Gavin wrongly writes (and it startles me that he does so. I am shocked,
shocked!):

> Keep in mind that "standard" checksum algorithms like CRC codes were
>  designed to detect thermal noise in a transmission line, which has
different
>  characteristics than deliberate changes to binary data.  The CRC codes rely
>  to some degree on the fact that typical transmission line noise comes in
>  bursts and never changes just one bit, whereas many operations on a
database
>  record might only change a single bit, which would increase the possibility
>  that the CRC code would miss the "error".

In contrast to the above paragraph, CRC codes are designed so that they will
always be able to detect single-bit errors, and they are also able to detect
double-bit and all odd numbers of errors. A quick & dirty
more-than-any-sane-human-would-want-to-know application note on CRC codes is
at:

     http://www.cypress.com/pub/appnotes/crc.pdf

In general, CRC's can't be used to correct errors once a packet of
information is received, only that they're present. But if the packet of data
has redundant bits for the transmitted word organized as a Hamming code,
single-bit (and sometimes double-bit) errors can be recovered on the fly
without having to request a retransmission of the original packet.

Here's a similar 2001 note from Agilent that says much the same thing:


http://www.ietf.org/internet-drafts/draft-cavanna-iscsi-crc-vs-cksum-01.txt

Hamming code redundancies (which are different than CRC's) are also the basis
of all error-correcting memory (ECC RAM) and have been a part of the HP3000
since its beginning.

Wirt Atmar

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