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,

Can you share the link to the paper if it is publicly available? or send me a private message so that I can share it internally with our team.

As you know, our development team intend to keep the current state of the HPPLMN search design on the nRF9160 modem now. I can not promise you any change in the future, but it is better to have more discussion on this topic, so it will help both the user and developer to understand HPPLMN search and how to optimize it further.

I will return back for your question later.

Best regards,

Charlie 

[Achim Kraus]

"As you know, our development team intend to keep the current state of the HPPLMN search design on the nRF9160 modem now."

"so it will help both the user and developer to understand HPPLMN search and how to optimize it further."

One very easy first step would be:

- add an event for such HPPLMN searchs (see lte_lc_nw_reg_searching and lte_lc_modem_evt_search_done ), that will make it much more obvious, and the users will not longer asking indirect about the extra power consumption (which may be also caused by other topics.)

- report also the SIM data used by the modem, e.g. the interval and the highest PPLMN. (I added that in the meantime by reading the SIM card in my app).

Both will help the users to understand, if they are affected or not.

My experience so far:

iBASIS: 20h, no HPPLM

flolive: 10h, no HPPLM

1nce: 2h, HPPLMN 26201

I need to retest and verify my observed behavior, that will take some time, because I currently would like to focus on an other project (Eclipse/Californium), but I hope I can spend some time at least at the begin of next year.

What I observed:

- my nRF9160 still records such HPPLMN searchs, even if the HPPLMN is already chosen. That may be caused by the signal level, if that's considered.

- using "%XDATAPRFL" and "%REDMOB" both limits the HPPLMN search, in difference to your information. For me, this would be brilliant.

I mainly focus on "long term, static located, battery powered sensors". If I'm able to reduce the HPPLMN searchs, then that works for me. Please consider: using the right protocol (CoAP /DTLS 1.2 CID) really enable user to run such use-cases. My current results even with the Thingy:91 are very promising.

77-17:54:04 [d-hh:mm:ss], Thingy:91 v0.5.99, 0*1791, 1*74, 2*1, 3*0, failures 1
3990 mV 73% battery (low-power)
!Network: CAT-M1,roaming,Band 20,PLMN 26201,TAC 67B9,Cell 01CC2B03,RSRP -94 dBm
!PSM: TAU 86400 [s], Act 8 [s], Released: 10851 ms
Stat: tx 934kB, rx 126kB, max 535B, avg 289B, searchs 82, PSM delays 0

The Thingy:91 is now running for 77 days. It uses PSM and exchanged a encrypted message of about 400 bytes every hour. The battery reached 73%, the amount of data is about 1.1 mBytes.

If you add a "60s 30mA every 2h", then this doesn't work.  

In my experience, the MVNOs are not trying to control the costs by such HPPLM lists, they control that by subscriptions. There also MVNOs, where your subscription is a "general global" one, but even there, I don't see, that a HPPLMN search really helps. 

And the energy consideration depends then a lot from the assumed communication model. If the idea is to search and then use the HPPLMN for many many messages, then there maybe a win. But if you exchange two messages in 2h (e.g. each about 100mC in bad condition, maybe 20mC in perfect conditions), and then a HPPLM search takes about 60s (30mA => 1800 mC) then that search task will not pay off.

So, let's see, what the answer about the consideration of the signal level is.

Maybe a compromise would be, to offer a way to overwrite the SIM card's interval with values in a range from "2h to 240h". 

Anyway, adding the functions above, would make it transparent and with that I guess, you will receive much more useful user feedback. 

[Charlie]

Hi Achim,

Thanks for sharing your experience and observation. I understand your points and your advice makes sense.

I will forward this to our modem team to see if we can do something in the future.

Best regards,

Charlie

[Charlie]

Hi Achim,

Just get more comments from our modem developer, and we will have more internal discussions on this topic.

