HPPLMN search - Please consolidate advice.

[Achim Kraus]

Since being aware of the power consumption of the "HPPLMN searchs", I'm struggling to see the right strategy.

What makes me really wondering, is that the first questions are raising already two years ago:

nrf9160 psm and hplmn search on roaming sim cards

unfortunately, the answer is missing there.

3 months ago in lte_lc_nw_reg_searching and lte_lc_modem_evt_search_done the recommendation was to lock the PLMN.

2 weeks ago in measured scanning procedure a few minutes after connection establishment the recommendation was "use only home network".

Using "only home network", really? Isn't the downside of that, that SIM cards of MVNOs are always roaming?

Therefore I would really appreciate, if Nordic spend some more time in investigate, what users should do.

The first question 2 years ago, mentions, that other modems offers to disable the HPPLMN search. Maybe that's considered.

[Charlie]

Hi Achim,

I have got it from my colleague, I will give you a reply later.

Best regards,

Charlie

[Charlie]

Hi Achim,

Comments from our modem developer:

1. I already commented the visibility of HPPLMN search. For debugging reasons it might be good idea to be notified by modem, but not that much for the product design (extra signalling etc.).

2. I still stick in the fact that %REDMOD has nothing to do with HPPLMN search (it affects to totally different protocol than EMM which controls HPPLMN search). And %XDATAPRFL affects only when modem does periodic searches (when not camped on cell or camped on limited service cell) - does not affect while modem is in normal service. No need for modem traces to confirm this.

3. I can assure that modem does not search for PLMN which it is already camped on. If there is a search, it is for some other PLMN. I suspect there is a misunderstanding what is actually the highest priority PLMN at that point.

4.  I'm pretty sure, just no-one there did reconsider the effects of such a HPPLMN search for IoT devices.

Yes, the HPPLMN search was originally designed for smartphones, but please note that the 3GPP 23.122 chapter "Automatic and manual network selection modes" specifies the search periods and has taken IoT devices into account: for smartphones the unit is 6 minutes whereas for IoT devices the unit is 2 hours. The actual HPPLMN search period is the unit x value T. Therefore, the minimum search period for smartphones is 6 minutes and for IoT devices 2 hours. If we followed the smartphone part, the search period would be 6 minutes instead of the current 2 hours. So 3GPP has considered IoT devices in this, but I agree that not enough.

5. 3GPP compliance & HPPLMN configuration at SIM card. As I have said, HPPLMN configuration cannot be modified by anyone else than the operator. 3GPP 31.102 chapter "EFHPPLMN (Higher Priority search period)" specifies that admin PIN code is needed for updating the file. There is no question about it.

We have a clear understanding what is the problem. I don't think there is more than I can do for this. As it has already been mentioned, changing this would violate 3GPP. Nordic has to consider different aspects such as acceptance tests, conformance tests, operator relations etc. Therefore, continuing the discussion with a modem developer does not seem beneficial at this point. 

I would still like to remind that HPPLMN search would be avoided with manual network selection mode. This would work like this:

1. Activate modem in automatic network selection mode
2. Wait for modem to Attach
3. Change the network selection mode to manual (the PLMN modem just Attached).

Then if modem notifies that cell has been lost or UE is in limited service (CEREG status is not roaming or home).

1. Change the network selection mode back to automatic
2. Wait for modem to find service again
3. Change the network selection mode to manual (the PLMN modem just found).

Hi Achim, at this time point, I have included both our modem developers and PMT team in this discussion. All the questions have been answered and a couple of times some of them. I do not think we will have any change to the current HPPLM search method now. I will let you know if we have any improvements in the future.

Best regards,

Charlie

[Achim Kraus]

Hi Charlie,

thanks a lot to all, who have answered.

Beside of the general question, I have already a specific question about a specific case.

> I suspect there is a misunderstanding what is actually the highest priority PLMN at that point.

Agreed, yes, therefore an event with the reason helps to understand, which PLMN is considered to be the HPPLMN. Not that in some cases the search is caused not by that misunderstanding.

So, please check the trace in hpplmn search - reason unknown .

[Achim Kraus]

Thanks, I read the answer on the other ticket.

[Achim Kraus]

With mfw 1.3.5, the modem will not longer search for the 90140.

See mfw_nrf9160_1.3.5_release_notes.txt:

- Fix for higher priority PLMN search. Global PLMN IDs (start with digit 9) are no more
  automatically considered to be suitable for higher priority PLMN search and selection.