Weird LTE-M PSM behaviour

RE: Weird LTE-M PSM behaviour

ante — Thu, 30 Apr 2020 10:30:17 GMT

Ok, so I did miss something. The active time was set to 48 min, if I am not mistaking, and I didn't do power measurements for that long. By decreasing the active time significantly I could verify that the nRF did actually enter PSM mode.

RE: Weird LTE-M PSM behaviour

ante — Mon, 27 Apr 2020 08:01:38 GMT

Hi again Luliana!

yes I have read that different operators have different minimum PSM periods, but I want to get a better understanding of what is happening since, to me, I think the nRF91 is acting unexpectedly. If I try to activate PSM with an operator that does not support it this result in %XMONITOR and +CEREG returning a periodic TAU and active time values "11100000", and the devices then continues communicating like before. This sounds reasonable to me. When I try a PSM that, I guess is too low, I would expect that the device either would enter PSM and return the negotiated PSM values with CEREG/XMONITOR or that it would not enter PSM and return "11100000" for both timers and continue communicating like before - instead I get the weird behavior I mention above.

To test the suggested values you sent me I switched to the ATClient sample application and used manual AT-commands. After executing:

AT+CPSMS=1,"","","00111000","01001000"

I read back the values:

AT+CEREG?
+CEREG: 5,5,"0027","018A0116",7,,,"01001000","00111001"
OK

AT%XMONITOR
%XMONITOR: 5,"Telia","Telia","24001","0027",7,20,"018A0116",211,6300,59,28,"","01001000","00111001"
OK

As far as I have read on the forum here this would suggest that PSM is now active. However, by measuring the current it very much looks like it is not the case. This is the current consumption from the time I send AT+CPSMS:

So is there a way to get more detailed information on what is happening between base station and modem? Maybe I am missing something, but as far as I see it, only looking at CEREG/XMONITOR is not a reliable way of checking if PSM is active/working.

Kind regards

/Ante

RE: Weird LTE-M PSM behaviour

iBasis Tech Support — Fri, 24 Apr 2020 15:02:37 GMT

Hi,

As you mentioned, this result means that PSM mode is not enabled. The recommendation will be to increase the timers because each MNO has different timers configured. Usually, PSM works with timer higher than 4 hours, so please use this values:

AT+CPSMS=1,"","","00111000","01001000"

Also, to verify PSM, execute these commands:

AT+CEREG=5

AT+CEREG?

%XMONITOR: 5,"Telia","Telia","24001","0027",7,20,"0188D616",,,,,"","00000000","11100000"

Kind regards,

Iuliana