CHAPTER CHAPTER

ModulationModulation

Chapter Objectives

• Explain amplitude, frequency and phase shift modulation

• Give an example of a modulation technique used in modems

• Discuss modem standards– Communication, compression etc.

Continued

Continuation of Chapter Objectives

• Differentiate between bps and Baud that are units used for measuring communication speed

• Describe analog-to-digital modulation• Explain digital-to-digital interface• Summarize the different types of signal

conversions– Digital-to-analog, analog-to-digital, analog-

to-analog and digital-to-digital

Chapter Modules

• Amplitude modulation• Frequency and phase shift modulation• Modems and modulation• FM modulation in modems• Speed of modulated signals• Analog-to-digital modulation• Digital-to-digital interfacing

Overview

• Digital-to-analog modulation– Computer-to-telephone interface

• Analog-to-digital modulation– Digitization of audio

• Digital-to-digital interface– Computer-to-ISDN interface

Modulation

Amplitude Modulation

Overview of Modulation

Computer Modem

Serial linkRS -232

PhoneLineRJ-11

Digital Analog

Amplitude Modulation (AM)

10

10

Amp. 1 Amp. 2

1 = Amp. 10 = Amp. 2

A B

Characteristics of Amplitude Modulation

• Amplitude of the analog signal is modulated

• One amplitude represents a 0• Another amplitude represents a 1• Frequency remains unchanged in both

cases• Signals that are modulated at one end

are demodulated at the other end

Usage

• Amplitude is susceptible to interference– This technique in not normally used in

modems

• A variation of this technique is used in AM radio transmission– Analog-to-analog modulation takes

place

AM and Radio Transmission

Modulated Amplitude

Voice

CarrierWave

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Frequency Modulation

Frequency Modulation (FM)

10

10

Freq. 1 Freq. 2

1 = Frequency F10 = Frequency F2

Characteristics of Frequency Modulation

• Frequency is modulated • Frequency f1

– Represents 1• Frequency f2

– Represents 0• The amplitude remains unaltered

in both cases

Usage• Variations in frequency are easy to detect

– They are less susceptible to interference

• FM and variations of this technique are used in modems

• Easy to implement full duplex transmission under FM

• A variation of the FM technique described here is used in FM radio transmission

Use of FM in Early Day Modems

F10

F21

F30

F41

VoiceBand-Width

Full-duplex CommunicationA B

Modulation in Modern Day Modems

• Modern day modems may not use the FM technique for modulation

• They may be using a technique known as Phase Shift Modulation (or Phase Shift Keying)

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Phase Shift Keying (PSK) Modulation

Phase of an Analog Signal

YStrength

XTime Frame

0 90 180 270 360

The Concept of Phase Shift

90 degrees phase shift

180 degrees phase shift

0 90

0 180

Phase Modulation Technique

10

90 Degrees phase shift

0 Degree phase shift

•This is also known as phase shift keying.

Characteristics of Phase Shift Modulation

• Phase is modulated• Phase shift of 0 represents a 0• Phase shift of 90 degrees represents a 1• Both amplitude and frequency remain

unaltered is both cases• Also known as Phase Shift Keying, it is

used in a number of modern modems as well

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Module

FM Modulation in Modems

Module Objectives

• Explain the basic concept of modem communication

• Provide an example of frequency modulation used in modems

• Discuss the importance of call mode setting– Call mode and receive mode settings

Basic Concepts of Modem Communication

F10

F21

F30

F41

VoiceBand-Width

Full-duplex CommunicationA B

FM Details

• Different frequencies are used for transmission

• At node A– F1 for 0– F2 for 1

• At node B– F3 for 0– F4 for 1

Call and Receive Modes

• Setting for communication– Set one side on call mode– Set the other side on receive mode– The above would ensures proper

assignment of frequencies

Mode Setting Rule

• Calling mainframes or on-line services– Set the calling computer on call mode

• In general– Set the home computer on the call mode

• Fortunately, in a number of cases, the modems poll and set themselves dynamically for communication between the receiver and the sender

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Terms Used in Measuring the Communication speed

Overview• In general, the terms used for measuring speed

are bps and Baud• The former is being used more widely than the

latter• bps is the accurate measure of the speed of

communication • In the past, Baud was being used interchangeable

with bps– Both are not interchangeable

• Only in certain circumstances they amount to the same

Definition of bps and Baud

• bps represents the number of bits transmitted per second

• Baud represents the number of times the signal changes its state during a given period of time

