Modulation - Communication Theory - Lecture Slides, Slides of Data Communication Systems and Computer Networks

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CHAPTER Modulation docsity.com 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 docsity.com Overview • Digital-to-analog modulation • Computer-to-telephone interface • Analog-to-digital modulation • Digitization of audio • Digital-to-digital interface • Computer-to-ISDN interface docsity.com Modulation Amplitude Modulation docsity.com Overview of Modulation Computer Modem Serial link RS -232 Phone Line RJ-11 Digital Analog docsity.com 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 docsity.com AM and Radio Transmission Modulated Amplitude Voice Carrier Wave docsity.com End of Module docsity.com Characteristics of Frequency Modulation • Frequency is modulated • Frequency f1 • Represents 1 • Frequency f2 • Represents 0 • The amplitude remains unaltered in both cases docsity.com 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 docsity.com Use of FM in Early Day Modems F1 0 F2 1 F3 0 F4 1 Voice Band- Width Full-duplex Communication A B docsity.com Module Phase Shift Keying (PSK) Modulation docsity.com Phase of an Analog Signal Y Strength X Time Frame 0 90 180 270 360 docsity.com The Concept of Phase Shift 90 degrees phase shift 180 degrees phase shift 0 90 0 180 docsity.com End of Module docsity.com Module FM Modulation in Modems docsity.com 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 docsity.com 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 docsity.com 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 docsity.com End of Module docsity.com 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 docsity.com Example Where bps and Baud Represent the Same 1 0 1 Second F1 F2 bps = 1 Baud = 1 docsity.com Example Where bps and Baud are Different 0 0 01 10 11 bps = 2 Baud = 1 1 second docsity.com End of Module docsity.com Module Modem Standards docsity.com 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 docsity.com 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 docsity.com 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 docsity.com 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 docsity.com Module Analog-to-Digital Mapping docsity.com 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 docsity.com Steps Involved in the Representation of Analog Signals by Digital Signals Analog Signals PAM or PDM PCM Computer Processing * See earlier slides for details on PAM and PCM Digitize Encode docsity.com 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 docsity.com Reference • More information on Asynchronous Transmission docsity.com End of Module docsity.com 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 docsity.com Digital-to-Digital Interfacing Computer ISDN Adapter Digital RS232C Adapter Converts From Computer To ISDN Format Digital ISDN Phone Line docsity.com 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 docsity.com