MCIS has placed 22 14.4kbps modems in service on 529-4999.
The old 9600bps modems appear at 529-2161.
 
The new modems will answer MNP level 5 and V.42bis error correction
requests.  They negotiate communications at data rates from 2400 kbps
to 14,400 kbps.  MNP level 5 data compression can double the effective
throughput of a modem.  V.42bis LAPM can quadrouple the effective throughput
of a modem.  Ultimately you are constrained by the speed of the modem attach-
ment to the host computer.  In the case of the Gandalf data switch (VM, VAX,
Sherlock), the maximum throughput will be 19.2 kpbs because that is the
maximum data rate that our asychronous data system can accommodate.
 
This document discusses modem control for those who want to optimize
performance or who are having problems with their current modem setup.
A section describing common problems and resolution is followed by:
recommended modem commands.
A recommended Disaster Fallback command string,
A discussion of proper modem cables for external modems,
and a glossary of common Hayes commands.
Please note, Hayes compatible means similiar to a Hayes modem.
Your modem may vary in some detail from the information presented here.
Check your modem manual.
 
--Symptom
Letters I type are delayed and "spit" back in batches of three or more.
-Discussion
Error correction and compression features work by "packetizing" data and
sending the packet to the other modem.  The rule is to wait until either the
input buffer is full or a timer elapses to send the buffer contents to the
other modem.
-Resolution
This is a feature, not a bug.  You can get used to this feature or use the
discussion below to defeat error correction and data compression.  Then
characters will echo immediately, but your effective data rate will be lower.
 
--Symptom
When I dial, the telephone rings with no answer.
-Resolution
Problem with our modem bank.  Call 7900 to report the problem.
 
--Symptom
When I dial the modem answers, but never issues the CONNECT message.
-Resolution
Try a non-error correcting connection using discussion below.
If that doesn't solve the problem, call 7900 to report the problem.
 
--Symptom
I am able to log onto the computer properly, but when at some point the
computer "freezes" and won't communicate with me.
-Resolution #1
You typed a control-s.  Type a control-q and the system should return
characters to your screen.
-Resolution #2
If you use the administrative or academic IBM mainframes the terminal
controller is confused.  Type control-g to reset.  Your screen should refresh.
-Resolution #3
If you are using an Error Corrected/Compressed session, you may have flow
control problems.  Either reconfigure not to use error control or see the
discussion of flow control below.
 
--Symptom
When I dial in, the modem answers and sends a long string of garbage back to
the terminal.  It does not respond to my "enter class" choice.
-Resolution
Defective answer modem.  Call 7900 and report the problem.
 
--Sympton
When I attempt a download with Ymodem or Zmodem either the session hangs, or
the received file is corrupt.
-Resolution
This is usually a flow control problem.  To successfully use true binary
(as opposed to Kermit binary) file transfers you MUST NOT use software flow
control.  Use hardware flow control, negotiate a non-Error Corrected session,
or use Kermit.  VAX users please note:  The new VAX Kermit allows large packet
lengths.  This has a dramatic (and positive) effect upon Kermit file transfer
throughput.  If you have a lot of downloading to do, consider visiting a
microcomputer lab.  Then you benefit from network speeds.
 
            Discussion of Hayes compatible modem settings
 
The following are common Hayes compatible modem settings you should use.
You would normally use "ATDT529-4999".  (or "ATDP" for pulse service).
Place the following strings between "AT" and "DT", if necessary.
NOTE: NEVER TYPE QUOTES (").
 
All compatible modems:
"N5" or if that fails "N1" negotiate highest common speed.
"&Q5" V.42 Error control.  This offers 2 to 4 times compression on bulk
                           text transfer.
 
External modems also require:  (See External modem cable note below.)
"&K1" RTX/CTS flow control.    Required for negotiated sessions.
                               Requires appropriate cable.
                               Exp. Macintosh - or -
"&K2" XON/XOFF flow control.   Less desirable, but may be the only option
                               if your cable doesn't support RTS/CTS.
                               Will probably hose any true binary transfer
                               mechanism, i.e., zmodem and ymodem.
 
Disaster fallback:
Set terminal emulator to rated modem speed.  Then insert the following in
the dial string:  "N1&K0&Q0".
For example, "ATN1&K0&Q0DT5294999".
This will defeat negotiated error control and data compression.
Alternatively, dial 529-2161, these are the modems that were in service
last semester.
 
External modem cable note:
To use hardware flow control, your cable must support the RTS/CTS signals.
On a DB25 connector, these correspond to pins 4 and 5.  If your cable does
not pass these pins through, you cannot use hardware flow control.  If you
use MCIS supported software you can use software flow control.  If you use
ymodem or zmodem file transfer, you should use hardware flow control, or defeat
negotiated error control and data compression as mentioned in Disaster Fallback
 
MACINTOSH users special note:  There are two flavors of Mac modem cables.
One flavor passes DTR.  The other flavor supports hardware flow control.
It is difficult to determine which type of cable you have without a cable
tester.  If your session "hangs" after working normally during login, you
probably have a flow control problem.  Either use software flow control and
avoid use of ymodem or zmodem, or negotiate a non-corrected session.
 
Hayes compatible modem command summary (check your manual, if available)
Flow control:  Note for internal modems, use RTS/CTS software flow control.
"&K0"  Disable local flow control.
"&K1"  Enable RTS.CTS hardware local flow control.              <=REC.
                                                     If your cable supports it.
"&K2"  Enable XON/XOFF software local flow control.
"&K3"  Same as &K1
"&K4   Same as &K2
"&K5"  Transparent XON/XOFF software flow control.              <= Not Rec.
"&K6"  Enable XON/XOFF (local) with pass th. Ack not supp.      <= Not Rec.
"&K7"  Enable RTS/CTS (local) with XON/OFF pass through.        <= Not Rec.
 
Modem to Modem session negotation
"&Q0"  Select asynchronous non-EC(Error Correcting) mode.       <= disables
                                                                smart features.
"&Q5"  Select V.42 EC operation (requires flow control).        <= Recommended
"&Q6"  Asynchronous mode with ASB (req. flow control).
"&Q8"  Select alternate EC protocol (e.g. force MNP).
 
Result codes and messages (allow you to watch connection progress)
"Q0"  Return result code.                                       <= Recommended
"Q1"  Do not return result codes.
"Q2"  Return result codes in origionate, but not answer mode.   <= Alt. Rec.
 
"V0"  Return results in English.                                <= Recommended
"V1"  Return results as numeric codes.
 
"X0"  Basic all progress support.  Blind Dial                   <= Not Rec.
"X1"  Returns CONNECT messages only.  Blind Dial                <= Not Rec.
"X4"  Basic call progress.  Dial tone and busy recognition.     <= Rec.
 
Basic modem setting controls:
"&F"  Reset to factory defaults
"&W"  Write to local config. 0.  The default for dialing.
"&V"  Display current modem settings.
 
Questions and comments on this long and perhaps confusing document are welcome.
Send mail comments to jsimpson@miamiu or [log in to unmask]
 
Joe Simpson   January 12, 1994