Example Where bps and Baud Represent the Same

10

1 Second

F1 F2

bps = 1Baud = 1

Example Where bps and Baud are Different

00

01 10 11

bps = 2Baud = 1

1 second

Frequency Representation

00 1

01 2

10 3

11 4

Bits Frequency

In Summary

• bps measures the speed of communication correctly in bits per second

• Baud indicates he number of times the state of a signal changes in one second

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Modem Standards

Modem Standardization

• The International body that standardizes the modulation technique is known as the ITU

• ITU is also responsible for setting standards pertaining to:– Error correction– Data compression

Sample ITU Specifications

• Modulation – ITU V.34

• Error correction – ITU V.42– MNP 5

• Data compression – ITU V.42 bis– MNP 2 to 4

Bell Standard and its Implications

• At 1200 bps and below there were two standards– CCITT (ITU at present)– Bell

• A Bell modem cannot communicate with a CCITT modem

• Bell standard at that time was used predominantly in the US

• Today, all modems fall under the ITU specifications

Sample Protocols and Speed

• V.92 for 56,000 bps• V.90 for 56,000 bps• V.34 for 28,800 bps• V.32 bis for 14,400 bps• V.32 for 9,600 bps• A high speed modem could also operate

at the lower speed– High speed modems can thus communicate

with a low speed modems

A Note on the Protocol Used in the Faster 56K

Modems

• When the 56K modems were first introduced there were two competing standards

• One was the X2 standard proposed by US Robotics that is now part of 3Com

• The competing protocol was knows as the Kflex56 standard– A joint effort between Lucent and

Rockwell

ITU Standard for 56K Modems

• Both standards have now been superceded by the ITU V.90 standard

• The vendors now produce modems that operate under the ITU V.90 protocol

• The vendors also offer upgrades to the older X2 and Kflex modems so that they could operate under V.90

In Summary

• ITU specified protocols with respect to modems exist for the following.– Modulation– Error correction– Data compression

• Different protocols apply to different speeds of communication

• A high speed modem can communicate with a low speed modem

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Analog-to-Digital Mapping

An Overview of Analog-to-Digital Modulation

• Representation of analog signals by digital signals is known as analog-to-digital modulation

• Often the digitized information is further coded into binary form for computer processing

• Sample applications include the encoding of audio for computer processing

Steps Involved in the Representation of Analog Signals by Digital Signals

Analog Signals

PAM or PDM

PCM

ComputerProcessing

* See earlier slides for details on PAM and PCM

Digitize

Encode

Modulation Techniques

• Pulse Amplitude Modulation (PAM)• Pulse Code Modulation (PCM)• Pulse Duration Modulation (PDM)

Pulse Duration Modulation

16

Note: pulse duration is proportional to The height of the analog wave

5

110 001 101

Salient Points of Pulse Duration Modulation

• Sample the analog signal at predetermined time intervals– Sampling rate

• Generate digital pulses of duration proportional to the amplitude of the analog signal at the sampling point

• Encode the information into binary form

Reference

• More information on Asynchronous Transmission

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Digital-to-Digital Interfacing

Module

Module Objectives

• Explain the difference between signal modulation (conversion) and digital-to-digital signal transformation

• Explain the concept of digital-to-digital interfacing using ISDN as an example

• Provide a summary of the different modulation processes

Overview

• Analog-to-Digital signal conversion requires modulation

• Digital-to-Digital interfacing– Requires conversion and not modulation– In this case, digital signals are converted from

one digital format to another digital format– Hence, the need for an interface unit even

though the signals at both ends are represented in digital form

• An example is the Computer-to-ISDN link

Digital-to-Digital Interfacing

ComputerISDN

Adapter

DigitalRS232C

Adapter ConvertsFrom Computer ToISDN Format

DigitalISDNPhoneLine

Summary of Modulation

• Digital-to-analog– FM used in modems

• Analog-to-digital– PAM and PCM used in the digitization of audio

• Analog-to-analog– AM used in radio transmission

• Digital-to-digital– This is not a modulation process– Used by the ISDN interface to the computer– Used in DSL communication

End of ModulationEnd of Modulation

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