It should be easy to verify that this multiplication

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Unformatted text preview: n − k of them) so that the code word is in systematic form. It should be easy to verify that this multiplication produces a polynomial whose coefficients correspond to original message bits followed by all zeroes (for the check bits we’re going to add in below). Then, let’s divide xn−k m(x) by g (x). If the remainder from the polynomial division is 0, then we have a valid codeword. Otherwise, we have a remainder, R. We know that if we subtract this remainder from the polynomial xn−k m(x), we will obtain a new polynomial that will be a multiple of g (x). Remembering that we are in F2 , we can replace the subtraction with an addition, getting: w(x) = xn−k m(x) + R{xn−k m(x)/g (x)}, (7.2) where the notation R{a(x)/b(x)} stands for the remainder when a(x) is divided by b(x). The encoder is now straightforward to define. Take the message, construct the message 4 LECTURE 7. DETECTING BIT ERRORS Figure 7-1: CRC computations using “long division”. polynomial, multiply by xn−k , and then divide that by g (x). The remainder forms the check bits, acting as the digest for the entire message. Send these bits appended to the message. ￿ 7.2.2 Decoding Step The decoding step is essentially identical to the encoding step, one of the advantages of using a CRC. Separate each code word received into the message and remainder portions, and verify whether the remainder calculated from the message matches the bits sent together with the message. A mismatch guarantees that an error has occurred; a match suggests a reasonable likel...
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This document was uploaded on 02/26/2014 for the course CS 6.02 at MIT.

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