Question : Problem: Odd modem baud rate problem

I'm trying to connect to a modem which will only respond at 2400 baud.  If I try from an old standalone desk modem, it manages this fine.  Through a Cisco MICA modem in our access router, it seems to connect at 21600, and freezes after the connect line.  Any higher requested rate (using modem registers S30 and S31 to restrict the baud rate) connects at 33600 with the same response - nothing.  Connecting at lower than 2400 just gives me gobbledegook back.

Most similar devices I'm connecting to manage this fine, it's just this one that seems anomalous.  Anyone seen anything like this before and can suggest anything to try to fix it?  Getting the modem at the far end replaced is not an option I'm afraid, as I can get access - just not via the route I want to.

Thanks
B

Answer : Problem: Odd modem baud rate problem

But it does tell you that it is connecting at 2400 bps.

LAPM: Link access Procedure for Modems.  This is a protocol the modems use to talk to each other for error correction.

In the "old" days modems were truly asynchronous and transmitted data between each other, as well as the computer, asynchronously.  However, this lead to a LOT of errors and corrupted data.  So the modem manufactures started developeing protocols so that the modems actually talked in a semi-synchronous mode.  That is, instead of sending just the start-bit, data, parity-bit, stop-bit, it actually send a "packet/frame" that included a header, trailer, and some type of "check" byte.  LAPM uses cyclic redundancy checking (CRC).  LAPM is defined in the V.42 standard.

So your computer talks asynchronously to your modem, but modem to modem it talks semi-synchronously.

What I am assuming is that the remote modem is configured withOUT flow control and is connected to the device at 2400 bps.  That means if you connect higher than 2400 bps the data get garbled on the remote end.

You need to find away to get the MICA modem to only connect at 2400 bps.  It looks like if you set:

     S29=3
     S30=2400
     S41=0
     S42=0 (this is the default)

It may work.  If that does not work, you may need to set S29=4.