NRF9160 - LTE-M takes a long time for first connection

[Eneko]

Hi! We are currently testing our custom device using the nrf9160 modem. Our use case is for an ultralow power IoT device that connects to the Internet each 30 minutes to send some data and then goes back to sleep. We don’t use PSM, when the device goes back to sleep, we disconnect from the cloud and set the modem to flight mode. The device will be in a place with no coverage (a maritime container) for days or weeks, in which the device will be in “hibernation mode”, trying to connect to LTE-M each 4 hours.

We have seen that after the first connection, the device connects to the LTE-M network in less than 5 seconds but before that, the first connection takes a long time, around two minutes.

We are using the command AT+CGDCONT before every connection to set the APN hoping that it will decrease the time it takes for that first connection, but it doesn’t make a lot of difference. The device travels around the world, so we cannot lock the LTE-M bands nor the country.

Why does the first connection take a long time and the rest doesn’t? What are the best practices to reduce the time of that first connection? Are there any other AT commands that we can use to reduce that time?

Thank you and best regards

[Didrik Rokhaug]

Hi,

Yes, the initial connection with a new SIM card/in a new location might take a long time.

One thing you can try, is to use the %XCOUNTRYDATA AT command to give the modem a bit more information about what bands it should search first in the given country.

However, much of the cell and network selection procedure is controlled by the LTE spec., so there isn't much that can be done.

Why does the first connection take a long time and the rest doesn’t?

The modem will store information about the connected network (in flash if you disconenct with AT+CFUN=0), and use this information when re-connecting to the network.

That way, it knows what network to search for, and where to find it, and it doesn't have to try other networks first.

Best regards,

Didrik

[Eneko]

Hi! Thank you for your quick reply. Unfortunately, the devices travel all around the world so we cannot know in which country the device is in when it’s trying to connect. 

Is there any other thing we can do to help the modem connect that first time?

Best regards

[Eneko]

Hi Dridik,

Thank you so much for the explanation. 

I updated the modem firmware to v1.3.5 as the modem won't search for PLMN started with 901.

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.

I'm using an iBasis SIM for comparation. For emulate the first connection I'm switching both SIMs to see the time. The iBasis SIM takes less than 20s to make the first connection.

But with 1nce SIM don't see any improvement. It takes the same time (around 1:20 min) to connect using automatic network selection (+COPS=0).

The second connection (resetting the MCU) takes almost no time with both SIM cards.

If I change to 1nce SIM after connecting using iBasis it takes again 1:20 mins even if it's connecting to the same PLMN used by iBasis SIM.

Could this behaviour mean the modem is still trying to connect to the Global home network (901)?

Thank you for your help.

Best regards.

[Eneko]

Hi Didrik,

Have you been able to look at this?

Best regards!

[Didrik Rokhaug]

Sorry for the late response.

There might be other reasons for why the 1once SIM takes longer to connect to a network than just the time it takes to search for it.

Could you take a modem trace, so we can look at what happens on the modem and SIM side?

[Eneko]

Hi Didrik,

Attached the modem trace:

 1nce_modem_trace.bin

Best regards.

[Didrik Rokhaug]

Thanks.

I have forwarded the trace to our modem team, so they can take a look at it.

They typically answer quite quickly, so I should have an answer within a day or two.

[Eneko]

Hi Didrik, any feedback from the modem team?

Best regards.

[Didrik Rokhaug]

Yes, and sorry for this taking a bit longer than expected. There were some additional details I had to check.

First of all, the modem team pointed out that, in general, swapping the SIM cards does not simulate scenario where a new SIM card is used for the first time. Typically SIM card contains the last registered PLMN in EF_EPSLOCI file, so when such card is inserted to whatever device, the last registered PLMN will be searched first. In addition, factory reset would be needed for clearing the cell history modem has stored earlier.

