Module 2

Data Communication

Fundamentals

Lesson 4

Transmission of Digital Signal

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

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.

Line coding - digital data to digital Line coding - digital data to digital signalsignal

Line Coding CharacteristicsLine Coding Characteristics

Number of signal levels

Bit rate versus Baud rate

DC components

Signal Spectrum

Synchronization

Cost of Implementation

Line Coding TechniqueLine Coding Technique

Unipolar Line EncodingUnipolar Line Encoding

Polar encoding technique uses two voltage levels – one positive and the other one negative.

It is simple but obsolete.

Polar EncodingPolar Encoding

Polar encoding technique uses two voltage levels – one positive and the other one negative.

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

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

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

Biphase RZBiphase RZ

Bipolar Alternate Mark Inversion Bipolar Alternate Mark Inversion EncodingEncoding

1 is the “mark” and 0 is the “space”

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

HDB3HDB3

High Density Bipolar-3 Zeros

Used in Europe and Japan

It replaces a sequence of 4 zeros by a code

Bipolar Encoding AdvantagesBipolar Encoding Advantages

No DC components

No long sequences of 0-level line signal

No increase in bandwidth

Error detection capability

Summary