How are operators added to the FPLMN list?

Modem: SARA R410M

The only two hologram operators available in my area are Verizon and AT&T. Verizon registration is instantly denied and added to the FPLMN list. I would assume this is because of the reasons listed here:

That has left AT&T as the only option. Recently, under weak signal conditions, AT&T has been added to the FPLMN list. Why?

I am starting to lose sight of the advantage of hologram.

Any info on this?

In your article about clearing the FPLMN you recommend not clearing it within a program, but I don’t see how to avoid this if the only network available is added to the list when signal is poor?

weak signal conditions should not lead to FPLMN, have you changed how the module signals? If it is over signalling then that could be a source for FPLMN.

As that issue points out there isn’t much we can do regarding Verizon connectivity with the module since certification is required. Is there anything you can do to boost the signal for your device? There isn’t anything we can really do to increase signal strength but yeah a weak signal should not lead to FPLMN

What would you consider over-signaling? Not familiar with term. I am opening a tcp socket and posting data ever 10 mins. Sometimes it takes several tries to open the socket if signal is weak.

over signalling would be if you kept asking the tower to attach for a data session while its in the middle of working out the handshake with the module. We’ve had this happen before where a customer was so adverse to losing a connection that any time the module would lose a connection they would hammer the tower with connection requests until it connected instead of of waiting for the cellular state machine to resolve itself. It doesn’t sound like that is the case with you.

Since you are using CAT-M my suggestion if you are having issues is you might want to make the timeout for your TCP requests quite long, like multiple minutes long… I think our timeout for CAT-M devices is 90 seconds but we might have made it even longer than that

Also feel free to reach out to regarding this and they can look more into the sim and see if there is something weird happening :slight_smile:

As far as I know, this modem will attach itself. I do check registration with CEREG? before attempting to open socket.

Yes this is CAT-M. I am using a 120 second timeout on the USOCO socket connect. Sometimes it replies with CME Error several times and I have to try again.

I have seen this on two different sims.

I would reach out to success like I mentioned then. This sounds more like something on the network side rather than the module. They should be able to look into that for you as from what you have described sounds like normal operations from the device in terms of signalling.

I ended up adding some code to clear out the FPLMN list if registration is denied on AT&T. This happens several times a day.

I have ordered a AT&T CAT-M sim. It will be interesting to see if it has similar behavior.

I believe I was wrong on this. The network that was being added to the forbidden list was not AT&T. It was another network that I have not been able to identify. When signal is very weak, the modem cannot find AT&T. When this happens, sometimes the modem replies “denied” instead of “searching” to AT+CEREG? The “denied” is in reference to the unknown network in the forbidden list, not AT&T.