Cyclic Redundancy Check (CRC) - Part 2 — Transcript

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1. 0:00in part one of the CRC we have seen how
2. 0:03Center creates the CRC in part two of
3. 0:06the cyclic redundancy check we will see
4. 0:08how the receiver detects the errors
5. 0:11using the CRC technique let's start the
6. 0:13session with the outcomes in today's
7. 0:16session we are going to understand how
8. 0:18the receiver detects the error using CRC
9. 0:21technique let's take the same example
10. 0:23what we have taken in the previous
11. 0:25lecture in the previous lecture we took
12. 0:27this example with the data block one
13. 0:29double zero and one double zero with the
14. 0:32divisor 1 1 0 1 and we solve the problem
15. 0:35using polynomial long division and we
16. 0:38found out that the CRC is 0 0 1 and the
17. 0:41data that is transmitted is 1 double 0 1
18. 0:44double 0 0 0 1 so where 1 double 0 1
19. 0:48double 0 is the actual data and 0 0 1 is
20. 0:51the CRC and this is what the message
21. 0:54that is transmitted now let's say this
22. 0:57transmitted message is received by the
23. 0:59receiver without any transmission errors
24. 1:02it means this is what the information
25. 1:04that is transmitted later the same data
26. 1:07be received by the receiver without
27. 1:09transmission errors now we will be
28. 1:12focusing on how the receiver comes to
29. 1:14know that there are no errors in the
30. 1:16transmission so to do this what the
31. 1:18receiver does he takes the data what he
32. 1:21has received so this is what the data
33. 1:23that he has received and this is what
34. 1:26the divisor as he already mentioned in
35. 1:28the previous lecture that the divisor is
36. 1:30common for both the sender and the
37. 1:32receiver the protocol decides the
38. 1:34divisor and normally this divisor will
39. 1:36be a polynomial expression there are
40. 1:38standard polynomials
41. 1:40I have taken 1 1 0 1 as the divisor in
42. 1:42the previous lecture the crc was created
43. 1:45with this devices only and hence the
44. 1:47same divisor must be used by the
45. 1:49receiver in order to detect whether
46. 1:51there is an error or not to do this the
47. 1:53same polynomial division whatever the
48. 1:56receiver receives just put it here and
49. 1:58the divisor here as usual the same
50. 2:00polynomial division 1 1 0 1 1 times 1 1
51. 2:040 1 and just divide let's ignore this
52. 2:07part when we do this division for same
53. 2:10input it will be 0 for same input here
54. 2:12also 0
55. 2:13four different input it will be 1
56. 2:15because X our operation we are
57. 2:16performing so the answer for this is 1 0
58. 2:190 so it is 1 0 0 and let's bring the
59. 2:22next bit so 0 will take this place again
60. 2:25we will divide 1 1 0 1 1 times its 1 1 0
61. 2:291 and the result is 1 0 1
62. 2:32let's bring the next bit that is 0 here
63. 2:35again 1 1 0 1 one time it's 1 1 0 1 and
64. 2:39the intermediate result is 1 1 1
65. 2:42let's bring the next bit it's 0 and 1
66. 2:45time it's 1 1 0 1 and the result here is
67. 2:490 1 1
68. 2:51let's bring the next 0 here now if you
69. 2:54observe it's starting with 0 so we are
70. 2:56required to use a 0 and bring the next
71. 2:58one and I'll simplify this problem by
72. 3:01just putting 0 at the top and bringing
73. 3:03the next digit 1 and doing the same
74. 3:05operation 1 1 0 1 1 times it's 1 1 0 1
75. 3:09and the final result is 0 when the
76. 3:13receiver gets all zeroes has the
77. 3:15remainder the receiver understands that
78. 3:18there are no transmission errors and the
79. 3:20receiver accepts the data what it was
80. 3:22sent and this is all the receiver
81. 3:25concludes that there are no errors
82. 3:26during transmission I hope now you are
83. 3:29clear with how the receiver detects the
84. 3:31error if we get a non-zero value here it
85. 3:34means there are errors in the
86. 3:35transmission to understand things better
87. 3:37I recommend you to solve the homework
88. 3:40problem just take this homework problem
89. 3:43suppose we want to transmit the message
90. 3:46double 1 double 0 1 double 0 1 and we
91. 3:50want to protect it from errors using the
92. 3:53CRC polynomial and the polynomial
93. 3:55expression is given that is X cubed plus
94. 3:571 use the polynomial long division to
95. 4:00determine the message duck should be
96. 4:02transmitted so center have to create the
97. 4:05crc and append that crc with this
98. 4:07message and transmit that message when
99. 4:11the receiver gets this message and the
100. 4:13crc without any errors many does the
101. 4:16same polynomial division the remainder
102. 4:17will be obviously 0 it means there are
103. 4:20no transmission errors but the question
104. 4:22is not complete just to see this
105. 4:24carefully corrupt
106. 4:26leftmost third bit of the transmitted
107. 4:28message that is if this is the message
108. 4:31the leftmost third bit this is the
109. 4:33left-hand side and the leftmost third
110. 4:35bit should be corrupted it means when we
111. 4:37see one here it should become zero or if
112. 4:40we see zero here it must be one so in
113. 4:43this case zero will be made as one so
114. 4:45deliberately corrupt the leftmost third
115. 4:47bit of the transmitted message and show
116. 4:50that the error is detected by the
117. 4:52receiver using CRC technique after you
118. 4:55deliberately make the error in the
119. 4:56transmitted message the receiver does
120. 4:59the same polynomial long division
121. 5:01operation but the remainder of that
122. 5:04operation done by the receiver will not
123. 5:06be zero if the remainder of the division
124. 5:09performed by the receiver is a non zero
125. 5:11value obviously there are errors and
126. 5:14that's it about the CRC technique I hope
127. 5:17now you know how to detect error using
128. 5:19CRC technique in the previous lecture we
129. 5:21understood how CRC code is generated and
130. 5:24in this lecture we know how this CRC
131. 5:27technique is used to detect the errors I
132. 5:28hope you guys enjoyed the session and
133. 5:31thank you for watching
134. 5:33[Applause]
135. 5:35[Music]
136. 5:40you
137. 5:43[Music]