As for why the 1nce SIM card takes longer to connect to a network, this is what the modem team said:

1nce log show that the SIM card does not contain EF_EPSLOCI file, thus the home network (HPLMN) must be searched first. The HPLMN was not found and the only preferred PLMN configured to the SIM card was not either found (262 01). The only option left for modem is to try to Attach to some random network with high quality signal (3 of them were found). At first modem tries to Attach to 214 01 and luckily the Attach succeeds immediately.

They also commented on the use of %XCOUNTRYDATA:

In general, for devices that travel a lot we recommend to use the %XCOUNTRYDATA which can be used for prioritizing certain bands and channels in certain countries. This is very useful if preferred networks for many different countries have been configured to the SIM card. So for example, 1nce SIM card had 262 01 configured as preferred network. If certain band had been configured with %XCOUNTRYDATA for MCC 262, that band would have been scanned first when in country with MCC 262 resulting typically much faster Attach. Unfortunately, 1nce card didn’t have more than one preferred network configured, so benefit of using %XCOUNTRYDATA would limited to country with MCC 262 only.

Eneko said:

I updated the modem firmware to v1.3.5 as the modem won't search for PLMN started with 901.

Unfortunately, the fix introduced in mfw 1.3.5 doesn't impact the initial network search. However, it does mean that the modem will not search for the home network unless it is connected to a different global PLMN.

[Eneko]

Hi Didrik,

Thank you so much for the explanation. Now I understand better what is going on.

As you said, it seems the %XCOUNTRYDATA won't help.

Didrik Rokhaug said:

1nce log show that the SIM card does not contain EF_EPSLOCI file, thus the home network (HPLMN) must be searched first. The HPLMN was not found and the only preferred PLMN configured to the SIM card was not either found (262 01). The only option left for modem is to try to Attach to some random network with high quality signal (3 of them were found). At first modem tries to Attach to 214 01 and luckily the Attach succeeds immediately.

Although the SIM does not have EF_EPSLOCI file, is there any data about the last PLMN saved in the modem? It seems so, since after initial registration the modem takes less than 10s to connect.

Didrik Rokhaug said:

Unfortunately, the fix introduced in mfw 1.3.5 doesn't impact the initial network search. However, it does mean that the modem will not search for the home network unless it is connected to a different global PLMN.

What does initial network search means in this case? Using a new, unused SIM card?

I can reduce the time (locking bands) the modem takes to register with a new SIM card (then unlock bands). This way the modem have data about a PLMN -correct me if I'm wrong-.

What happens if the modem can't register to the last PLMN? eg: different country (MCC change) or area (same MCC but MNC/operator not available in that area).

As far as I understood, it will start searching the HPLMN (global) and then the Prefered PLMN (26201). After that try to register in nearby networks. Same procedure as the initial network search. Am I right?

Thank you for your help.

Best regards.

[Didrik Rokhaug]

Eneko said:

is there any data about the last PLMN saved in the modem?

Yes, the modem will save information about the connected network. First in RAM, but it will eventually be stored in flash as well. The application can also trigger the modem to store the information in flash with AT+CFUN=0 (note that this will turn the modem off, not just write the information to flash).

Eneko said:

What does initial network search means in this case? Using a new, unused SIM card?

By initial network search, I meant when the modem is searching for a network when it is not already connected to a network.

Eneko said:

I can reduce the time (locking bands) the modem takes to register with a new SIM card (then unlock bands).

Correct.

Eneko said:

As far as I understood, it will start searching the HPLMN (global) and then the Prefered PLMN (26201). After that try to register in nearby networks. Same procedure as the initial network search. Am I right?

Yes. Though the modem will only search for networks potentially deployed in the country it is in. As the home PLMN is global, it can potentially be found anywhere, so the modem will always search for it. However, it will only search for 26201 if it is in Germany.

[fernan]

Hi everyone,

At this moment I have the same problem, but in the past, I had not.

About one year ago, the connection spent about 3 seconds. Now it is very difficult to connect and register. The time to connect is too high.

Eneko, have you solve it?

I am doing test in Spain too, with Orange and Emnify SIMs, but with the same result.