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Chapter 2: Fundamentals of Data and Signals

Data Communications and Computer Networks: A Business User’s Approach Third Edition. Chapter 2: Fundamentals of Data and Signals. Objectives. After reading this chapter, you should be able to:

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Chapter 2: Fundamentals of Data and Signals

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  1. Data Communications and Computer Networks: A Business User’s Approach Third Edition Chapter 2: Fundamentals of Data and Signals

  2. Objectives • After reading this chapter, you should be able to: • Distinguish between data and signals, and cite the advantages of digital data and signal over analog data and signals • Identify the three basic components of a signal • Discuss the bandwidth of a signal and how it relates to data transfer speed Data Communications & Computer Networks: A Business User's Approach, Third Edition

  3. Objectives (continued) • Identify signal strength and attenuation, and how they are related • Outline the basic characteristics of transmitting analog data with analog signals, digital data with digital signals, digital data with analog signals, and analog data with digital signals • List and draw diagrams of the basic digital encoding techniques, and explain the advantages and disadvantages of each Data Communications & Computer Networks: A Business User's Approach, Third Edition

  4. Objectives (continued) • Identify the different shift keying (modulation) techniques and describe their advantages, disadvantages, and uses • Identify the two most common digitization techniques and describe their advantages and disadvantages • Discuss the characteristics and importance of spread spectrum encoding techniques • Identify the different data codes and how they are used in communication systems Data Communications & Computer Networks: A Business User's Approach, Third Edition

  5. Introduction – Data and Signals • Data - entities that convey meaning • Examples: computer file, music on a CD, results from a blood gas analysis machine • Signals - electric or electromagnetic encoding of data • Examples: telephone conversation, web page download • Computer networks and data / voice communication systems transmit signals • Data and signals can be analog or digital Data Communications & Computer Networks: A Business User's Approach, Third Edition

  6. Introduction – Data and Signals (continued) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  7. Analog versus Digital • Analog - continuous waveform • Examples: (naturally occurring) music and voice Data Communications & Computer Networks: A Business User's Approach, Third Edition

  8. Analog versus Digital (continued) • Harder to separate noise from an analog signal than from a digital signal Data Communications & Computer Networks: A Business User's Approach, Third Edition

  9. Analog versus Digital (continued) • Digital - discrete or non-continuous waveform • Examples: computer 1s and 0s Data Communications & Computer Networks: A Business User's Approach, Third Edition

  10. Analog versus Digital (continued) • Despite noise in this digital signal • You can still discern a high voltage from a low voltage Data Communications & Computer Networks: A Business User's Approach, Third Edition

  11. Analog versus Digital (continued) • If there is too much noise • You cannot discern a high voltage from a low voltage Data Communications & Computer Networks: A Business User's Approach, Third Edition

  12. Fundamentals of Signals • All Signals Have Three Components • Amplitude • Frequency • Phase Data Communications & Computer Networks: A Business User's Approach, Third Edition

  13. Fundamentals of Signals (continued) • Amplitude • Height of the wave above or below a given reference point Data Communications & Computer Networks: A Business User's Approach, Third Edition

  14. Fundamentals of Signals (continued) • Frequency • Number of times a signal makes complete cycle within a given time frame • Spectrum - Range of frequencies that a signal spans from minimum to maximum • Bandwidth - The absolute value of the difference between the lowest and highest frequencies of a signal Data Communications & Computer Networks: A Business User's Approach, Third Edition

  15. Fundamentals of Signals (continued) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  16. Fundamentals of Signals (continued) • Frequency (continued) • For example, consider an average voice: • Average voice has a frequency range of roughly 300 Hz to 3100 Hz • The spectrum would thus be 300 - 3100 Hz • The bandwidth would be 2800 Hz Data Communications & Computer Networks: A Business User's Approach, Third Edition

  17. Fundamentals of Signals (continued) • Phase • Position of the waveform relative to a given moment of time or relative to time zero • A change in phase can be any number of angles between 0 and 360 degrees • Phase changes often occur on common angles, such as 45, 90, 135, etc. Data Communications & Computer Networks: A Business User's Approach, Third Edition

  18. Fundamentals of Signals (continued) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  19. Loss of Signal Strength • All signals experience loss (attenuation) • Denoted as a decibel (dB) loss • Decibel losses (and gains) are additive Data Communications & Computer Networks: A Business User's Approach, Third Edition

  20. Loss of Signal Strength (continued) • So if a signal loses 3 dB, is that a lot? • A 3 dB loss indicates the signal lost half of its power • dB = 10 log10 (P2 / P1) • -3 dB = 10 log10 (X / 100) • -0.3 = log10 (X / 100) • 10-0.3 = X / 100 • 0.50 = X / 100 • X = 50 Data Communications & Computer Networks: A Business User's Approach, Third Edition

  21. Converting Data into Signals • Converting Analog Data into Analog Signals • Often necessary to modulate analog data onto a different set of analog frequencies • Two common examples are broadcast radio and television Data Communications & Computer Networks: A Business User's Approach, Third Edition

  22. Converting Data into Signals (continued) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  23. Converting Data into Signals (continued) • Converting Digital Data into Digital Signals • Numerous techniques – let’s examine four: • NRZ-L • NRZ-I • Manchester • Differential Manchester • Bipolar AMI Data Communications & Computer Networks: A Business User's Approach, Third Edition

