[TROUBLESHOOTING-GUIDE] Sierra Wireless Cellular Modems

[tiermutter]

[PROLOGUE]
Ever since I became a member of this forum, I keep reading about problems with cellular modems from Sierra Wireless (which doesn't mean that Sierra Wireless modems are bad).

In the meantime I have switched to a different, external modem because I also had to struggle with these problems for some time. Over the years, I created a small troubleshooting guide that I could use when nothing worked again. Now I don't need this help anymore, but I don't want to let it get lost in my documents either, but share my findings and make them available here, maybe it will help someone after all.
Please note that it has been a year since I last used my MC7304 before I replaced it with an external modem. This time OPNsense 21.x was present with hardened BSD.

[GENERAL]
Below I describe what I found out for my setup, which doesn't mean that it works in all cases.
Most of the problems occur because the OS cannot communicate with the modem or when connecting to the mobile network itself.

1) Check modem support!
BSD not really seems to support cellular modems very well, so you should always check if someone has already reported that the modem has been successfully set up in BSD or OPNsense. Also remember that there are different cellular bands used by different geographical regions: Your modem must be compatible with your region! MC7304 will perform fine in Europe/ Germany and Australia, but not really good in USA as the modem only supports 2100MHz for that region.

2) Be patient!
After initial connection attempt, e.g. after reboot, it may take some attempts over a few minutes until the connection will be established.

3) Read the logs!
The point to point connection logs are essential to debug such problems, but be prepared that there may be created tons of entries within a few seconds while the modem is trying to connect. You´re good to go using filters for some specific keywords I´ll refer to later in „Reading the logs“ section.

4) Don´t worry!
Some PPP configuration settings such as service provider settings may dissapear in the GUI occasionally, that should not be the problem, in most cases the connection will work without these settings displayed in GUI.


As mentioned in the docs, the SIM PIN definitely should be deactivated. Do this with a cellular phone, or using the CLI if the access to the modem already works. See „AT-commands“ section for further information.

Sierra Wireless cellular modems are regulary devices called „cuaU0.2“, where „0“ can also differ or may change occasionally for unknown reasons. "cuaU0.1" may be the GPS device included in most cellular modems.

The AT-command / init string &F0E1Q0 +CMEE=2 is often mentioned, which is supposed to prevent or fix problems. For me that AT-command never has fixed any issues and if I translated the individual commands correctly, this command may only provide additional debug information if the parameters differ from default. The command at&v will show you all current parameters.
&F1 will set all parameters to default, &F0 should have no effect, but I am not really sure.
E1 turns echo mode on, but it should be on by default.
Q0 turns result code presentation mode off, but it is by default.
+CMEE=2 reports mobile termination errors, again that’s the default behaviour.

[AT-COMMANDS]
The docs states this way to locate the correct port while setting up the first time, but I recommend to do this step each time when issues occur, because it is (for me) the fastest way to see if any communication with the modem itself is possible:
Enter CLI / command prompt and enter cu -l /dev/cuaU0.2 to call the device, where „0.2“ is your cellular device. If the output is „connected“ the communication to the hardware is fine so far.
Now enter AT which should say „OK“ and enter AT+CPIN. An „OK“ states that access to SIM is also working. If „AT“ says „all ports are busy“ (ICP stage failure) you can do as follows:
Exit cu with ~ or start a new CLI session, then enter
cd ..
cd ..
rm var/spool/lock/LCK..cuaU0.2
where „0.2“ is your device and reboot. Sometimes editing the interface worked form me without requiring a reboot.
If „AT+CPIN“ gives an error you should check SIM card an PIN.
To disable SIM PIN, after calling your device, you can use AT+CLCK="SC",0,"xxxx" where xxxx is the current PIN.

For further troubleshooting you can use the following AT commands:
Reset modem (I experienced best results using this command in interface config as init string (without „AT“)): AT!GRESET
Show operation status of the modem: AT!GSTATUS
Determine signal strength (2-9 marginal, 10-14 OK, 15-19 good, 20-30 excellent): AT+CSQ
Network registration status: ATt+CREG?
Show ISP: AT+COPS?
List supported transmission types in actual network: AT*CNTI=1
Show used transmission type: AT*CNTI=0

[READING THE LOGS]
Instead of starting troubleshooting in CLI, you can check the PtP logs in the GUI. I recommend to filter the output using the following keywords one by one:

„Ack_Sent“
followed by „->opened“ indicates that the internal hardware communication (ICP stage) is fine. If it shows an error / failure such as „The modem is not responding to „AT“ …“, see „all ports are busy (ICP stage)“ in AT-commands section further up.

„MTU“, „MRU“, „ACCM“
followed by corresponding parameters indicates that parameter negotiation (LCP stage) was successful. I never experienced errors in this stage. If there are no parameters shown, the signal quality may be poor or modem parameters are misconfigured. It may be worth a try to reset parameters to factory defaults using AT&F1.

„LCP“
followed by „authorization successful“ indicates that the registration with the APN using username and password was successful (Authentication stage). If it shows an error / failure check your config.

„IPCP“
followed by „state change Ack-Sent --> Opened“ and an assigned IP address indicates that the cellular connection is established and ready to use (IPCP stage). If it shows no IP or an error / failure, check your PtP connection, select your country, ISP and all required options and save. This happened to me most times after a reboot, sometimes it took a while until the connection was established, sometimes it failed even after some hours of waiting. If „edit and save“ don’t do the trick, try a reset (AT!GRESET) in CLI or add !GRESET as init string in the interface configuration. Note that the init string may dissapear after some time. There is no need to add it again as long as everything works as expected.
I also remember that I had some issues at this stage (?) when I tried to get IPv6 to work on the cellular interface, maybe you're good to go disabling IPv6 temporarily.

[EPILOGUE]
I hope this little guide can help someone troubleshooting problems with Sierra Wireless modems. Maybe you can do something with it when using other modems, although my recent experiences show that there can be significant deviations here.
Please excuse my partly bad English, I read a lot in English, but that doesn't mean I can express myself better in that language, but what are online translators for?!

Now while you guys solve your problems, I'll just sit back and watch my external modem at work... I'll have a drink to that, cheers!