RF modem problem / CRC

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Article #40

#40

From: Marc Kaufman
Date: Fri, 25 Jul 1986 22:04

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Newsgroups: mod.protocols
Subject: Re: PROB CRC-FAIL on VMSHARE
Summary: 
Expires: 
References: <WANCHO.12224727681.BABYL@SIMTEL20.ARPA>
Sender: 
Reply-To: kaufman@Shasta.UUCP (Marc Kaufman)
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Distribution: 
Organization: Stanford University
Keywords: 

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 newer high-speed modems use 64-QAM or 256-QAM modulation schemes,
so the right kind of error in 2 or 3 adjacent frames can cause a
problem that still passes CRC.  The solution is to post-scramble the
bits before the modem sees them, so that adjacent bits are separated
by many frames, thereby reducing the chances that a burst error of a
few frames will allow the CRC to pass.  Unfortunately there is no
hardware to do this that I know of.

Marc Kaufman (kaufman@shasta.stanford.edu)

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From: kaufman@shasta.stanford.edu (Marc Kaufman)
Newsgroups: mod.protocols
Subject: RF modem problem / CRC
Message-ID: <8607252354.AA15690@ucbvax.Berkeley.EDU>
Date: Fri, 25-Jul-86 18:04:27 EDT
Article-I.D.: ucbvax.8607252354.AA15690
Posted: Fri Jul 25 18:04:27 1986
Date-Received: Fri, 25-Jul-86 23:48:23 EDT
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Approved: protocols@red.rutgers.edu