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From: ken@argus.UUCP (Kenneth Ng)
Newsgroups: comp.unix.wizards
Subject: Re: Filtering Everything
Message-ID: <826@argus.UUCP>
Date: Tue, 21-Apr-87 07:12:22 EST
Article-I.D.: argus.826
Posted: Tue Apr 21 07:12:22 1987
Date-Received: Wed, 22-Apr-87 02:21:06 EST
References: <919@watmum.UUCP> <6447@bu-cs.BU.EDU> <65@dalcsug.UUCP> <3011@sdcsvax.UCSD.EDU>
Distribution: comp
Organization: NJ Instit. of Tech: TEIES Project
Lines: 32

In article <3011@sdcsvax.UCSD.EDU>, hutch@sdcsvax.UCSD.EDU (Jim Hutchison) writes:
> <>
> Well, after you get your streams/pty/tty code finished, you will still
> be stuck with a truly nasty problem.  Line errors.  If you compress
> the signal (characters) at 2:1, you will be multiplying the effects of
> line noise.  If you use a state table driven compressor ala LZ you will
> have to write code to recover from noise (interesting trick that will be),
> or else go into hopeless noise mode until manual intervention.

Actually its worse, you don't multiply the effects of line noise, you
propagate it!  The one saving grace is that truely adapative LZ methods
tend to isolate, and then remove the effects of line noise, in most cases.
Shortly after a local flood from upstairs, I saw the effects of REALLY
BAD lines, the screen threw a fit, and then the program reset the table,
and displayed the screen correctly.  Last year I saw an LZ based program
on a telephone call across the country via ordinary 1200 baud modems, with
only ocasional line problems.  But you must remember, if you can reduce
the bytes sent by half, how expensive is a byte to do a CRC check?  Granted
there will be problems with finding optimal packet size, but the possibilities
are interesting.

>     Jim Hutchison   		UUCP:	{dcdwest,ucbvax}!sdcsvax!hutch
> 		    		ARPA:	Hutch@sdcsvax.ucsd.edu

-- 
Kenneth Ng: Post office: NJIT - CCCC, Newark New Jersey  07102
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