Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.3 4.3bsd-beta 6/6/85; site ucbvax.BERKELEY.EDU Path: utzoo!decvax!ucbvax!nrl-aic.ARPA!hoey From: hoey@nrl-aic.ARPA (Dan Hoey) Newsgroups: mod.protocols Subject: CRC-16 Message-ID: <524160487.hoey@nrl-aic> Date: Mon, 11-Aug-86 12:08:06 EDT Article-I.D.: nrl-aic.524160487.hoey Posted: Mon Aug 11 12:08:06 1986 Date-Received: Mon, 11-Aug-86 18:59:12 EDT Sender: daemon@ucbvax.BERKELEY.EDU Organization: The ARPA Internet Lines: 38 Approved: protocols@red.rutgers.edu A couple of errors appeared in recent postings about CRC-16 error checking. Date: Fri, 25 Jul 86 15:04:27 PDT From: Marc Kaufman Subject: RF modem problem / CRC CRC-16 is extremely susceptible to even-number-of-bit errors. For example, the following error pattern will not change the CRC: 10000100000010001 (binary), where '1' indicates a bit error.... The example applies to the CCITT V41 polynomial. The CRC-16 polynomial is different, so the example should be 10100000000000011 (binary), where '1' indicates a bit error. or, if you are a big-endian, 11000000000000101 (binary), where '1' indicates a bit error. Subject: Re: Interview with MNP protocol author Date: 05 Aug 86 17:09:30 EDT (Tue) From: John Robinson ...>>> Using the 16-bit redundancy check, it will detect every >> error which is 16 bits or smaller, with 100% probability. No! No! No! Any error in an odd number of bits, and all one-, two-, and three- bit errors will be detected. 16 bits in a row which are inverted are detected, yes (I think!), but a sequence of 16 bits in which some bits are in error is NOT necessarily detected. CRCs aren't that good.... The original statement is correct. Both CRC-16 and CCITT V41 will detect any error that is a subset of sixteen consecutive bits. (Not to say that's all that good, as we have seen.) This can be seen by noticing that nonzero multiples of a degree sixteen polynomial must have degree at least sixteen. The only undetected pattern that spans seventeen bits is the polynomial itself, as shown above. Dan Hoey