ModemManager : 3G dongle disconnection

[Stéphane Régnier <sregnier ubiquiet com>]

Dear programmer,

I have difficulties to solve my disconnection problem, I hope you can
help.

I'm using a 3G usb key, huawei e180, with NetworkManager 0.8 and
ModemManager 0.3. Once connected, without using the network and after
exactly 30 minutes, it disconnects and never comes back until I restart
ModemManager.

Looking to ModemManager traces I found : 
** Message: (ttyUSB1) closing serial device...
** Message: (Huawei): GSM
modem /sys/devices/platform/ehci-omap.0/usb1/1-1 claimed port ttyUSB1
** Message: (ttyUSB0) closing serial device...
** Message: Modem /org/freedesktop/ModemManager/Modems/0: state changed
(connected -> disconnecting)
** Message: Modem /org/freedesktop/ModemManager/Modems/0: state changed
(disconnecting -> connected)
** Message: Modem /org/freedesktop/ModemManager/Modems/0: state changed
(connected -> disconnecting)
** Message: Modem /org/freedesktop/ModemManager/Modems/0: state changed
(disconnecting -> connected)

In NetworkManager, traces corresponding to a dbus activate :
NM: <info>  Activation (ttyUSB0) starting connection 'bouyguestel'
NM: <info>  (ttyUSB0): device state change: 3 -> 4 (reason 0)
NM: <info>  Activation (ttyUSB0) Stage 1 of 5 (Device Prepare)
scheduled...
NM: <info>  Activation (ttyUSB0) Stage 1 of 5 (Device Prepare)
started...
NM: <info>  Activation (ttyUSB0) Stage 1 of 5 (Device Prepare) complete.
NM: <WARN>  stage1_prepare_done(): GSM modem connection failed: (32)
Sending command failed: device is not enabled
NM: <info>  (ttyUSB0): device state change: 4 -> 9 (reason 1)
NM: <info>  Marking connection 'bouyguestel' invalid.
NM: <info>  Activation (ttyUSB0) failed.
NM: <info>  (ttyUSB0): device state change: 9 -> 3 (reason 0)
NM: <info>  (ttyUSB0): deactivating device (reason: 0).

And lsof confirms that modem-manager has no ttyUSB opened.
It seems that a modem-manager restart or a simple AT command each 30
minutes is efficient to keep connection alive but I prefer correct cause
than consequence. It could help me to know the reason (and the answer?)
of that 30 minutes timeout, and where it comes from (3Gkey,ehci,mm,nm?).

Thanks for the help

Stéphane