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From: jones@fortune.UUCP
Newsgroups: net.physics
Subject: Re: Cyclical Redundancy Check - (nf)
Message-ID: <2762@fortune.UUCP>
Date: Wed, 14-Mar-84 23:04:24 EST
Article-I.D.: fortune.2762
Posted: Wed Mar 14 23:04:24 1984
Date-Received: Thu, 15-Mar-84 07:27:16 EST
Sender: notes@fortune.UUCP
Organization: Fortune Systems, Redwood City, CA
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#R:minn-ua:-101900:fortune:8600014:000:1036
fortune!jones    Mar 14 18:57:00 1984

Cyclical Redundancy Check (CRC - also referred to as Cyclical
Redundancy Code) is a enhanced method of parity checking data.
It is typically found in tape and disk magnetic storage devices.
Essentially, the data stream is divided by a selected polynomial
and the remainder is appended to the data block.  When reading
the data the data stream is divided (the divide is performed by
an exclusive-or and shift) by the same polynomial.  In a binary
system, feeding in the remainder, in addition to the data, zeroes
the result.  Therefore, if the result is nonzero, then the data
is in error.  Clearly, a multibit failure which matches the
generator polynomial would not be caught.  However a cleverly
selected polynomial will catch burst errors of up to 3/4 the
length of the generator polynomial.  Floppy disks use a 16 bit
CRC for data fields of up to 1 Kbytes.

Dan Jones

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