Modem event of HPPLMN searches could be possible, but the only thing application could do when the HPPLMN search begins is to deactivate modem. I don't see any other way. That would mean extra signalling to network (Detach and then a new Attach). Also, the same cell may not be even available anymore, which is why searching might be needed anyway. Of course, usually this is not the case with stationary devices. Anyway, deactivating modem when notification of started HPPLMN search is received is not always the optimal either. One cannot either predict how long the HPPLMN search takes, it could be just a few seconds.

I think the value of HPPLMN search timing is already readable via AT command. Might be good idea to add the command to some document, as it requires knowing specifically correct file IDs etc.

If UE is already camped on a cell of the highest priority PLMN in current country, there is no HPPLMN search regardless what is the signal strength of the current cell as long as it still fulfills cell suitability criteria (which is another topic). But if the HPPLMN search is initiated (i.e. the current PLMN is not the highest priority PLMN in current country), the signal strengths in HPPLMN search results are considered so that it needs to be over certain threshold for being suitable to be selected as higher priority PLMN.

%XDATAPRFL and %REDMOB do not limit the HPPLMN search. The latter one has nothing to do with it, but to be exact the data profile has minor affect: When UE has lost the cell or is in limited service (data transfer not possible) UE will search for suitable cell periodically (according to %PERIODICSEARCHCONF). If the HPPLMN timer expires between the periodic searches (while not searching), modem will not immediately trigger the HPPLMN search but waits until it is time for the next periodic search. So, in normal operation there is no affect.

Operators have often separate roaming agreements with other operators. Some agreements cost more than another. This one reason why the operators have prioritized the roaming networks in certain order - there may be other reasons too (alliances etc.). Anyway, by configuring the operator HPPLMN list and the HPPLMN search periods to SIM card, operator can control how often device must search for other PLMNs that are "better" from the operator's perspective.

Overwriting SIM card's HPPLMN search configuration is not possible. That is something that only the operator is allowed to do.

I understand the problem, this is not optimal. I'll still have some internal discussions whether there is anything we could do while still being 3GPP compliant.

Best regards,

Charlie

[Achim Kraus]

> Modem event of HPPLMN searches could be possible, but the only thing application could do when the HPPLMN search begins is to deactivate modem.

That's not my intention. My intention is to make this more visible to the users. Currently only users, which use a PPK (or something similar), are aware of that HPPLMN search. Once the event is available, user may be easier able to see, how much time (and so energy) that takes. If even the reason/trigger is added as parameter, then also irritations (as in my case, because my experience differs from the information here) may be reduced. I didn't want this event to switch off the modem. If someone plans to do that, then it will just be switched off, even before RRC Idle and so no HPPLMN search at all will happen.

> I think the value of HPPLMN search timing is already readable via AT command. Might be good idea to add the command to some document, as it requires knowing specifically correct file IDs etc.

Sure, someone can use the "AT+CRSM". I would prefer to have these values easier visible by a "high-level API". It's the same argument: if users know, how their SIM-card is configured, the behavior gets transparent and results in less surprisingly empty batteries.

> %XDATAPRFL and %REDMOB do not limit the HPPLMN search.

Once I have some more time, I will provide the traces. In my experience, the modem search for a HPPLMN even if that is already active, and using %XDATAPRFL (for now I only used it together with %REDMOB) increases the interval and so less HPPLMN searchs are used. Therefore I think, add the API the read the configuration values the modem is using, and report the event HPPLMN search including the reason.

> Overwriting SIM card's HPPLMN search configuration is not possible. That is something that only the operator is allowed to do.

Sorry, I think the developers stick to the wrong arguments! This HPPLMN search was introduced long ago for Smartphones by google. That kind of devices have a very different "communication profile". There the trade off between the cost for a search and the later costs, when transfer pictures and websites is completely different. I'm pretty sure, just no-one there did reconsider the effects of such a HPPLMN search for IoT devices. At least not for really rare communicating devices.

> 3GPP compliant.

Then explicit state, that overwriting the SIM card configuration is not 3 GPP compliant and violating it may result in being blocked by the M(V)NO. In the end, it's anyway the (M(V)NO, which need to decide what should happen. The users will just then go for the SIM cards, which offers, what the users need, once it gets transparent.

[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.