Path: utzoo!attcan!uunet!husc6!mailrus!cwjcc!neoucom!wtm
From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: comp.dcom.modems
Subject: Re: ATI 2400etc with MNP-5: any opinions?
Summary: Really gripes about MNP
Keywords: 2400bps internal, good noisy-line behavior
Message-ID: <1449@neoucom.UUCP>
Date: 7 Jan 89 13:58:08 GMT
References: <12975@duke.cs.duke.edu>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 40


This is a comment really about MNP protocol, rather than the ATI
modem in particular.  I initially tried a number of different MNP
modems between our sites impulse and neuocom.  Impulse are
separated by about 13 miles, both in Bell territory.  There is some
sort of line impairment that renders normal 1200 and 2400 modems
useless on about 80% of the calls placed between the two machines.
On rare occasions late at night, a clean connect can be obtained.
The symptom is a steady barrage of ~r _{, etc.  Apparently, there
is some sort of in-band impariment that is clobbering dibit groups
several times each second.

I've tried MNP modems from a number of sources, and found that I
didn't have very good luck with MNP fixing things.  I tried both
short and long packets without good results.  What would happen is
the modems would connect seemingly normally, and then lock-up with
the noise occuring frequently enough to not even let a single
packet through.

There is one standard 1200 baud modem that will work most of the
time:  the microchannel 1200 baud modem from IBM (It had a Racal
sticker on it, so you know who made it).  Unfortunately, that
doesn't do a whole lot of good for impulse and neoucom, which are
Unix PCs.  The IBM docs don't say that the modem has any sort of
special hardware in it; perhpas it has some sort of line equalizer.
I've measured the carrier power recieved at both neoucom; with a
-10 dBm output from the distant modem, the received power on the
other end is about -18 dBm -- well within acceptible limits.  The
local telco tested the lines and claimed they were fine.

We finally bit the bullet (encouraged by the Telebit Usenet offer)
and tried Trailblazer pluses on our machines.  We've had very good
luck with the Trailblazers.  My xfers are usually between 950 and
1200 char/sec with the Trailblazers.  I guess the PEP mode is able
to knock out the carriers where the noise that is killing the 1200
modems lies.  I don't like to sound like an advertisement, but
after several years of 1200 buad frustration, the Trailblazers
really did seem like magic.

--Bill