  24. Converting Data into Signals (continued) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  25. Manchester Digital Encoding Schemes • Note that with a Differential Manchester code, every bit has at least one signal change • Some bits have two signal changes per bit (baud rate is twice the bps) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  26. 4B/5B Digital Encoding Scheme • Converts four bits of data into five-bit quantities • Five-bit quantities are unique • No five-bit code has more than 2 consecutive zeroes • Five-bit code is then transmitted using an NRZ-I encoded signal Data Communications & Computer Networks: A Business User's Approach, Third Edition

  27. 4B/5B Digital Encoding Scheme (continued) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  28. Transmitting Digital Data with Analog Signals • Three basic techniques: • Amplitude shift keying • Frequency shift keying • Phase shift keying Data Communications & Computer Networks: A Business User's Approach, Third Edition

  29. Amplitude Shift Keying • One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  30. Amplitude Shift Keying (continued) • Some systems use multiple amplitudes Data Communications & Computer Networks: A Business User's Approach, Third Edition

  31. Transmitting Digital Data with Analog Signals (continued) • Multiple Signal Levels • Why use multiple signal levels? • We can represent two levels with a single bit, 0 or 1 • We can represent four levels with two bits: 00, 01, 10, 11 • We can represent eight levels with three bits: 000, 001, 010, 011, 100, 101, 110, 111 • Note that the number of levels is always a power of 2 Data Communications & Computer Networks: A Business User's Approach, Third Edition

  32. Frequency Shift Keying • One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  33. Phase Shift Keying • One phase change encodes a 0 while another phase change encodes a 1 (a form of phase modulation) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  34. Phase Shift Keying (continued) • Quadrature Phase Shift Keying • Four different phase angles are used: • 45 degrees • 135 degrees • 225 degrees • 315 degrees Data Communications & Computer Networks: A Business User's Approach, Third Edition

  35. Phase Shift Keying (continued) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  36. Phase Shift Keying (continued) • Quadrature Amplitude Modulation • 12 different phases are combined with two different amplitudes • Since only 4 phase angles have 2 different amplitudes, there are a total of 16 combinations. • With 16 signal combinations, each baud equals 4 bits of information (2 ^ 4 = 16) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  37. Phase Shift Keying (continued) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  38. Higher Data Transfer Rates How do you send data faster? 1. Use a higher frequency signal (make sure the medium can handle the higher frequency) 2. Use a higher number of signal levels In both cases, noise can be a problem Data Communications & Computer Networks: A Business User's Approach, Third Edition

  39. Maximum Data Transfer Rates • How do you calculate a maximum data rate? • Use Shannon’s equation: • S(f) = f log2 (1 + W/N) • Where f = signal frequency (bandwidth), W is signal power, and N is noise power Data Communications & Computer Networks: A Business User's Approach, Third Edition

  40. Maximum Data Transfer Rates (continued) • For example, what is the data rate of a 3400 Hz signal with 0.2 watts of power and 0.0002 watts of noise? • S(f) = 3400 x log2 (1 + 0.2/0.0002) • = 3400 x log2 (1001) • = 3400 x 9.97 • = 33898 bps Data Communications & Computer Networks: A Business User's Approach, Third Edition

  41. Transmitting Analog Data with Digital Signals • To convert analog data into a digital signal, there are two basic techniques: • Pulse code modulation (used by telephone systems) • Delta modulation Data Communications & Computer Networks: A Business User's Approach, Third Edition

  42. Pulse Code Modulation • Analog waveform is sampled at specific intervals • “Snapshots” are converted to binary values Data Communications & Computer Networks: A Business User's Approach, Third Edition

  43. Pulse Code Modulation (continued) • Binary values are later converted to an analog signal • Waveform similar to original results Data Communications & Computer Networks: A Business User's Approach, Third Edition

  44. Pulse Code Modulation (continued) • The more snapshots taken in the same amount of time, or the more quantization levels, the better theresolution Data Communications & Computer Networks: A Business User's Approach, Third Edition

  45. Pulse Code Modulation (continued) • Because the human voice has a fairly narrow bandwidth • Telephone systems digitize voice into either 128 levels or 256 levels • Called quantization levels • If 128 levels, then each sample is 7 bits (2 ^ 7 = 128) • If 256 levels, then each sample is 8 bits (2 ^ 8 = 256) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  46. Pulse Code Modulation (continued) • How fast do you have to sample an input source to get a fairly accurate representation? • Nyquist says 2 times the bandwidth • Thus, if you want to digitize voice (4000 Hz), you need to sample at 8000 samples per second Data Communications & Computer Networks: A Business User's Approach, Third Edition

  47. Delta Modulation • An analog waveform is tracked using a binary 1 to represent a rise in voltage and a 0 to represent a drop Data Communications & Computer Networks: A Business User's Approach, Third Edition

  48. Spread Spectrum Technology • A secure encoding technique that uses multiple frequencies or codes to transmit data • Two basic spread spectrum technologies: • Frequency hopping spread spectrum • Direct sequence spread spectrum Data Communications & Computer Networks: A Business User's Approach, Third Edition

  49. Spread Spectrum Technology (continued) Data Communications & Computer Networks: A Business User's Approach, Third Edition

  50. Spread Spectrum Technology (continued) • Direct Sequence Spread Spectrum • This technology replaces each binary 0 and binary 1 with a unique pattern, or sequence, of 1s and 0s • For example, one transmitter may transmit the sequence 10010100 for each binary 1, and 11001010 for each binary 0 • Another transmitter may transmit the sequence 11110000 for each binary 1, and 10101010 for each binary 0 Data Communications & Computer Networks: A Business User's Approach, Third Edition

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