Level 0 is used only for forward error detection.
 Gigabit LANs use this technique to send 1-Gbps
data over four copper cables that can handle
125 Mbaud.
4.47

4.48
Figure 4.12 Multilevel: 4D-PAM5 scheme

4.49
Multitransition Coding
 Because of synchronization requirements we force
transitions. This can result in very high bandwidth
requirements -> more transitions than are bits (e.g.
mid bit transition with inversion).
 Codes can be created that are differential at the bit
level forcing transitions at bit boundaries. This results
in a bandwidth requirement that is equivalent to the
bit rate.
 In some instances, the bandwidth requirement may
even be lower, due to repetitive patterns resulting in
a periodic signal.

MLT-3 scheme
 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 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.
4.50

4.51
Figure 4.13 Multitransition: MLT-3 scheme

4.52
Table 4.1 Summary of line coding schemes

4.53
Block Coding
 For a code to be capable of error detection, we need
to add redundancy, i.e., extra bits to the data bits.
 Synchronization also requires redundancy -
transitions are important in the signal flow and must
occur frequently.
 Block coding is done in three steps:
division, substitution and combination.
 It is distinguished from multilevel coding by use of
the slash - xB/yB.
 The resulting bit stream prevents certain bit
combinations that when used with line encoding
would result in DC components or poor sync. quality.

4.54
Block coding is normally referred to as
mB/nB coding;
it replaces each m-bit group with an
n-bit group.
Note

4.55
Figure 4.14 Block coding concept

4.56
Figure 4.15 Using block coding 4B/5B with NRZ-I line coding scheme

 The five bit codes were selected so that there is no more
than one leading zero and no more than two trailing
zeros.
 When the code are strung together, there can be no
more than three consecutive zeros.
4.57
Table 4.2 4B/5B mapping codes

4.58
Figure 4.16 Substitution in 4B/5B block coding

4.59
Figure 4.17 8B/10B block encoding
 The 8B/10B encoding is similar to 4B/5B encoding except that a
group of 8 bits of data is substituted by a 10-bit code.
 It provides greater error detection capability than 4B/5B.
 The most five significant bits is fed to the 5B/6B encoder; the
least 3 significant bits is fed into a 3B/4B encoder.

4.60
More bits - better error detection
 The 8B10B block code adds more
redundant bits and can thereby choose
code words that would prevent a long
run of a voltage level that would cause
DC components.

4.61
Scrambling
 The best code is one that does not increase
the bandwidth for synchronization and has no
DC components.
 Scrambling is a technique used to create a
sequence of bits that has the required c/c‟s for
transmission - self clocking, no low
frequencies, no wide bandwidth.
 It is implemented at the same time as
encoding, the bit stream is created on the fly.
 It replaces „unfriendly‟ runs of bits with a
violation code that is easy to recognize and
removes the unfriendly c/c.

4.62
Figure 4.18 AMI used with scrambling

Bipolar 8-Zero Substitution (B8ZS)
 B8ZS works in a similar way to AMI by
changing poles for each binary 1.
 B8ZS attempts to tackle the problem with
synchronisation being lost when there is a
stream of binary 0s being sent by making
artificial signal changes.
 These signals are known as violations and
occur when eight consecutive 0s occur in
the bit stream.
4.63

4.64
For example: B8ZS substitutes eight
consecutive zeros with 000VB0VB.
The V stands for violation, it violates the
line encoding rule
B stands for bipolar, it implements the
bipolar line encoding rule

4.65
Figure 4.19 Two cases of B8ZS scrambling technique

High Density Bipolar of Order 3 (HDB3)
 The high density bipolar of order 3
(HDB3) code replaces any instance of 4
consecutive 0 bits with one of the patterns
"000V" or "B00V".
 The choice is made to ensure that
consecutive violations are of differing
polarity, i.e. separated by an odd number
of normal + or - marks.
4.66

4.67
HDB3 substitutes four consecutive
zeros with 000V or B00V depending
on the number of nonzero pulses after
the last substitution.
If # of non zero pulses is even the
substitution is B00V to make total # of
non zero pulse even.
If # of non zero pulses is odd the
substitution is 000V to make total # of
non zero pulses even.

HDB 3 coding of "0000"
4.68
Number of +/-
bits
since last V
Pattern
Polarity
of last Pulse
Coded
odd 000V
+ 000+
− 000−
even B00V
+ −00−
− +00+

4.69
Figure 4.20 Different situations in HDB3 scrambling technique

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