CHAPTER 4d Encoding

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CHAPTER 4: ENCODING TECHNIQUES


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Chapter 4/EncodingTechniques

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Data must be transformed into signal form in order to send themfrom one place to another.

How the information is transformed is depends on its original

format and the format used by the communication hardware.A simple signal by itself does not carry information any more than astraight line conveys words. The signal must be manipulated sothat it contains identifiable changes that are recognizable to thesender and receiver as representing the information intended.

Data stored in a computer are in the form of 0s and 1s. To becarried form one place to another, data are usually converted todigital signal.

INTRODUCTION


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Digital-to-digital conversion is representing of digital information bya digital signal.

For example; when a data is transmitted from your computer to

your printer, both the original data and the transmitted data aredigital signals.

In this type of encoding, the binary 1s and 0s generated by acomputer are translated into a sequence of voltage pulse that canbe propagated over a wire.

The three most useful categories for digital-to-digital conversion indata communications are: Unipolar

Polar

Bipolar

DIGITAL-TO-DIGITAL CONVERSION


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DIGITAL-TO-DIGITAL CONVERSION

Digital encoding

Unipolar BipolarPolar

NRZ

RZ

Biphase

AMI

B8ZS

HDB3


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Unipolar encoding is very simple and very primitive.

Although it is almost obsolete today, its simplicity provides an easyintroduction to the concepts developed with the more complex systems.

Unipolar encoding is so named because it uses only one polarity.

This polarity is assigned to one of the two binary states, usually the 1 forpositive voltage and the other state usually is 0, which representing byzero voltage.

UNIPOLAR


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Polar encoding uses two voltage levels: one positive and one

negative.

By using both levels, in most polar encoding methods the average

voltage level on the line is reduced and the dc component problem

of unipolar encoding is alleviated.

POLAR


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In NRZ encoding, the level of the signal is always either positivevoltage or negative voltage. The signal will never return to the zerolevel.

There are two most popular methods of NRZ transmission: NRZ L (Nonreturn to zero level)

NRZ I (Nonreturn to zero invert)

In NRZ-L encoding, the level of the signal depends on the type ofbit it represents. A positive voltage usually means the bit is 0 and a

negative voltage means the bit is 1 (or vice versa).Thus, in NRZ-L, the level of thesignal is dependent upon thestate of the bit.

NONRETURN TO ZERO (NRZ)


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Chapter 4/EncodingTechniques 8

In NRZ I encoding, any inversion of the voltage level will berepresented by bit 1. It happens when there is a transitionbetween a positive and negative voltages.

If there are no changes on the voltage level, the signal will berepresented by bit 0.

NRZ-I is superior to NRZ-L due to the synchronization providedby the signal change in time a bit 1 is encountered.

In NRZ-I thesignal is inverted if bit 1 isencountered.

NRZ L and NRZ I


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Figure below shows the NRZ L and NRZ I representations of

the same series of bits.

NRZ L and NRZ I


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It used to solve the problem in NRZ which in RZ encoding it usesthree voltage values: positive, negative and zero (Bipolar).

The changes of the signal occurs during each bit interval. In NRZ-

L, a positive voltage means 1 and a negative voltage is 0.Otherwise in RZ, halfway through each bit interval, the signalreturns to zero.

A bit 1 is actually represented by positive-to-zero and a bit 0 isrepresented by negative-to-zero, rather than by positive or negative

alone.The main disadvantage of RZ encoding is that it requires twosignal changes to encode one bit and therefore occupies morebandwidth.

RETURN TO ZERO (RZ)


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Figure below shows the RZ representation of a series of bits.

RETURN TO ZERO (RZ)


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Biphase is the best existing solution to the problem of

synchronization.

In this method, the signal changes at the middle of the bit interval

but does not return to zero. Instead, it continues to the opposite

pole.

As in RZ, these mid interval transition allow for synchronization.

There are two types of biphase encoding:

Manchester

DifferentialManchester

BIPHASE


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Manchesteruses the inversion at the middle of each bit intervalfor both synchronization and bit representation.

A negative-to-positive transition represents binary 1 and a

positive-to-negative transition represents binary 0.DifferentialManchesteruses the inversion at the middle of thebit interval for synchronization, but the presence or absence of anadditional transition at the beginning of the interval is for bitidentifying.

The transition means binary 0 and no transition means binary 1.Differential Manchester requires two signal changes to representbinary 0 but only one to represent binary 1.

BIPHASE


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Figure below shows the Manchester and differential Manchester

signals for the same bit pattern.

BIPHASE

Amplitude

Time

Time

Manchester

Differential

Manchester

0 1 0 0 1 1 0


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Bipolar encoding is similar to the RZ, uses three voltage levels:positive, negative and zero.

The zero level in bipolar encoding is used to represent binary 0.

The 1s are represented by alternating positive and negativevoltages.

If the first bit 1 is represented by the positive amplitude, thesecond bit 1 will be represented by the negative amplitude, thethird bit 1 represented by the positive amplitude and so on.

This alternation occurs even when bit 1s are not consecutive.

BIPOLAR


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Three types of bipolar encoding that popular in data

communication are shown below:

BIPOLAR

Bipolar

AMIHDB3B8ZS


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AMI is the simplest type of bipolar encoding.

The word MARK came from telegraphy and it means 1. So AMImeans alternate 1 inversion.

A neutral, zero voltage represented by binary 0. Binary 1 representspositive and negative voltages alternately.

A variation of bipolar AMI is called pseudo-ternary, in which binary 0alternates between positive and negative voltages.

BIPOLAR ALTERNATE MARK INVERSION (AMI)


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B8ZS is the convention to provide synchronization of long stringsof 0s.

The difference between AMI and B8ZS occur whenever eight or

more consecutive 0s are encountered in the data stream.The solution provided by B8ZS is to force artificial signal changescalled violation, within the 0 string.

Anytime eight 0s occur in succession, B8ZS introduces changes inthe pattern based on the polarity of the previous 1 (the occurring

just before the 0s).

BIPOLAR 8-ZERO SUBSTITUTION (B8ZS)


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If the previous bit 1 was positive, the eight 0s will be encoded aszero, zero, zero, positive, negative, zero, negative, positive. Thereceiver looking for alternating polarities to identify 1s.

When it finds two consecutive positive charges surrounding three0s, is recognizes the pattern as a deliberately introducedviolation and not an error.

It then looks for the second pair of the expected violations. Whenit finds them, the receiver translates all eight bits to 0s and

reverts back to normal bipolar AMI mode.If the polarity of the previous bit 1 is negative, the pattern ofviolation is the same but with inverted polarities.



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Both positive and negative pattern are shown in figure below.



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By using B8ZS, encode the bit stream of 10000000000100.

Assume that the polarity of the first 1 is positive.

EXAMPLE


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HDB3 introduces changes into the bipolar AMI pattern every timefour consecutive 0s are encountered instead of waiting for theeight expected by B8ZS.

The pattern of violations is based on the polarity of the previousbit 1. And it is also looks at the number of 1s that have occurredin the bit stream since the last substitution.

In HDB3 if four 0s come one after another, we change thepattern in one of four ways based on the polarity of the previous1 and the number of 1s since the last substitution.

HIGH-DENSITY BIPOLAR 3 (HDB3)


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HIGH-DENSITY BIPOLAR 3 (HDB3)


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By using HDB3, encode the bit stream of 10000000000100.

Assume that the number of 1s so far is odd and the first 1 is

positive.

EXAMPLE

CHAPTER 4d Encoding

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