Transcript 6 Lecture 6 Unipolar & Polar Coding

Unit 1
Lecture 6
1
Before we discuss various line coding schemes, let us first
have an idea of different data conversion schemes.
Different Conversion/Transmission Schemes
2
Different Techniques Used in Data Transmission/Conversion
• Digital to digital conversion
• Line Coding
• Block Coding
• Scrambling
• Analog to Digital Conversion
• PAM
• PCM
– Nyquist Theorem
• Digital To Analog Conversion
• ASK, FSK, PSK & QAM
– Constellation
• Analog to Analog Conversion
• AM, FM &PM
3
Line Coding Schemes
• We can roughly divide line coding schemes into
five broad categories, as shown in Figure.
• There are several schemes in each category.
4.4
Figure: Line coding scheme
4.5
Digital to Digital Encoding
6
Types of Digital to Digital Encoding
7
Unipolar Encoding
•Unipolar encoding uses only one voltage level or one polarity
•This polarity is assigned to one of the two binary states usually the 1
& other state is usually 0
•The average amplitude of a unipolar encoded signal is nonzero. This
creates a DC component with zero frequency. That means it can travel
only through media which can handle DC component.
•This is almost an obsolete method today.
8
Types of Polar Encoding
9
Types of Bipolar Encoding
10
Polar encoding
• Polar encoding uses two voltage levels
(positive and negative).
• By using both levels, the average voltage level
on the line is reduced & the DC component
problem of unipolar encoding is alleviated.
• There are three most popular variations of
polar coding
1. Non Return to Zero (NRZ)
2. Return to Zero (RZ) &
3. Biphase Encoding
11
Types of Polar Encoding
12
Non Return Zero (NRZ)
In NRZ encoding, the level of signal is always either
positive or negative. The two most popular methods
of NRZ transmission are:
1. NRZ-L (NRZ Level)
2. NRZ-I (NRZ Inversion)
NRZ-L : in NRZ-L encoding the type of the signal
depends on the type of bit it represents. A positive
voltage usually means the bit is 0 or negative voltage
means bit is 1 or vice versa. Thus the level of voltage
depends on the level of the bit.
13
• NRZ-I: it is a method, in which the inversion
of the voltage level represents a 1 bit. It is a
transition between a positive & a negative
voltage, not the voltages themselves that
represent a 1 bit. A 0 bit is represented by no
change.
• Out of two methods the NRZ-I is superior to
NRZ-L due to the synchronization provided by
the signal change each time a 1 bit is
encountered. The existence of 1s in the data
stream allows the reciever to resynchronize.
14
Tips
In NRZ-L the level of the signal is
dependent upon the state of the bit.
In NRZ-I the signal is inverted if a 1 is
encountered.
NRZ-I is used in USB, Compact
CD & Fast Ethernet
15
Figure NRZ-L and NRZ-I encoding
Amplitude
Binary Data
0
1
0
0
1
1
1
0
NRZ-L
time
NRZ-I
time
Transition because
next bit is 1.
Figure NRZ-L and NRZ-I encoding
VARIATION 2 : WHEN 0 IS
HIGH VOLTAGE
17
Figure: Polar schemes (NRZ-L and NRZ-I)
4.18
Example
A system is using NRZ-I to transfer 10-Mbps data. What are
the average signal rate and minimum bandwidth?
Solution
The average signal rate in NRZ-I coding is S = N/2
= 500 kbaud.
The minimum bandwidth for this average baud rate
is
Bmin = S = 500 kHz.
19
4.19
Return to Zero (RZ)
• RZ (return-to-zero) refers to a form of digital data
transmission in which the binary low and high states,
represented by numerals 0 and 1, are transmitted by
voltage pulses having certain characteristics. The signal
state is determined by the voltage during the first half of
each data viz 0 or 1. The signal returns to a resting state
(called zero) during the second half of each bit. The resting
state is usually zero volts
• In RZ transmission, the signal changes not between bits but
during each bit.
• To summarize, RZ uses three values Positive, negative &
zero
20
Figure: Polar schemes (RZ)
RZ scheme uses self clocking
4.21
Tip
A good encoded digital signal must
contain a provision for
synchronization.
22
Biphase Encoding
• The best solution to the problem of
synchronization is biphase encoding. In this
method the signal signal changes at the
middle of the bit but does not return to zero.
Instead it continues to the opposite pole. As in
RZ these mid intervals transitions allow for
synchronization.
• It has two variations
1. Manchester encoding
2. Differential Manchester Encoding
23
Manchester Encoding
• In telecommunication and data storage,
Manchester coding (also known as phase encoding,
or PE) is a line code in which the encoding of each
data bit has at least one transition and occupies the
same time. It therefore has no DC component, and
is self-clocking.
24
Manchester Encoding
• Manchester encoding uses the inversion at the
middle of each bit interval for both synchronization
and bit representation.
• A negative-to-positive transition represents binary 1
and a positive-to-negative transition represents
binary 0.
• By using a single transition for a dual purpose,
Manchester encoding achieves the same level of
synchronization as RZ but with only two levels of
amplitude.
25
Figure
Manchester encoding
26
Tip
In Manchester encoding, the transition
at the middle of the bit is used for
both synchronization and bit
representation.
27
Differential Manchester Encoding
• Differential Manchester encoding is a line code in
which data and clock signals are combined to form a
single 2-level self-synchronizing data stream. It is a
differential encoding, using the presence or absence of
transitions to indicate logical value.
• It is not necessary to know the polarity of the sent
signal since the information is not kept in the actual
values of the voltage but in their change:
• in other words it does not matter whether a logical 1 or
0 is received, but only whether the polarity is the same
or different from the previous value; this makes
synchronization easier.
28
Differential Manchester Encoding
• In differential Manchester, the inversion at the
middle of the bit interval is used for
synchronization, but the presence or absence of
an additional transition at the beginning of the
interval is used to identify the bit.
• A transition means binary 0 and no transition
means binary 1.
• Differential Manchester requires two signal
changes to represent binary 0 but only one to
represent binary 1.
29
Figure
Differential Manchester encoding
30
Tip
In differential Manchester encoding,
the transition at the middle of the bit
is used only for synchronization.
The bit representation is defined by
the inversion or noninversion at the
beginning of the bit.
31
Figure : A Combined Look to Manchester &
Differential Manchester Encoding
The minimum bandwidth of Manchester & Differential Manchester
is 2 times that of 802.3 token bus & 802.4 Ethernet
4.32
Various Encoding Schemes
More examples of encoding schemes
34
Figure : Various line codes for data stream 10110100101
Binary Data
1
0
1
1
0
1
0
0
1
0
1
Unipolar
0
Unipolar
NRZ
Polar
RZ
Polar
NRZ
Bipolar
NRZ
(AMI)
t
Tb
Tb
A
0
A/2
0
-A/2
A/2
0
-A/2
A
0
-A
Split phase A/2
Manchester
0
-A/2
t
t
t
t
t
Polar
Quaternar
y NRZ
11
3A/2
10
10
A/2
0
-A/2
01
-3A/2
2Tb
00
Practical Usage of these coding
• Manchester code has been specified for the IEEE
802.3 (Ethernet) standard for baseband coaxial cable
and twisted-pair bus LANs.
• Manchester encoding is also used in IEEE 802.4
(token bus)
• Differential Manchester is specified in the IEEE
802.5 standard for token ring LANs, and is used for
many other applications, including magnetic and
optical storage.
37