Long disconnects with empty PDP on nrf9160

[nicolae.georgescu]

Hello,

Our solution uses nrf9160 in smart parking devices that the user controls from a smartphone app, through the internet. Since commands come from the user to the device, we need fast response and very high availability.

We are closely monitoring the connection to be active. I.e. a confirmed ticket opened couple of months ago to confirm with you we are doing the best we can Modem connection state detection 

We still see some disconnects of >2 hours even though the device retries (from soft retry to reboot) every ~5 minutes.

We asked 1NCE to investigate on their side, and the conclusions were below (plus pic attached)

• The device intermittently worked on both Vodafone and Orange networks.
• At times, the device became stuck with all PDP contexts empty on both networks, meaning no data was sent or received.

Since the issue occurred simultaneously on both networks, it is unlikely to be a network problem. It appears to be related to the device’s logic, as it intermittently functions but sometimes gets stuck with empty PDP contexts.

Questions:
Q1: What do you make of the explanation from 1NCE? Is there anything we can do from app level related to PDP context? We are not directly using/configuring it in any way, we use high level initializers.

Q2: 1NCE suggested we could blacklist an operator at their level, so the device only connects to one operator. But they told us to make sure if we ever want to switch, we need to reset the saved context that can get stored on the SIM.
How can we do that?

Thanks,
Geo

 

pic from 1NCE

[Achim Kraus]

nicolae.georgescu said:

1NCE suggested we could blacklist an operator at their level,

There is also a blacklist (forbidden PLMN list) on the SIM. The nasty point will be, if you have two operators and you blacklist one, then an outage of the other (not blacklisted) will disconnect device. 

nicolae.georgescu said:

We still see some disconnects of >2 hours even though the device retries (from soft retry to reboot) every ~5 minutes.

Not sure, did you also try a modem factory reset before reboot? (Will only help, if your application always initialize the modem proper in restart.)

[nicolae.georgescu]

Hi Achim,

Indeed there is a blacklist on the SIM. The question 2 is about how to reset the saved blacklisted operator on the sim.

By reboot i mean from the code, we do a division by zero so it reboots. Modem factory reset doesn't sound like something the device could do on it's own and we cannot do something like this as a manual maintenance from time to time every few months.

Thanks

[Achim Kraus]

nicolae.georgescu said:

Indeed there is a blacklist on the SIM. The question 2 is about how to reset the saved blacklisted operator on the sim.

There are AT cmds ("AT+CRSM") to read and write such list from and to the SIM by the app. It requires AFAIK some know-how about the SIM card (e.g. 3GPP TS 31.102, I've implemented a set of functions for my client some time ago).

Some SIM card even delete the blacklist in a power-off/on cycle, there you need to frequently rewrite that list. If I remember well, the 1NCE SIM does so, so you mayl need to write it again after each modem restart.

nicolae.georgescu said:

By reboot i mean from the code, we do a division by zero so it reboots.

Strange solution. There is a "sys_reboot" function (may require CONFIG_REBOOT).

nicolae.georgescu said:

Modem factory reset doesn't sound like something the device could do on it's own

There are functions for that, first call "lte_lc_power_off" and then "lte_lc_factory_reset".

Anyway, that is just an idea, it may or may not help. In my case it helps to bring back some "blocked devices", but that's very rare (only few device, less than once every 150 days). More often (in my experience more cause by other MVNOs, not 1NCE) a "lte_lc_power_off" and "lte_lc_power_on" again helps to get over some sticky network rejection codes.

[Achim Kraus]

I retested the "forbidden PLMN list" with a 1NCE SIM. That clears the list on power off (at+cfun=0). So you would need to rewrite it on power on.

[nicolae.georgescu]

Thanks a lot! The blacklising of an operator would be set by 1NCE, the SIM only does a local save that can get in the way if in the future we remove the blacklisting from 1NCE side and the sim keeps the cache. But from your test it seems a restart is enough and the sim removes the cache, so that is definitely easy to do!