C Code For Error Detection Using Crc
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Crc Error Detection Probability
Series, Part 3: CRC Implementation Code in C/C++ Sat, 2000-01-01 00:00 - Michael Barr by Michael Barr CRCs are among the best checksums available to detect and/or correct errors in communications transmissions. Unfortunately, the modulo-2 arithmetic used to compute CRCs doesn't map easily into software. This article shows how to implement an efficient CRC in C. I'm going to complete my discussion of checksums by showing you how to implement CRCs in software. I'll start with a naive implementation and gradually improve the efficiency of the code as I go along. However, I'm going to keep the discussion at the level of the C language, so further steps could be taken to improve the efficiency of the final code simply by moving into the assembly language of your particular processor. For most software engineers, the overwhelmingly confusing thing about CRCs is their implementation. Knowing that all CRC algorithms are simply long division algorithms in disguise doesn't help. Modulo-2 binary
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Crc Error Detection And Correction
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Join them; it only takes a minute: Sign up Function to Calculate a CRC16 Checksum up vote 25 down vote favorite 8 I'm working on a library to provide simple reliable communication over an RS232 or RS485 connection. Part of this code involves using a CRC16 checksum on the data to detect corruption from line noise. I've crc error detection created a function to calculate a CRC16 checksum, but it doesn't seem to be outputting correct values. The relevant code I've written can be found here. I'm checking my output against this online CRC calculator. I've come to the conclusion that either my understanding of how to calculate a CRC16 is wrong, or the online calculator is wrong (the former seems more likely). Can someone tell me where I might be going wrong? Thanks. c crc crc16 share|improve this question edited May 12 '12 at 16:49 asked May 12 '12 at 14:13 Jonathan Lamothe 133125 See also my question stackoverflow.com/questions/4455257/… which was based on the same online calculator, and includes a handy matrix table comparing the various formulas. –Potatoswatter Apr 17 '14 at 3:37 add a comment| 5 Answers 5 active oldest votes up vote 26 down vote accepted +50 There are several details you need to 'match up' with for a particular CRC implementation - even using the same polynomial there can be different results because of minor difference Redundancy Check) Data is sent with a checksum. When arrives, checksum is recalculated. Should match the one that was sent. Bitstring represents polynomial. e.g. 110001 represents: 1 . x5 + 1 . x4 + 0 . x3 + 0 . x2 + 0 . x1 + 1 . x0 = x5 + x4 + x0 The order of a polynomial is the power of the highest non-zero coefficient. This is polynomial of order 5. Special case: We don't allow bitstring = all zeros. Easy to use framing or stuffing to make framed-and-stuffed transmission never all-zero, while still allowing payload within it to be all-zero. hash functions CRC Origin in research of W. Wesley Peterson: W.W. Peterson and D.T. Brown, "Cyclic codes for error detection", Proceedings of the IRE, Volume 49, pages 228-235, Jan 1961. W.W. Peterson, Error Correcting Codes, MIT Press 1961. Modulo 2 arithmetic We are going to define a particular field (or here), in fact the smallest field there is, with only 2 members. We define addition and subtraction as modulo 2 with no carries or borrows. This means addition = subtraction = XOR. Here's the rules for addition: 0 + 0 = 0 0 + 1 = 1 1 + 0 = 1 1 + 1 = 0 Multiplication: 0 * 0 = 0 0 * 1 = 0 1 * 0 = 0 1 * 1 = 1 Subtraction: if 1+1=0, then 0-1=1, hence: 0 - 0 = 0 0 - 1 = 1 1 - 0 = 1 1 - 1 = 0 Long division is as normal, except the subtraction is modulo 2. Example No carry or borrow: 011 + (or minus) 110 --- 101 Consider the polynomials: x + 1 + x2 + x ------------- x2 + 2x + 1 = x2 + 1 We're saying the polynomial arithmetic is modulo 2 as well, so that: 2 xk = 0 for all k. Digital Communications course by Richard Tervo Intro to polynomial codes CGI script for polynomial codes CRC Error Detection Algorithms What does this mean? Just consider this as a set of rules which, if followed, yield certain results. When the checksum is re-calculated by the receiver, we should get the same results. All sorts of rule sets could be used to detect error. It is useful here that the rules define a well-behaved field. Consider the polynomials with x as isomorphic to binary arithmetic with no carry. It is just easier to work with abstract x so we don't make the mistake of starting to add, say. 3 x3 to get x4 + x3 if we were thinking of x=2. We work in abstract x and keep "the coeff