nrf9160 LTE modem documentation

[Luis Alberto]

Hi,

Im working with serial LTE modem sampleApp which I have manage to compile, connect to my server and validate that it works using nrfConnect LTE Link Monitor to send/receive data over TCP IP socket (Commit from 17 Dec 2019 - a955d411)

Currently I'm implementing my own modem interface but as I do not have enough background on the LTE-M specification, it is being a bit difficult to validate they state of my device if connected or not... right now I'm using the output log from LTE Link monitor and the document  "nRF91 AT Commands Command Reference Guide v1.1 " as reference.

Does anybody have any other resource to use the AT Client? (documentation on the machine state to track connection/disconnections?), is it needed to perform any actions to ensure proper low power modes?

I also have notice that LTE link monitor states that the device is connected to the network once CGAT indicates that the ContextId is "active"  AT+CGACT?+CGACT: 0,1OK, (LTE Link monitor tooltip says that), however in the AT command reference guide the same command response is parsed as context Id 1 is inactive, which does not seems to makes sense as only the context ID 0 has been used during the session... I am guessing that this is a typo in the document, or am I missing something? Attached the log.

Fullscreen LTE Link monitor Log.txt Download

AT+CFUN?+CFUN: 0OK
AT+CFUN=1OK
AT+CFUN?+CFUN: 1OK
AT+CGSN=1+CGSN: "352656100381139"OK
AT+CGMINordic Semiconductor ASAOK
AT+CGMMnRF9160-SICAOK
AT+CGMRmfw_nrf9160_1.1.0OK
AT+CEMODE?+CEMODE: 2OK
AT%XCBAND=?%XCBAND: (1,2,3,4,5,8,12,13,14,17,18,19,20,25,26,28,66)OK
AT+CMEE?+CMEE: 0OK
AT+CMEE=1OK
AT+CNEC?+CNEC: 0OK
AT+CNEC=24OK
AT+CGEREP?+CGEREP: 0,0OK
AT+CGDCONT?+CGDCONT: 0,"IP","","",0,0OK
AT+CGACT?+CGACT: 0,0OK
AT+CGEREP=1OK
AT+CIND=1,1,1OK
AT+CEREG=2OK
AT+CEREG?+CEREG: 2,2,"370A","006A0503",7OK
AT%CESQ=1OK
AT+CESQ+CESQ: 99,99,255,255,19,54OK
AT%XSIM=1OK
AT%XSIM?%XSIM: 1OK
AT+CPIN?+CPIN: READYOK
AT+CPINR="SIM PIN"+CPINR: "SIM PIN",3OK
AT+CIMI204080813516888OK%CESQ: 54,2,24,3%CESQ: 54,2,18,2%CESQ: 54,2,13,1+CGEV: ME PDN ACT 0,0+CNEC_ESM: 50,0+CEREG: 5,"370A","006A0503",7
AT+CGDCONT?+CIND: "service",1+CIND: "roam",1+CGDCONT: 0,"IP","ibasis.iot","10.160.2.128",0,0OK
AT+COPS=3,2OK
AT+CGACT?+CGACT: 0,1OK
AT+COPS?+COPS: 0,2,"334020",7OK
AT%XCBAND%XCBAND: 4OK
AT+CGDCONT?+CGDCONT: 0,"IP","ibasis.iot","10.160.2.128",0,0OK
AT+CGACT?+CGACT: 0,1OK
%CESQ: 54,2,19,2%CESQ: 54,2,14,2%CESQ: 54,2,20,2

[Håkon Alseth]

Hi,

 

AT+CEREG? gives you the network status:

https://infocenter.nordicsemi.com/topic/ref_at_commands/REF/at_commands/nw_service/cereg_read.html?cp=2_1_6_9_1

The <stat> field will be either 1 or 5 when you're connected, depending on what type of SIM card you have. Since you're using iBasis, it returns roaming (5)

AT+CESQ will give you the signal quality and strength, as described here: https://infocenter.nordicsemi.com/topic/ref_at_commands/REF/at_commands/mob_termination_ctrl_status/cesq_set.html?cp=2_1_3_4_0

For the RSRP, you normally want 40 or higher (ie: -100 dBm or better), which you seem to get.

For the connection over IP, the socket operations will return an errno/err if the connection over LTE has gone down.

 

Kind regards,

Håkon

[Hadrien Kohl]

What errno/err should one expect to see if the modem connection went down? And what would be the appropriate recovery actions to take?

Alternatively, is there a way to query the modem for it's state (connected/disconnected) before using it?

[Håkon Alseth]

Hadrien Kohl said:

What errno/err should one expect to see if the modem connection went down? And what would be the appropriate recovery actions to take?

The errno depends on the socket call, please see the independent manual for each socket call (like "man setsockopt"). A common one is ENETDOWN if the network is down, and ENOTCONN if the socket was closed. I would still recommend that you parse the CEREG status to see the LTE network status. 

Hadrien Kohl said:

Alternatively, is there a way to query the modem for it's state (connected/disconnected) before using it?

AT+CEREG shall give you the network status, and you can check the signal strength RSRP with AT+CESQ (or subscribe to it via AT%CESQ)

[Hadrien Kohl]

I see. What should one do when getting ENETDOWN?

[Håkon Alseth]

Check the network connection status (using AT+CEREG), as per the former comment. In that case, it can be that you're out of range. You can for instance try to connect via NB1 (if former mode was M1). It all depends on your wanted application behavior.

Kind regards,

Håkon