Question on pppd / reset?

Hi
If I'm not on the good forum, or too specific with my questions, please provide me the address of the good mailing list/forum.

We have an application using ppp with a GPRS modem. The system is actually in the field and working 24/7 for several months.

Carrier disconnected modem often (IP renewal, maintenance etc.) and modem alway reconnected correctly, except last time. I went onsite for debugging and try different commands to make modem reconnect (ifdown/ifdown, poff/pon, shutdown -r etc...) All commands failed to reconnect the modem. I had to pull the plug to make it connect.

My question is about reset. I would like to understand more the stack of the system, and why it failed to reconnect. The GPRS modem is connected trought full RS-232 signals, and RS-232 chips is on an ISA bus. How does ppp deal with this kind of hadware configuration ? I read ppp deal with modem reset, but how ? Any idea what I should look at to understand more the root cause of this failure ?

Thanks,

Pierre