LTE Link connecting is blocking?

[ExAg]

Hi

In the mqtt_simple example there is a line "modem_configure();" which establishes the LTE Link connection. If connecting the LTE link is not possible (for example no signal), this function never returns.

1. Is this the intention of this function or is there a possibility to configure a timeout?

2. Is it normal, that this function needs several minutes to establish the LTE link, if a provider is present?

regards

ExAg

[Simon]

1. Is this the intention of this function or is there a possibility to configure a timeout?

The reason it is blocking is due to some semaphores inside w_lte_lc_init_and_connect(..) in lte_lc.c. It will wait for the at_handler() to run and if the response is appropriate it will give up the semaphore and the program continues. See code below

//From w_lte_lc_init_and_connect() in lte_lc.c

k_sem_init(&link, 0, 1);
        //k_sem_init(&link, 1, 2);

	at_cmd_set_notification_handler(at_handler);

	if (at_cmd_write(normal, NULL, 0, NULL) != 0) {
		return -EIO;
	}

	k_sem_take(&link, K_FOREVER);

You can set a timeout, by running k_sem_take(&link, timeout_in_ms); where you specify the timeout in timeout_in_ms. 

Be aware that this may cause some unwanted results/race conditions etc., but I am not sure specifically what may happen.

 

2. Is it normal, that this function needs several minutes to establish the LTE link, if a provider is present?

 I will look into this and come back to you.

Best regards,

Simon

[Simon]

ExAg said:

2. Is it normal, that this function needs several minutes to establish the LTE link, if a provider is present?

The time to establish an LTE link varies depending on the signal strength and the network, it may more than 5 minutes at worst. If the network settings are stored on the modem, the connection only takes some few seconds. The network settings are automatically stored to the modem when the modem is turned off, through the call to AT+CFUN=0.

Best regards,

Simon

[ExAg]

Hi Yusuke

Thank you for this tip. Unfortunatly for me, this is not really a solution because i do not know, which bands are aviable in all the locations where we want to deliver our devices.

But probably during development, this could speed up the connection a little bit...

[ExAg]

Hi Simon

Here are my latest observations according the modem connection time:
I have updated the modem firmware to 1.0.1.
It seems, that the connection time is much faster, if the eval-kit is runnig without debugger. After pushing reset, it takes about 5s to (re)connect the LTE link.
But if I use the debugger, it takes this 1:30 to 6 minutes to connect...

Are there any settings or configurations I have to set up for faster debugging?
(The CONFIG_DEBUG_OPTIMIZATIONS=y is allready set in prj.conf).

Regards
ExAg

[Simon]

Just to make things clear. You are talking about the time between here and here? 

I am not able to reprocuce your behaviour. What NCS tag are you using? Are you using the mqtt_simple example as it is or have you done any changes? 

Best regards,

Simon

[ExAg]

Hi Simon

No, I am not debugging on this low level of libraries (lte_lc.c). I see this behaviour on the line "modem_configure();" in main.c. (mqtt_simple or asset_tracker example does not matter).
If I single step over this line (or set a break point after this line), I see the text "LTE Link Connecting..." in the terminal. From now it takes 1:30 up to 6 minutes, until the text "LTE Link Connected!" appears in the terminal window and I am able to continue with single stepping.

I am using ncs_tag v1.0.0

No, I did not change the code. Its the original code provided by Nordic.

Regards
ExAg

[ExAg]

Hi Simon

I just did some additional testing. I reloaded the mqtt_simple project (this one with TLS from GIT), clean and rebuild the project and downloaded the HEX onto the eval-kit.
My tests are without TLS enabled and without debugger (but it seems to have the same behaviour with the debugger now).
I used the reset button on the eval-kit to restart the application and measured the time between "LTE Link Connecting..." and "LTE Link Connected!".

I have the impression, that it is much better than it was; probably because of the new modem firmware 1.0.1.

Here are the results of 30 resets:

<5s:   25 times
20s:     1 time
1min:   1 time
>7min: 2 times
>9min: 1 time

best regards
ExAg

[Simon]

I am sorry for the delay, I have been sick lately, which is why I haven't been able to answer you. I will look into your issue the next couple of days.

Best regards,

Simon

[Simon]

What is the signal strength when the connection establishment takes long time (7-9 minutes)? Check out this thread for more information about this and how to improve the performance.

Best regards,

Simon

[ExAg]

Hi Simon

I will check this thread.
I don't know the signal strength is, but it should always be the same because the dev-kit was on my table on the same position during all these tests.

Regards
Michael

[ExAg]

Hi Simon

The signals strength is between -86dBm and -83dBm on my desk.

Regards
Michael

[Simon]

That should be sufficient.

However, a reason for this might be that the network detects that the same device tries to connects many times in a row in a given period of time. This may cause the network to block the device for a short period, to avoid it from flooding the base station.

I will try to get some feedback internally from people that knows more about this, and get it confirmed whether this is the reason, or if there is another reason.

You could also try to do some testing after calling AT+CFUN=0 followed by a modem reset. As mentioned earlier, this will store the network settings, and the connection time may decrease. Read more about it in this ticket.

Best regards,

Simon