MNP protocol stuff

[Bill Mayhew]

One thing to keep in mind is that if you dial into the Unix PC On
Board Modem set for MNP, the MNP modem will freak out because the
On Board Modem echoes back the MNP negotiation string beause of the
way [uu]getty works.  Microcomm thoughtfully made the negotiation
handshake the same for both directions, so the caller modem thinks
that the Unix PC OBM is MNP-capable when, in fact, it is not.  The
result is a lock-up that can only be broken by hanging up and
restting the remote modem to disable MNP.  Keen, huh?  This MNP
feature/bug, of course, is not necessarily unique to the Unix PC.

In general, I've not been terribly excited by MNP.  It turns out
that any line that has even a modest amount of noise reults in so
many retransmits of packets that the throughput drops to the tens
of buad range.  I haven't had a whole lot of success by switching
to short MNP packets.  PEP mode modems deal with noise much better
than QPSK modems.  My experince has been that shopping for a modem
wiht adaptive equalization circuitry is money better spent than
buying MNP.  Adaptive EQ and MNP features aren't aways features
that occur as a pair.  It is possible to implement MNP with
software on the host system.  I use Mirror III with the MNP A-I
(Add-In) kit on my laptop, and it works quite well with the plain
vanilla Hayes-compatible internal modem.  Total cost for Mirror and
MNP A-I was less than $100.

With regards to the Trailblazer, its MNP is sort of strange.  It's
like MNP level 3-1/2.  Not all of the Level 4 MNP features are
implemented.


Bill