CHAPTER

Modulation

Chapter ObjectivesExplain amplitude, frequency and phase shift modulationGive an example of a modulation technique used in modemsDiscuss modem standardsCommunication, compression etc.

Continued

Continuation of Chapter ObjectivesDifferentiate between bps and Baud that are units used for measuring communication speedDescribe analog-to-digital modulationExplain digital-to-digital interfaceSummarize the different types of signal conversionsDigital-to-analog, analog-to-digital, analog-to-analog and digital-to-digital

Chapter ModulesAmplitude modulationFrequency and phase shift modulationModems and modulationFM modulation in modemsSpeed of modulated signalsAnalog-to-digital modulationDigital-to-digital interfacing

OverviewDigital-to-analog modulationComputer-to-telephone interfaceAnalog-to-digital modulationDigitization of audioDigital-to-digital interfaceComputer-to-ISDN interface

ModulationAmplitude Modulation

Overview of ModulationComputerModemSerial linkRS -232PhoneLineRJ-11DigitalAnalog

Amplitude Modulation (AM)1010Amp. 1Amp. 21 = Amp. 10 = Amp. 2AB

Characteristics of Amplitude Modulation Amplitude of the analog signal is modulatedOne amplitude represents a 0Another amplitude represents a 1Frequency remains unchanged in both casesSignals that are modulated at one end are demodulated at the other end

UsageAmplitude is susceptible to interferenceThis technique in not normally used in modems A variation of this technique is used in AM radio transmissionAnalog-to-analog modulation takes place

AM and Radio TransmissionModulated AmplitudeVoiceCarrierWave

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ModuleFrequency Modulation

Frequency Modulation (FM)1010Freq. 1Freq. 21 = Frequency F10 = Frequency F2

Characteristics of Frequency ModulationFrequency is modulated Frequency f1 Represents 1Frequency f2Represents 0The amplitude remains unaltered in both cases

UsageVariations in frequency are easy to detect They are less susceptible to interferenceFM and variations of this technique are used in modemsEasy to implement full duplex transmission under FMA variation of the FM technique described here is used in FM radio transmission

Use of FM in Early Day ModemsF10F21F30F41VoiceBand-WidthFull-duplex CommunicationAB

Modulation in Modern Day ModemsModern day modems may not use the FM technique for modulationThey may be using a technique known as Phase Shift Modulation (or Phase Shift Keying)

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ModulePhase Shift Keying (PSK) Modulation

Phase of an Analog SignalYStrengthXTime Frame090180270360

The Concept of Phase Shift90 degrees phase shift180 degrees phase shift0900180

Phase Modulation Technique1090 Degrees phase shift0 Degree phase shiftThis is also known as phase shift keying.

Characteristics of Phase Shift ModulationPhase is modulatedPhase shift of 0 represents a 0Phase shift of 90 degrees represents a 1Both amplitude and frequency remain unaltered is both casesAlso known as Phase Shift Keying, it is used in a number of modern modems as well

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ModuleFM Modulation in Modems

Module ObjectivesExplain the basic concept of modem communicationProvide an example of frequency modulation used in modemsDiscuss the importance of call mode settingCall mode and receive mode settings

Basic Concepts of Modem CommunicationF10F21F30F41VoiceBand-WidthFull-duplex CommunicationAB

FM DetailsDifferent frequencies are used for transmissionAt node AF1 for 0F2 for 1At node BF3 for 0F4 for 1

Call and Receive ModesSetting for communicationSet one side on call modeSet the other side on receive modeThe above would ensures proper assignment of frequencies

Mode Setting RuleCalling mainframes or on-line servicesSet the calling computer on call mode In generalSet the home computer on the call modeFortunately, in a number of cases, the modems poll and set themselves dynamically for communication between the receiver and the sender

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ModuleTerms Used in Measuring the Communication speed

OverviewIn general, the terms used for measuring speed are bps and BaudThe former is being used more widely than the latterbps is the accurate measure of the speed of communication In the past, Baud was being used interchangeable with bpsBoth are not interchangeable Only in certain circumstances they amount to the same

Definition of bps and Baudbps represents the number of bits transmitted per secondBaud represents the number of times the signal changes its state during a given period of time

Example Where bps and Baud Represent the Same101 SecondF1F2bps = 1Baud = 1

Example Where bps and Baud are Different00011011bps = 2Baud = 11 second

Frequency Representation001

012

103

114

BitsFrequency

In Summarybps measures the speed of communication correctly in bits per secondBaud indicates the number of times the state of a signal changes in one second

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ModuleModem Standards

Modem StandardizationThe International body that standardizes the modulation technique is known as the ITUITU is also responsible for setting standards pertaining to:Error correctionData compression

Sample ITU SpecificationsModulation ITU V.34Error correction ITU V.42MNP 5Data compression ITU V.42 bisMNP 2 to 4

Bell Standard and its ImplicationsAt 1200 bps and below there were two standardsCCITT (ITU at present)BellA Bell modem cannot communicate with a CCITT modemBell standard at that time was used predominantly in the USToday, all modems fall under the ITU specifications

Sample Protocols and SpeedV.92 for 56,000 bpsV.90 for 56,000 bpsV.34 for 28,800 bpsV.32 bis for 14,400 bpsV.32 for 9,600 bpsA high speed modem could also operate at the lower speedHigh speed modems can thus communicate with a low speed modems

A Note on the Protocol Used in the Faster 56K ModemsWhen 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 3ComThe competing protocol was knows as the Kflex56 standardA joint effort between Lucent and Rockwell

ITU Standard for 56K ModemsBoth standards have now been superceded by the ITU V.90 standardThe vendors now produce modems that operate under the ITU V.90 protocolThe vendors also offer upgrades to the older X2 and Kflex modems so that they could operate under V.90

In SummaryITU specified protocols with respect to modems exist for the following.ModulationError correctionData compressionDifferent protocols apply to different speeds of communicationA high speed modem can communicate with a low speed modem

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ModuleAnalog-to-Digital Mapping

An Overview of Analog-to-Digital ModulationRepresentation of analog signals by digital signals is known as analog-to-digital modulationOften the digitized information is further coded into binary form for computer processingSample applications include the encoding of audio for computer processing

Steps Involved in the Representation of Analog Signals by Digital SignalsAnalog SignalsPAM or PDMPCMComputerProcessing* See earlier slides for details on PAM and PCMDigitizeEncode

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

Pulse Duration Modulation16Note: pulse duration is proportional to The height of the analog wave5110001101

Salient Points of Pulse Duration ModulationSample the analog signal at predetermined time intervalsSampling rateGenerate digital pulses of duration proportional to the amplitude of the analog signal at the sampling pointEncode the information into binary form

Reference More information on Asynchronous Transmission

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Digital-to-Digital InterfacingModule

Module ObjectivesExplain the difference between signal modulation (conversion) and digital-to-digital signal transformationExplain the concept of digital-to-digital interfacing using ISDN as an exampleProvide a summary of the different modulation processes

OverviewAnalog-to-Digital signal conversion requires modulationDigital-to-Digital interfacingRequires conversion and not modulationIn this case, digital signals are converted from one digital format to another digital formatHence, 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 InterfacingComputerISDNAdapterDigitalRS232CAdapter ConvertsFrom Computer ToISDN FormatDigitalISDNPhoneLine

Summary of ModulationDigital-to-analogFM used in modemsAnalog-to-digitalPAM and PCM used in the digitization of audioAnalog-to-analogAM used in radio transmissionDigital-to-digitalThis is not a modulation processUsed by the ISDN interface to the computerUsed in DSL communication

End of Modulation

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