cyclic redundancy check

algorithm (CRC or "cyclic redundancy code") A number derived

from, and stored or transmitted with, a block of data in order

to detect corruption. By recalculating the CRC and comparing

it to the value originally transmitted, the receiver can

detect some types of transmission errors.

A CRC is more complicated than a checksum. It is calculated

using division either using shifts and exclusive ORs or

table lookup (modulo 256 or 65536).

The CRC is "redundant" in that it adds no information. A

single corrupted bit in the data will result in a one bit

change in the calculated CRC but multiple corrupted bits may

cancel each other out.

CRCs treat blocks of input bits as coefficient-sets for

polynomials. E.g., binary 10100000 implies the polynomial:

1*x^7 + 0*x^6 + 1*x^5 + 0*x^4 + 0*x^3 + 0*x^2 + 0*x^1 + 0*x^0.

This is the "message polynomial". A second polynomial, with

constant coefficients, is called the "generator polynomial".

This is divided into the message polynomial, giving a quotient

and remainder. The coefficients of the remainder form the

bits of the final CRC. So, an order-33 generator polynomial

is necessary to generate a 32-bit CRC. The exact bit-set used

for the generator polynomial will naturally affect the CRC

that is computed.

Most CRC implementations seem to operate 8 bits at a time by

building a table of 256 entries, representing all 256 possible

8-bit byte combinations, and determining the effect that each

byte will have. CRCs are then computed using an input byte to

select a 16- or 32-bit value from the table. This value is

then used to update the CRC.

Ethernet packets have a 32-bit CRC. Many disk formats

include a CRC at some level.

(1997-08-02)