Question regarding connection and Fallback

[Giannis Anastasopoulos]

Dear All,

In my application I need to switch on and off the modem using AT+CFUN=0 and AT+CFUN=4 respectivelly.

My question is the following. Let's assume that the device cannot find the preferred type of network and uses the fallback one. Then the modem is switched off. What will happen once the modem is back on again?

Will it try first to connect to the preffered network and then try the fallback, or will it try to connect to the previously working network first?

Thank you very much for your response

[Martin Lesund]

Hi Giannis,

Please make sure that you are using the correct functional modes in your question for the +CFUN command.

0 – Power off                  [Sets the device to minimum functionality. Disables both transmit and receive RF circuits and deactivates LTE and GNSS services.]
1 – Normal mode            [Sets the device to full functionality.]
4 – Offline mode             [Sets the device to "airplane mode". Disables both transmit and receive RF circuits and deactivates LTE and GNSS services.]
20 – Deactivate LTE       [Deactivates LTE without shutting down GNSS services]
21 – Activate LTE           [Activates LTE without GNSS]
30 – Deactivate GNSS    [Deactivates GNSS without shutting down LTE services]
31 – Activate GNSS        [Activates GNSS without LTE]
44 – Offline mode without shutting down UICC [Sets the device to airplane mode without shutting down UICC]

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I assume that you mean AT+CFUN=1 and AT+CFUN=4 in your question:
I am pretty sure it will choose the last known working network e.g. the fallback in your case and try to connect to that one once you turn ON the modem again.
but let me double-check that functionality internally and come back to you.

Best regards,
Martin L.

[Giannis Anastasopoulos]

Dear Martin,

Yes, you are right. I meant AT+CFUN=1.

Please let me know of the results of your check

[Martin Lesund]

Hi Giannis,

I just got it confirmed internally.

The last registered network is always the highest priority when you turn ON the modem.

Note the word "registered", i.e. we have obtained full service (IP connectivity and so on).

Best regards,
Martin L.

[Giannis Anastasopoulos]

Thank you for your response. I wonder this though:

If you enable the modem and there is no network to connect to, how will the device try the fallback?
To be clear here:

Initially, one can call the lte_lc_init_and_connect(), that will try initially to connect to the preferred network and if that is not available, it will try the fallback. Then one can call the AT+CFUN=4 to go offline. And then one can call AT+CFUN=1.

At this point the modem can either connect to the last network it connected succesfully and this is very clear based on your comments. But what will happen in the case that the previous succesfully-connected-to network is no longer available (e.g. moved to a different region) while in the offline mode? Will the modem execute again the lte_lc_init_and_connect() or something similar?

So basically what I need to know is ho the modem behaves when it cannot find the previously succesfully-connected-to network.

What I know for sure is that if I call the lte_lc_init_and_connect(), every time after CFUN=1 the modem first tries the preferred network and then the fallback, so if I was in an area that only the fallback is available the modem would scan first until the time out for the preferred and then try the fallback.

Thank you very much for your assistance.

[Martin Lesund]

Hi Giannis,

The modem takes care of this automatically.

e.g. Regarding the cell search done after cell lost (i.e. nrf91 moves out of range or does not find cell after startup),

• The modem will periodically try to find a cell after a cell is lost.
• No actions required from the application (i.e. no need for new AT+CFUN=1)
• If there is a cell available, it will eventually be found, but depending on the current search periods it may take some time.
• The modem does not just continuously search until the cell is found, it takes breaks in between searches.

Also, the search itself takes the benefit of a large number of parameters gathered earlier in order to find a cell efficiently.

• The current Data Profile affects the search periods (set with AT%XDATAPRFL): 4 (= High performance) means searching more often (and aggressively) than 0 (= Ultra low power).
• In this aspect, the Data profile is not about connection reliability vs power consumption, it is about how fast the device finds a cell (after cell lost) vs power consumption. So when on cell, Data profile does not affect to connection reliability.

Note: The modem indicates the state of service using AT+CEREG. Unsolicited indications can be subscribed to or the current status can be read with "AT+CEREG?".

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A sidenote, please notice that you do not need to manually send AT+CFUN=1 or 4 when using the LTE Link Controller library.
If you look into the function lte_lc_init_and_connect() it will send the AT command inside the function. (normal = at+cfun=1)

Best regards,

Martin L.