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From: kaufman@SHASTA.STANFORD.EDU (Marc Kaufman)
Newsgroups: mod.protocols
Subject: Re:  CRC-16
Message-ID: <8608120853.AA06492@ucbvax.Berkeley.EDU>
Date: Tue, 12-Aug-86 00:20:28 EDT
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Posted: Tue Aug 12 00:20:28 1986
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Thanks for the correction. Note that my example was 17 bits, even if I
used the wrong polynomial.  The problem is, that this represents only
three (3) symbols in the newer RF modems (64- or 256- QAM, etc.).  In
addition, a symbol error in a differentially encoded modem will cause
an error in the next symbol (as the phase is corrected).  With trellis
coding, the error may propogate to two or more symbols before the codes
get back to normal.  Just possibly (I have not looked in detail) the
generated errors may just cancel the CRC error in some cases.  In any
event, short polynomials become less useful as the number of bits in
a symbol increases.  Either the makers of modems should select their
transition rules to preserve effectiveness of the CRC, or we should
look for better error checking polynomials.