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Module 2
Data Communication
Fundamentals
Lesson 4
Transmission of Digital Signal
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Specific Instructional ObjectivesSpecific Instructional Objectives
On completion, the students will be able to:
Explain the need for digital transmission
Explain the basic concepts of Line Coding
Explain the important characteristics of line coding
Distinguish among various line coding techniques
Unipolar
Polar
Bipolar
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Why use digital transmission?Why use digital transmission?
Digital transmission has several advantages over analog transmission:
Analog circuits require amplifiers, and each amplifier adds distortion and noise to the signal.
In contrast, digital amplifiers regenerate an exact signal, eliminating cumulative errors. With analog circuits, intermediate nodes amplify the incoming signal, noise and all.
Voice, data, video, etc. can all by carried by digital circuits.
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Line coding - digital data to digital Line coding - digital data to digital signalsignal
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Line Coding CharacteristicsLine Coding Characteristics
Number of signal levels
Bit rate versus Baud rate
DC components
Signal Spectrum
Synchronization
Cost of Implementation
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Line Coding TechniqueLine Coding Technique
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Unipolar Line EncodingUnipolar Line Encoding
Polar encoding technique uses two voltage levels – one positive and the other one negative.
It is simple but obsolete.
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Polar EncodingPolar Encoding
Polar encoding technique uses two voltage levels – one positive and the other one negative.
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Non Return to zero (NRZ)Non Return to zero (NRZ)
- voltage is used to represent one binary value and a + voltage to represent the other.
NRZ-L
NRZ-I
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NRZ Pros and ConsNRZ Pros and Cons
Advantages
Detecting a transition in presence of noise is more reliable than to compare a value to a threshold.
NRZ codes are easy to engineer and it makes efficient use of bandwidth.
Disadvantages
Presence of a DC component
Lack of synchronization capability
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RZ (Return to zero)RZ (Return to zero)
Key Characteristics
Three levels
Bit rate is double than that of data rate
No DC component
Good synchronization
Main limitation is the increase in bandwidth
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Biphase RZBiphase RZ
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Bipolar Alternate Mark Inversion Bipolar Alternate Mark Inversion EncodingEncoding
1 is the “mark” and 0 is the “space”
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B8ZSB8ZS
Bipolar with 8 Zeros Substitution
A sequence of eight zero’s is replaced by
000+ - 0 + -, if the previous pulse was positive.
000 - + 0 + -, if the previous pulse was negative
Used in North America
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HDB3HDB3
High Density Bipolar-3 Zeros
Used in Europe and Japan
It replaces a sequence of 4 zeros by a code
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Bipolar Encoding AdvantagesBipolar Encoding Advantages
No DC components
No long sequences of 0-level line signal
No increase in bandwidth
Error detection capability
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Summary