Path: utzoo!yunexus!maccs!cs4g6ag
From: cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn)
Newsgroups: comp.sys.ibm.pc
Subject: Re: MNP level 5
Summary: error correction and some data compression
Keywords: MNP modems
Message-ID: <25350C7F.8192@maccs.dcss.mcmaster.ca>
Date: 12 Oct 89 22:13:19 GMT
Article-I.D.: maccs.25350C7F.8192
References: <10767@phoenix.Princeton.EDU>
Reply-To: cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn)
Organization: McMaster University, Hamilton, Ontario
Lines: 52

James Webster Birdsall writes:
$   This went by on the net a few months ago, but I've forgotten. Is MNP
$level 5 simply hardware error-correction or is it also 2:1 compression?

   MNP has five levels.  One through four implement various error correction
schemes.  Level five incorporates error correction and data compression.  It
is not 2:1 compression - don't let those who advertise 2400 baud MNP5 modems
as "4800 baud throughput" fool you.  I find typically that MNP5 squeezes
about 3500 baud average throughput out of my 2400 baud modem ON TEXT.  If
you're going to be up/downloading binary files (especially archives of
various sorts), you'll actually lose throughput with MNP5.  Obviously, the
amount of analysis that can be done on the fly by the modem is limited, so
the data compression algorithm must be fairly simple.  It seems to be tuned
to the kind of distribution found in text (note that I'm not saying that it
uses a fixed compression scheme - it doesn't - but rather that it uses a
dynamically variable scheme which generally works well for text).

   BTW, MNP is not restricted to hardware implementations.  I am writing this
on a link using a standard 2400 baud, non-MNP modem with software (FlashLink)
that implements MNP.  More on this later.

$   More importantly, how common is it now? Is it likely to be more
$common in the future? Is it worth spending $165 (minus selling my
$current modem) to get it (no increase in speed, just MNP)?

   How common is it?  Well, our university's phone-up data lines all have
MNP level 5.  Of the BBSs in this area, around 15-25% have MNP.  This number
will no doubt grow slowly, but that's how it is at the moment.

   Is it worth trading in for an MNP modem?  Depends on how many of the
BBSs you're calling have MNP.  Why don't you ask the sysops of them all
if they have MNP, and then decide based on that and the cost you'd incur
by switching?

   As for FlashLink, it came with my modem (from Cardinal Technologies
in Lancaster, PA).  It implements MNP levels 2, 4 and 5.  It doesn't
always get the correct level, though; there are some BBSs who swear they
have level 5 but with which I can only connect at level 4 or 2, and
sometimes calling the university I end up connected with no MNP.  The
advantage of this software is that it saves money on the modem; the
disadvantage is that FlashLink is a fairly simple-minded program,
certainly not as powerful as Procomm Plus, Mirror, or whatever - for
example, the only file transfer protocols I can use are Xmodem, Xmodem/CRC,
Ymodem, and Ymodem Batch; I can't, for example, add Zmodem.  Also, the
terminal emulation isn't quite VT100.  Anyway, you get the idea.

   Hope this helps.
-- 
Stephen M. Dunn                             cs4g6ag@maccs.dcss.mcmaster.ca
         <std_disclaimer.h> = "\nI'm only an undergraduate!!!\n";
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Maybe if we're lucky they will show it again, such a terrible thing to see