Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site fortune.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxl!houxm!ihnp4!fortune!jones 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 Lines: 24 #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 UUCP: {sri-unix,amd70,hpda,harpo,ihnp4,allegra}!fortune!jones DDD: (415)595-8444 x 440 USPS: Fortune Systems Corp, 101 Twin Dolphins Drive, Redwood City, CA 94065