This chapter delves into the essential concepts of modulation, specifically amplitude, frequency, and phase shift modulation. It provides examples of modulation techniques utilized in modems and discusses standards that govern modem communication. The difference between Bps and Baud is explained, highlighting their significance in measuring communication speed. Furthermore, this chapter addresses analog-to-digital modulation, digital-to-digital interfacing, and the various types of signal conversions including digital-to-analog and analog-to-analog modulation.
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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
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
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
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)
Phase Modulation Technique 1 0 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
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
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
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
In Summary • bps measures the speed of communication correctly in bits per second • Baud indicates the number of times the state of a signal changes in one second
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
