CHAPTER Modulation

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 Analog-to-digital 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 Phone Line RJ-11 Serial link RS -232 Modem Computer Digital Analog

Amplitude Modulation (AM) 1 1 A B Amp. 2 Amp. 1

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 Voice Carrier Wave Modulated Amplitude

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Module Frequency Modulation

Frequency Modulation (FM) 1 = Frequency F1 0 = Frequency F2 1 1 Freq. 2 Freq. 1

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 Voice Band- Width F3 F4 1 F1 F2 1 B A Full-duplex Communication

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 Module Phase Shift Keying (PSK) Modulation

Phase of an Analog Signal Y Strength 90 180 270 360 X Time Frame

The Concept of Phase Shift 90 degrees phase shift 90 180 degrees phase shift 180

Phase Modulation Technique 1 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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FM Modulation in Modems 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 Voice Band- Width F3 F4 1 F1 F2 1 B A Full-duplex Communication

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 In general 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 Module 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 1 bps = 1 Baud = 1 F2 F1 1 Second

Example Where bps and Baud are Different 1 second 11 10 00 01 bps = 2 Baud = 1

Frequency Representation Bits Frequency 00 1 01 2 10 3 11 4

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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Module 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 Module 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 * See earlier slides for details on PAM and PCM PAM or PDM Digitize Computer Processing PCM Encode

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

Pulse Duration Modulation 110 001 101 6 1 5 Note: pulse duration is proportional to The height of the analog wave

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 Digital-to-Digital Interfacing

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 RS232C Digital ISDN Phone Line Computer ISDN Adapter Adapter Converts From Computer To ISDN Format

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

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