UMTS usb stick MC998D (U998D): disconnect after 30-45 seconds

[Christian Jaeger <chrjae gmail com>]

Hi

I've got a persistent problem with this modem disconnecting in about
50-70% of cases when I plug it in, after about half a minute. This is
after first connecting and delivering data during that half minute.

The process is absolutely consistent:

(1) I plug it in; my udev scripts do the usb mode switching and reload
the kernel modules (a hack, see
http://forum.nginx.org/read.php?31,107067). (That process is pretty
slow, it takes about 45 seconds; but these are not the 30-45 seconds I
mention above, and this part works reliably, i.e. it's always getting
to step 2.) The stick has an LED that blinks intermittently (about 1/3
Hz, on:off time about 1:10 or so) at this point.

(2) network manager recognizes it; I choose to connect from the applet

(3) network manager brings up the connection (pretty quickly, in about
5 seconds). The LED of the stick lights up constantly at this point. I
can check for new email, surf the web, open ssh connections, then:

(4a) the connection stays up just fine, and does so for hours. I can
even move around in the city in a bus, or walk around, and the
connection can become spotty of course when the signal is getting weak
in places, but when the signal comes back the connection is again
there without any intervention from me (as long as it's not too weak
for too long). Also, I can choose "disconnect" from the nm applet, and
connect again later and it goes immediately back through 3 -> 4a,
never to 4b. I can even put my laptop to sleep, wake it up, connect,
and it goes to 4a. If I unplug and replug the stick, it begins with
(1) and may end up in 4b instead.

*or*

(4b) the connection drops after 30-45 seconds. The LED goes back to
blinking mode. Network Manager still shows the connection as
"connected". I choose "disconnect", but cannot connect again. So the
only thing I can do is unplug the stick and replug it, so as to
restart at (1). There's a +-good chance to land at 4a then. (But it
depends, I've had cases where I went through this cycle 10 times and
haven't been successful.)


So, this can make me spend ~3-10 minutes just connecting to the
network; once I'm successful, I try to just leave the stick plugged in
for as long as possible.

One detail that I thought interesting at some point was that, in that
first phase of 30-45 seconds, nm shows it as being in UMTS mode, and
at about the same time as it can loose connection, it also switches
over to HSPA or HSUPA (or the + versions? Tell me if you need it
exactly). But I'm not sure these are related.

Another detail is that the stick seems to work under the bundled
drivers from Bell in Windows XP without this issue (but I've barely
tested).

This is on Debian stable. My laptop is an HP NC2400. (Well I tested
with Windows on another machine, but that shouldn't play a role,
right?.. also, the NC2400 has 2 USB ports and I tested on both with no
difference.)

Any idea? Do you need more info?

Thanks,
Christian.