Unit 1

Lecture 6

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Different Conversion/Transmission Schemes

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Before we discuss various line coding schemes, let us first have an idea of different data conversion schemes.

• 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

Different Techniques Used in Data Transmission/Conversion

4.4

Line Coding SchemesLine Coding Schemes

• We can roughly divide line coding schemes into five broad categories, as shown in Figure.

• There are several schemes in each category.

4.5

Figure: Line coding scheme

Digital to Digital Encoding

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Types of Digital to Digital Encoding

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Unipolar Encoding

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•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.

Types of Polar Encoding

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Types of Bipolar Encoding

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Polar encodingPolar encoding• Polar encoding uses two voltage levels Polar encoding uses two voltage levels

(positive and negative).(positive and negative).• By using both levels, the average voltage level By using both levels, the average voltage level

on the line is reduced & the DC component on the line is reduced & the DC component problem of unipolar encoding is alleviated. problem of unipolar encoding is alleviated.

• There are three most popular variations of There are three most popular variations of polar codingpolar coding1.1. Non Return to Zero (NRZ)Non Return to Zero (NRZ)2.2. Return to Zero (RZ) &Return to Zero (RZ) &3.3. Biphase Encoding Biphase Encoding

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Types of Polar Encoding

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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.

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• 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.

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In NRZ-L the level of the signal is In NRZ-L the level of the signal is dependent upon the state of the bit.dependent upon the state of the bit.

TipsTips

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In NRZ-I the signal is inverted if a 1 is In NRZ-I the signal is inverted if a 1 is encountered.encountered.

NRZ-I is used in USB, Compact NRZ-I is used in USB, Compact CD & Fast EthernetCD & Fast Ethernet

0 1 0 0 1 1 1 0 NRZ-L

NRZ-I

Binary Data

Figure NRZ-L and NRZ-I encodingFigure NRZ-L and NRZ-I encoding

time

time

Transition because next bit is 1.

Amplitude

Figure NRZ-L and NRZ-I encoding

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VARIATION 2 : WHEN 0 IS HIGH VOLTAGE

4.18

Figure: Polar schemes (NRZ-L and NRZ-I)

A system is using NRZ-I to transfer 10-Mbps data. What are the average signal rate and minimum bandwidth?

SolutionThe 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.

4.1919

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

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4.21

Figure: Polar schemes (RZ)

RZ scheme uses self clocking

A good encoded digital signal must A good encoded digital signal must contain a provision for contain a provision for

synchronization.synchronization.

TipTip

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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 variations1. Manchester encoding2. Differential Manchester Encoding

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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.

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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.

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Figure Manchester encoding

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In Manchester encoding, the transition In Manchester encoding, the transition at the middle of the bit is used for at the middle of the bit is used for

both synchronization and bit both synchronization and bit representation.representation.

TipTip

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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.

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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.

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Figure Differential Manchester encoding

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In differential Manchester encoding, In differential Manchester encoding, the transition at the middle of the bit the transition at the middle of the bit

is used only for synchronization. is used only for synchronization. The bit representation is defined by The bit representation is defined by the inversion or noninversion at the the inversion or noninversion at the

beginning of the bit.beginning of the bit.

TipTip

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4.32

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

Various Encoding Schemes

More examples of encoding schemes

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1 0 1 1 0 1 0 0 1 0 1 0 Tb Tb

A

0 A/2 0 -A/2 A/2 0 -A/2

A 0 -A

A/2 0 -A/2

Binary Data

Unipolar

Unipolar NRZ

Polar RZ

Polar NRZ

Bipolar NRZ (AMI)

Split phase Manchester

Figure : Various line codes for data stream 10110100101Figure : Various line codes for data stream 10110100101

t

t

t

t

t

t

10

11

01 00

10

2Tb

A/2

0

-A/2

-3A/2

3A/2

Polar Quaternary NRZ

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.

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