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From: ch@dce.ie (Charles Bryant)
Newsgroups: comp.dcom.modems
Subject: Re: Help: Why MNP5?
Message-ID: <1991Jan29.105612.17395@dce.ie>
Date: 29 Jan 91 10:56:12 GMT
References: <0cD8V3w163w@ozonebbs.UUCP> <1991Jan27.160653.7104@nstar.rn.com> <15126@celit.fps.com>
Organization: Datacode Communications Ltd, Dublin, Ireland
Lines: 26

In article <15126@celit.fps.com> billd@fps.com (Bill Davidson) writes:
>In article <1991Jan27.160653.7104@nstar.rn.com> larry@nstar.rn.com (Larry Snyder) writes:
>>Sorry - but incorrect with MNP5.  MNP5 over a 2400 baud connection
>>if properly installed will produce transfer rates of 280 cps even
>>sending GIFs and ZIPs
>
>I find this *VERY* difficult to believe.  GIF's are already LZW
>compressed and so in general cannot be further compressed by *ANY* one
>dimmensional digital compression method I am aware of without data
>loss.  If the MNP modems are really running a 2400 baud link, 280cps
>seems impossible.  The theoretical max for 2400 baud is something like
>266cps (8 data + 1 stop) and anything higher would require compression
>and as I said, I don't believe you can compress a GIF => 280cps
>impossible.

When MNP level 3 is used, the data is sent synchronously so no start or
stop bits are required. The raw throughput is therefore 2400/8 = 300.
Normally an MNP modem will allow the compression to be disabled which may
result in an increase in throughput for data that cannot be compressed by
MNP5. If another protocol is run on top of MNP (e.g. XMODEM) the
throughput may be reduced by MNP2/3/4 because a small extra delay must be
added (the whole packet must arrive before the CRC can be examined).
-- 
Charles Bryant (ch@dce.ie)
--
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