Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!convex!egsner!vector!jonb
From: jonb@vector.Dallas.TX.US (Jon Buller)
Newsgroups: comp.mail.uucp
Subject: Re: Using UUCP over international phone lines
Keywords: satellite packet-size acknowledgment delays
Message-ID: <1154@vector.Dallas.TX.US>
Date: 28 Jan 91 23:08:49 GMT
References: <267@fgh.fgh.oz.au> <1991Jan25.214706.185@spock.UUCP>
Reply-To: jonb@vector.Dallas.TX.US (Jon Buller)
Organization: Dallas Semiconductor
Lines: 30

In article <1991Jan25.214706.185@spock.UUCP> lancelot@spock.UUCP (Thor Lancelot Simon) writes:

>If you're sending megabytes of data, I would assume you're compressing it.
>But MNP is not really 4800 bps.  It is 2400bps with a data compression
>scheme which will get, *at best* 4800bps throughput.  If you're sending data
>that is already compressed, throughput should drop to BELOW 2400 bps.  The only
>reason it wouldn't is that MNP makes slight improvements on 2400 bps using a
>link scheme that eliminates stop bits.  If you stop compressing your data on
>the sending host, throughput will rise, but the files will be proportionately
>larger, and your compression at the sending site is probably superior to the
>modem's.  To me, MNP modems have always been a fairly obvious waste of money
>if used for file transfer - I can do far better compression before I send the
>data, and only have to buy a 2400 bps modem.

MNP class 4 does no compression, class 5 and class 7 do roughly 2 to 1
and 2.5-3 to 1 compression respectively.  This is all independent of the
transmission speed.  Some Telebit modems (T2500?) can do MNP5 at 9600bps
or do v42bis which is a superset of MNP with (your favorite and mine) LZW
compression, very similar to what the 'compress' program does.

So if you buy a 4800bps MNP4 modem, you will get (almost) 4800bps.  A bit
will be lost to packet overhead (about 10 bytes/packet in fact, but I
don't have my docs & code at the moment...  If you buy a 4800bps MNP5
modem you can get between 4800bps and 9600bps depending on your data.
Don't tell the guy he doesn't know what he's saying UNLESS YOU ARE ABSOLUTELY
SURE about it.  Of course he may have a 2400bps MNP5 modem, in which case
your statements would be correct.
-- 
Jon Buller       jonb@vector.dallas.tx.us       ..!texsun!vector!jonb
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