Unit 4

DIGITAL-TO-DIGITAL CONVERSION or
ENCODING
•We can represent digital data by using digital signals.We can represent digital data by using digital signals.
The conversion involves three techniques:The conversion involves three techniques:
1.1.line codingline coding
2.2.block codingblock coding
3.3.scramblingscrambling
Line coding is always needed; block coding andLine coding is always needed; block coding and
scrambling may or may not be needed.scrambling may or may not be needed.

Line coding
 Line coding is the process of converting
digital data to digital signal.
 It converts a sequence of bits to a digital
signals.

Figure 4.1 Line coding and decoding

Signal element Vs Data element
In data communication our aim is to send data
elements.
 Data element: it is the smallest entity that can
represent a piece of information: this is the bit.
These are “what we need to send”.
 Signal element: it carries data elements. it is the
shortest unit(timewise) of a digital signal. These are
“what we can send.”
 Data elements are being carried and signal
elements are the carriers.

Signal element versus data element

 Data rate: defines the number of data
elements (bits) sent in 1 sec. Unit is bps called
as bit rate.
 Signal rate: it is the number of signal elements
sent in 1 sec .unit is baud. Also called baud
rate, pulse rate or modulation rate.

Relationship between bit rate &
signal rate
Where,
N is the data rate(bps);
c is the case factor, which varies from worst,
average and best case and generally c=1/2
S is the number of signal elements
r is the ratio of number of data elements carried by
each signal element.

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

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

Effect of lack of synchronization

Unipolar Non Return to Zero (NRZ) scheme:
All signal levels are on the same side of the time axis; either above or below.

Polar schemes:
In polar schemes, the voltages are on the both sides of the time axis.
Polar NRZ-Level (NRZ-L): the level of voltage determines the
value of the bit.
Polar NRZ-Invert (NRZ-I): the change or lack of change in the
level of the voltage determines the value of the bit.
If there is no change, the bit is 0; if there is a change the bit is 1

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

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

NRZ-L and NRZ-I both have a DC
component problem.
Note

Polar Return to Zero (RZ) scheme:
It uses three values: positive, negative and zero.
The signal changes not between the bit but during the bit i.e. signal goes to zero
in middle of each bit.

Polar biphase: Manchester and differential Manchester schemes
Manchester Scheme: Combines idea of RZ & NRZ-L
•In this duration of bit is divided into two halves. The voltage remains at one level
during first half and moves to other level in second half.
•Transition at the middle of the bit provides synchronization.
Differential Manchester Schemes: Combines idea of RZ & NRZ-I
•There is always a transition at the middle of the bit, but the values are
determined at the beginning of the bit.
•If next bit is ‘0’, there is transition and if next bit is ‘1’ there is no transition.

Polar biphase: Manchester and differential Manchester schemes

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

The minimum bandwidth of Manchester
and differential Manchester is 2 times
that of NRZ.
Note

Bipolar schemes:
We use three levels: positive, zero, and negative.
The voltage level of one data element is at zero, while the voltage level for the
other element alternates between positive and negative.
Alternate mark inversion (AMI): AMI means alternate 1 inversion.
A neutral zero voltage represents binary 0 and binary 1 is represented by
alternating positive and negative voltages.
Pseudoternary: in this the ‘1’ bit is encoded as a zero voltage and the ‘0’
bit is encoded as alternating positive and negative voltages.

Bipolar schemes: AMI and pseudoternary

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

Figure 4.10 Multilevel: 2B1Q scheme

Figure 4.11 Multilevel: 8B6T scheme

Figure 4.12 Multilevel: 4D-PAM5 scheme

Figure 4.13 Multitransition: MLT-3 scheme

Table 4.1 Summary of line coding schemes

Scrambling technique: used for proper synchronization
• Bipolar with 8 zero substitution (B8ZS):
In this, eight consecutive zero level voltages are replaced by the
sequence 000VB0VB.
The V denotes ‘violation’; this is a nonzero voltage(opposite from the
previous )
The B denotes ‘bipolar’; which means a nonzero level voltage.

Two cases of B8ZS scrambling technique

High Density Bipolar 3 zero (HDB3)scrambling
technique:
In this technique, four consecutive zero level voltages are replaced
with a sequence of 000V or B00V.
The reason for two different substitution is to maintain the even
number of nonzero pulses after each substitution. The two rules are:
1.If the number of nonzero pulses after the last substitution is odd, the
substitution pattern will be 000V, which makes the total number of
nonzero pulses even.
2.If the number of nonzero pulses after the last substitution is even,
the substitution pattern will be B00V, which makes the total number
of nonzero pulses even.

Different situations in HDB3 scrambling technique

HDB3 substitutes four consecutive
zeros with 000V or B00V depending
on the number of nonzero pulses after
the last substitution.
Note

TRANSMISSION MODESTRANSMISSION MODES
The transmission of binary data across a link can beThe transmission of binary data across a link can be
accomplished in either parallel or serial mode. Inaccomplished in either parallel or serial mode. In
parallel mode, multiple bits are sent with each clockparallel mode, multiple bits are sent with each clock
tick. In serial mode, 1 bit is sent with each clock tick.tick. In serial mode, 1 bit is sent with each clock tick.
While there is only one way to send parallel data, thereWhile there is only one way to send parallel data, there
are three subclasses of serial transmission:are three subclasses of serial transmission:
asynchronous, synchronous, and isochronous.asynchronous, synchronous, and isochronous.
Parallel Transmission
Serial Transmission
Topics discussed in this section:Topics discussed in this section:

Figure 4.31 Data transmission and modes

Figure 4.32 Parallel transmission

Figure 4.33 Serial transmission

In asynchronous transmission, we send
1 start bit (0) at the beginning and 1 or
more stop bits (1s) at the end of each
byte. There may be a gap between
each byte.
Note

Asynchronous here means
“asynchronous at the byte level,”
but the bits are still synchronized;
their durations are the same.
Note

Figure 4.34 Asynchronous transmission

In synchronous transmission, we send
bits one after another without start or
stop bits or gaps. It is the responsibility
of the receiver to group the bits.
Note

Figure 4.35 Synchronous transmission

Unit 4

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