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Various CRC standards extend the polynomial division algorithm by specifying an initial shift register value, a final Exclusive-Or step and, most critically, a bit ordering . As a result, the code seen in practice deviates confusingly from "pure" division, [ 2 ] and the register may shift left or right.
The CRC and associated polynomial typically have a name of the form CRC-n-XXX as in the table below. The simplest error-detection system, the parity bit, is in fact a 1-bit CRC: it uses the generator polynomial x + 1 (two terms), [5] and has the name CRC-1.
Download as PDF; Printable version; ... Name Length Type cksum (Unix) 32 bits CRC with length appended CRC-8: 8 bits CRC: CRC-16: 16 bits CRC:
These inversions are extremely common but not universally performed, even in the case of the CRC-32 or CRC-16-CCITT polynomials. They are almost always included when sending variable-length messages, but often omitted when communicating fixed-length messages, as the problem of added zero bits is less likely to arise.
The checksum is therefore 0x0403. It could be transmitted with the byte stream and be verified as such on the receiving end. Another option is to compute in a second step a pair of check bytes, which can be appended to the byte stream so that the resulting stream has a global Fletcher-16 checksum value of 0.
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A CRC has properties that make it well suited for detecting burst errors. CRCs are particularly easy to implement in hardware and are therefore commonly used in computer networks and storage devices such as hard disk drives. The parity bit can be seen as a special-case 1-bit CRC.
Low-density parity-check (LDPC) codes are a class of highly efficient linear block codes made from many single parity check (SPC) codes. They can provide performance very close to the channel capacity (the theoretical maximum) using an iterated soft-decision decoding approach, at linear time complexity in terms of their block length.