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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Module
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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Module
Phase Shift Keying (PSK) Modulation
Phase of an Analog Signal
YStrength
XTime Frame
0 90 180 270 360
The Concept of Phase Shift90 degrees phase shift
180 degrees phase shift
0 90
0 180
Phase Modulation Technique
10
90 Degrees phase shift0 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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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
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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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
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
Computer ISDNAdapter
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
END OF CHAPTEREND OF CHAPTER