Getting +CEREG: 5,"FFFE","FFFFFFFF" while disconnected from the network

[Lukáš]

Hi,

In one of our private issues where we asked how to detect a lost cell, a Nordic engineer told us that: 

You can detect if the cell is lost from +CEREG but would not be able to detect it if this actually is about cell reselection. If +CEREG where <stat> is something else than 2 or 5 then it means the device is not in service at that moment.

However, we noticed that when we lose the signal/cell, then we receive CEREG notification with stat 5 (registered, roaming). Particularly:

+CEREG: 5,"FFFE","FFFFFFFF",9,,,"11100000","00101000"

So what is the correct procedure to detect when the modem is not attached to the network and it is not connected to any cell? Sometimes we also receive this CEREG: 4 with FFFE, so it is confusing.

 

[ovrebekk]

Hi 

How do you know that the signal to the cell tower is lost?

How long after you lose the connection do you get the +CEREG notification?

An alternative way to detect if you are connected or not is to use the +CESQ command, as documented here. 

If you are unable to figure out the issue it would be helpful to get a modem trace and the serial log output when the issue occurs, so we can investigate more closely what is going on. 

Best regards
Torbjørn

[Lukáš]

Hi Torbjørn,

ovrebekk said:

How do you know that the signal to the cell tower is lost?

Well, the +CEREG: 5,"FFFE","FFFFFFFF" doesn't seem like a valid cell. Besides that, we also receive this:

[00:00:36.594,451] modem_at_handler: Processing notification: +CSCON: 0
[00:00:37.215,637] modem_at_handler: Processing notification: %CESQ: 255,0,255,0
[00:00:37.216,766] modem_at_handler: Processing notification: +CEREG: 5,"FFFE","FFFFFFFF",9,,,"11100000","00101000"

So I am almost sure, that we have lost the cell. However, I don't get why we are still on CEREG 5, which is also contrary to what your colleague said in another ticket.

[ovrebekk]

Hi Lukáš

Apparently the current version of the modem is not behaving exactly like Øyvind stated in the earlier case. For the time being I would recommend looking at the cell ID instead to verify whether or not you are connected to a cell tower. 

We are planning to change this in the 1.3 version of the modem firmware, so that the +CEREG command would return state 4 instead of 5 when the cell is lost. 

Best regards
Torbjørn

[Lukáš]

Hi Torbjørn,

You overtook me with the answer :) Today morning, we had a call with your colleagues and we have clarified this issue. The resolution is, that right now we should relly on cell ID, but it will be fixed in the next mode firmware release.

Best regards,

Lukas

[ovrebekk]

Hi Lukas

Sounds good, I will close the case then

Best regards
Torbjørn