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CHAPTER Modulation

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.

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CHAPTER Modulation

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  1. CHAPTER Modulation

  2. 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

  3. 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

  4. 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

  5. Overview • Digital-to-analog modulation • Computer-to-telephone interface • Analog-to-digital modulation • Digitization of audio • Digital-to-digital interface • Computer-to-ISDN interface

  6. Modulation Amplitude Modulation

  7. Overview of Modulation Phone Line RJ-11 Serial link RS -232 Modem Computer Digital Analog

  8. Amplitude Modulation (AM) 1 = Amp. 1 0 = Amp. 2 1 1 0 0 A B Amp. 2 Amp. 1

  9. 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

  10. 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

  11. AM and Radio Transmission Voice Carrier Wave Modulated Amplitude

  12. End of Module

  13. Module Frequency Modulation

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

  15. Characteristics of Frequency Modulation • Frequency is modulated • Frequency f1 • Represents 1 • Frequency f2 • Represents 0 • The amplitude remains unaltered in both cases

  16. 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

  17. Use of FM in Early Day Modems Voice Band- Width F3 0 F4 1 F1 0 F2 1 B A Full-duplex Communication

  18. 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)

  19. End of Module

  20. Module Phase Shift Keying (PSK) Modulation

  21. Phase of an Analog Signal Y Strength 0 90 180 270 360 X Time Frame

  22. The Concept of Phase Shift 90 degrees phase shift 0 90 180 degrees phase shift 0 180

  23. Phase Modulation Technique 1 0 90 Degrees phase shift 0 Degree phase shift • This is also known as phase shift keying.

  24. 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

  25. End of Module

  26. Module FM Modulation in Modems

  27. 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

  28. Basic Concepts of Modem Communication Voice Band- Width F3 0 F4 1 F1 0 F2 1 B A Full-duplex Communication

  29. 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

  30. 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

  31. 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

  32. End of Module

  33. Module Terms Used in Measuring the Communication speed

  34. 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

  35. 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

  36. Example Where bps and Baud Represent the Same 1 bps = 1 Baud = 1 0 F2 F1 1 Second

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

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

  39. 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

  40. End of Module

  41. Module Modem Standards

  42. 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

  43. Sample ITU Specifications • Modulation • ITU V.34 • Error correction • ITU V.42 • MNP 5 • Data compression • ITU V.42 bis • MNP 2 to 4

  44. 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

  45. 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

  46. 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

  47. 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

  48. 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

  49. End of Module

  50. Module Analog-to-Digital Mapping

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