Path: utzoo!attcan!uunet!mitel!sce!cognos!dgbt!gandalf!carr
From: carr@gandalf.UUCP (Dave Carr)
Newsgroups: comp.dcom.modems
Subject: Re: Request for information (again)
Summary: MNP answers (for what their worth)
Message-ID: <2608@gandalf.UUCP>
Date: 19 Sep 89 13:03:04 GMT
References: <31194@ucbvax.BERKELEY.EDU>
Lines: 53

In article <31194@ucbvax.BERKELEY.EDU>, edward@ucbarpa.Berkeley.EDU (Edward Wang) writes:
> 
> What is MNP?  I gather it's a packet protocol, implemented
> on a synchronous line when it's built into a modem.  How
> can I find out the details, like packet format, error detection,
> handshake method?
> 

MNP is a synchronous layered protocol which handles parameter negotiation,
error correction, and compression.  The compression has 2 layers, 5 and 7.
MNP level 5 is commonplace.  MNP Level 7 is relatively new and expensive.

MNP level 5 is character based and achieves 2:1 compression *usually*.
For the single user interactive sessions, and maybe for file transfer and
mail you get the 2:1.  Don't use it for instance on a multiplexer backbone.
It won't get even 1.7:1.

The details for MNP aren't free, you have to buy them from Microcom.  We're
under license from them and that's about all the details I can give.  
Level 5 is something you could write in 2 days if you have the specs.

Level 7 is quite a bit more complicated and would takes several weeks to
implement (and lots of memory).

> What kind of protocol does the Telebit use?  It seems to do
> some sort of data compression as well.  What algorithm?
> 
We're not under contract with these guys, but I don't know the details.  
I will hazard a guess.

What happened about 2 years ago was that most of the modem manufacturers who
were producing MNP modems got pissed at Microcom for releasing the MNP specs
6 months after releasing their own modems.  This left them 12 months 
behind in the market.  They got off the MNP bandwagon and went to a software
house called Adaptive Computer Technologies and bought a package called the
ACT CommPressor.  It gets about 3:1 in the same situations that MNP Level 5
gets 2:1.  It still suffers in some applications.

ACT CommPressor software (rights to reproduce) go for about $3,000 US.

Now the funny part.  CCITT called for papers to standardize modem 
compression (V.42 bis).  Microcom leapt up and said "Pick me. Pick me".
ACT and most other modem manufacturers said "Pick us".  But alas, CCITT
picked a modified Limpel-Ziv (Unix compress,etc) from British Telecom.
So, expect the new breed of modems to get about 4:1 compression ! wowzers !.

I'll bet the Telebit uses ACT CommPressor software.

----
Dave Carr
Gandalf Data Limited
Nepean, Ontario, Canada
(613) 723-6500