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Data Communications . 2. ERROR Any unwanted signal Any rem val or attenuation of a signal Any unwanted chhaangee in a signal. Richard W. Hamming . Data Communications . 3. Objectives. . After reading this chapter, you should be able to:Identify the different types of noise commonly found in computer networksSpecify the different error-prevention techniques, and be able to apply an error-prevention technique to a type of noiseCompare the different error-detection techniques in terms of efficiency and efficacy.
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1. Chapter 6:
Errors, Error Detection, and Error Control
2. Data Communications 2 ERROR Any unwanted signal Any rem val or attenuation of a signal Any unwanted chhaangee in a signal Richard W. Hamming
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9. Data Communications 9 Real Noise
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12. Data Communications 12 The bottom figure should show much more distortion, completely blowing out one or two bits of information.The bottom figure should show much more distortion, completely blowing out one or two bits of information.
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19. Data Communications 19 Real Jitter
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49. Data Communications 49 Block Interleaving In fading channels received data can experience burst errors that destroy large number of consecutive bits. This is harmful for channel coding
Interleaving distributes burst errors along data stream
A problem of interleaving is introduced extra delay
Example below shows block interleaving:
50. Data Communications 50 Detect Error On Credit Card
51. Data Communications 51 Formula for detecting error
52. Data Communications 52 Detect Error On Credit Card
53. Data Communications 53 Now the test
54. Data Communications 54 Credit Card Summary
55. Data Communications 55 The purpose of error correcting codes A message can become distorted through a wide range of unpredictable errors.
Humans
Equipment failure
Lighting interference
Scratches in a magnetic tape
56. Data Communications 56 Encoding
57. Data Communications 57 Why error-correcting code? To add redundancy to a message so the original message can be recovered if it has been garbled.
e.g. message = 10
code = 1010101010
58. Data Communications 58 Send a message
59. Data Communications 59 Take Naïve approach
60. Data Communications 60 Hamming [7,4] Code
61. Data Communications 61 Diagram of Hamming Code Using Odd Parity
62. Data Communications 62 Hamming Code with Data P1 protects D1, D2 and D3
63. Data Communications 63 Practice Hamming #1
64. Data Communications 64 Practice Hamming #1
65. Data Communications 65 Practice Hamming #2
66. Data Communications 66 Practice Hamming #2
67. Data Communications 67 Remember ! Hamming Code finds errors
Hamming Code lets you fix errors without retransmission
Hamming code has lots of overhead so only use when retransmission is difficult (It’s only 0.57142857142857…… efficient)
68. Data Communications 68 Hamming [7,4] Encoding
69. Data Communications 69 Hamming [7,4] Codes
70. Data Communications 70 Minimum Weight Theorem
71. Data Communications 71 Definitions
72. Data Communications 72 Definitions
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