Getty for ISC/Telebit T-2500 w/autobaud via the CONNECT msg?

[Bill Mayhew]

<< whether or not to lock the interface speed of Telebit modem >>

Locking the interface speed is pretty handy.  I typically run my
port getty at 19200 and let the modem, in my case a Trailblazer +,
handle the speed translation.

There are some things you have to watch out for.  Namely with speed
translation going on in the modem, there is inevitable need for
flow control.  In normal dial-up the default xon/xoff flow control
works quite well.  However, the flow control will zap uucp
connections by adding extra characters to the data stream.  At 1200
and 2400 bps, the modem will insert flow control often enough that
the window of 3 packets is sure to get zapped with the result being
a deadlock of infinite retries.  The easy way to get around the
flow control problem is for your site to dail out to outher sites
with the modem set at a speed appropriate to the connection type.
You could still get flow problems if you are running the interface
at 4800 bps with the assumption of getting an MNP-4 2400 bps
connection.  In such cases, I disable MNP compression (limit to
level 3).

The Trailblzer, and most v.32 modems, have the ability to use the
RTS/CTS leads for flow control.  This has the advantage of keeping
the flow out of the character data stream.  Also advantageous is
the fact that the flow control latency is reduced because the flow
message is not queued in input buffers, etc; it can go straight to
the driver.  I believe the stock ISC/ATT asy port driver does not
have RTS/CTS flow capabilities; I don't recall off hand, but the
fas port driver available at your friendly ftp connection may have
it.

Auto bauding getty certainly is possible, though I have not seen it
available for for ISC.  I am using a TB+ on our HP-9000 with HP's
uugetty.  Their uugetty looks for carriage returns initially to set
the speed.  It usually one takes one return to recognize 9600 or
19200 but may take up to 5 to recognize 1200 bps.  Obviously, here
I let the modem match the speed of the connection; no problems with
flow control.  No need to read the CONNECT message from the modem,
vi even gets the right window size.  In this case, I use Q3 for
the verbosity setting (no messages, ever).  I've tried the Q6 and
have occasionally had troubles with the modem getting confused
about what modality it is in, resulting getting getty babble.  I use
the fist entry in the dialer chat script to switch back to Q0 to allow
progress monitoring.

There is one last thing.  I've heard some complaints about people
that use TB modems with locked interface speeds of a phenomenon
called bit-shaving.  Apparently TB modems might have slight speed
mismatches when doing speed translation.  From my understanding,
newer modems aren't bothered, but older modems have problems
staying in sync.  I've also been told that the newer firmware for
TB modems mitigates the bit-shaving problem.

Hope this is of some help,
Bill

-- 
Bill Mayhew      NEOUCOM Computer Services Department
Rootstown, OH  44272-9995  USA    phone: 216-325-2511
wtm at uhura.neoucom.edu   ....!uunet!aablue!neoucom!wtm
via internet: (140.220.001.001)