3
$\begingroup$

Wikipedia says that CRC algorithm is based on cyclic codes, but it doesn't say that it is a cyclic code. If I understood correctly, a linear code of length $n$ called cyclic if and only if its generator polynomial divides $x^n-1$. So I think that in general CRC codes are not cyclic. Is it true? And if yes, why do they called it "Cyclic Redundancy Check/Code" rather than, for example, "Polynomial Redundancy Code"?

$\endgroup$
4
$\begingroup$

Minus typical bells and whistles such as first 16 bits are complemented etc., a CRC-encoded transmission ($k$ data bits followed by CRC bits) can be viewed as a codeword in a systematic cyclic code. In a practical implementation, $k$ is restricted to be a few thousand bits at most while the cyclic code has much larger block length.

For example, with CRC-16, the code is a $[2^{15}-1,2^{15}-17]= [32767, 32751]$ single-error-correcting double-error-detecting expurgated Hamming code whose generator polynomial (the CRC-16 polynomial) is of the form $(x+1)p(x)$ where $p(x)$ is a primitive polynomial of degree $15$. So, if $1600$ data bits are to be transmitted, the actual transmission is $1616$ bits, and this transmission is a codeword in the $[32767, 32751]$ systematic cyclic code. However, the leading $32751-1600 = 31151$ data bits in this codeword are $0$, and they are ignored at the transmitter as well as the receiver, that is, they are not transmitted at all. What is transmitted is just the $1600+16 = 1616$ bits which are at the low-order end of the codeword polynomial. At the receiver, the system knows that the high-order bits, however many they might be, are all $0$ and can be ignored: it simply processes the $1616$ bits it has received as a polynomial of degree $1615$, and checks whether or not the received polynomial is a multiple of the CRC-16 polynomial or not. The next transmission might have only $256$ data bits in it. No matter: the receiver processes only $256+16 = 272$ bits. So, how can the receiver tell if it is supposed to process $1616$ bits or $272$ bits? It doesn't need to know! The receiver simply processes the received bit stream until an "end-of-transmission" (EOT) or "end-of-file" (EOF) marker is reached. At that point, if the "division by CRC-16 polynomial" process has produced a $0$ remainder, the received data bits (everything in the received stream of bits except the last $16$ bits which are the CRC bits) are accepted as correct; if not, a re-transmission is requested. Note that no attempt is made to use the error-correcting capability of the code; it is used for error detection only.

TL; DR version Yes, CRC transmissions are codewords in a systematic cyclic code but nobody bothers to think about the transmissions in this way, or to exploit this notion in any way.

| cite | improve this answer | |
$\endgroup$
  • $\begingroup$ Indeed. I hadn't thought that CRC could be considered as a cyclic code of length greater than the length of the transmitted message. Thanks! $\endgroup$ – user241135 Sep 2 '15 at 6:42
  • $\begingroup$ Nice answer. For completeness, perhaps you should add what happens when we have more than 32751 bit to code. $\endgroup$ – leonbloy Jun 3 '16 at 20:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy