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From: tnixon@hsfmsh.UUCP (Toby Nixon)
Newsgroups: comp.dcom.modems
Subject: re: Data Compression
Message-ID: <3547@hsfmsh.UUCP>
Date: 16 Jul 90 02:19:48 GMT
Sender: daemon@hsfmsh.UUCP
Organization: Hayes Microcomputer Products, Inc.  Norcross, Georgia
Lines: 38

In article <269DE3BD.28935@ics.uci.edu>, John Roy asks:

- So if I'm sending ASCII digits would I do better with MNP5 or V.42bis?
- I would guess MNP5, but...

No!  Remember, MNP5 compresses individual characters into, at a 
minimum, 4 bits.  This means that the absolute BEST it can do is 
2-to-1 compression (unless you're sending long strings of repeating 
characters, then it's run-length-encoding feature kicks in, but you 
rarely send files with frequent long strings of the same character). 
But with V.42bis, the algorithm can compress from 1 to 250 
characters into a single "codeword" (a codeword is from 9 to, 
usually, 12 bits, depending on the maximum "dictionary size" 
supported).  V.42bis typically acheives an average of about 4-to-1 
compression on English text files.  You will always (unless you 
purposely construct a file that doesn't compress well with V.42bis) 
get better compression with V.42bis than with MNP5.

- Yes, I know this could be done by translating into binary or even BCD
- and then compressing.  But if I can get similar results by just
- purchasing the right modem, I'd be happy.

You'll be better off with a V.42bis modem.  All of the ones on the 
market now also support MNP5, so you'll have backward compatibility 
with the installed base.  It uses MNP5 when talking to modems that 
don't have V.42bis, but automatically uses V.42bis for better 
performance when connected with another V.42bis modem.

	-- Toby

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