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From: geoff@desint.UUCP (Geoff Kuenning)
Newsgroups: net.sources.d
Subject: Re: \"FAST\" protocol proposal from Hayes
Message-ID: 
Date: Sun, 24-Aug-86 15:51:20 EDT
Article-I.D.: desint.249
Posted: Sun Aug 24 15:51:20 1986
Date-Received: Wed, 27-Aug-86 21:34:50 EDT
References:   
Reply-To: geoff@desint.UUCP (Geoff Kuenning)
Organization: SAH Consulting, Manhattan Beach, CA
Lines: 21
Keywords: errors reliability end-to-end CRC

In article  campbell@maynard.UUCP (Larry Campbell) writes:

> Note also that FAST just has one CRC for the entire file.  The longer
> the packet, the greater the probability that CRC-16 will fail to
> detect an error.  For almost any reasonably large file, CRC-16 is
> nearly worthless.

This is quite simply untrue.  CRC-16 will detect *all* error bursts that
are less than 16 (17?) bits long, and a remarkably large fraction (e.g.,
99.97% or some such) of all longer bursts.  The length of the packet is
not a factor.  (This is similar to statistical sampling, where a sample
of about 1200 is sufficient no matter how large the sampled population.)

Note that widely-separated errors can be thought of as a very long
error burst with most of the bits unchanged.  Thus, you can count on
CRC-16 picking up 99.97% (or whatever the right number is) of all error
patterns, regardless of packet size.
-- 

	Geoff Kuenning
	{hplabs,ihnp4}!trwrb!desint!geoff