RF modem problem / CRC

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1 total messages Started by Marc Kaufman Fri, 25 Jul 1986 22:04

#40

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

25 lines
1561 bytes

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)
Followup-To: 
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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