Modem open after ended transmission

[thireo]

Dear Devzone

I have an NB-IoT application running on a Thingy:91, that wakes up every two minutes and sends a small package over TCP to a remote server.  
I'm using a Keithley DMM6500 to measure the current consumption, and from that it is clearly visible that the modem doesn't not power down after sending the data. The current consumption is high (~30+mA) for at least 20 seconds after transmission has been completed. This of course ruins the average current consumption.
I have tried using different Power Saving Modes (PSM) with an active time as low as possible (a couple of seconds). The PSM mode is accepted by the network, but does not change the described behaviour. I have also tried different network vendors but without success.

I see similar behaviour when using the Asset Tracker example project.

Is this intended behaviour?

Modem firmware: 1.1.0

Best Regards, Andreas (Thireo)

[Håkon Alseth]

Hi,

 

The current consumption is high (~30+mA) for at least 20 seconds after transmission has been completed

PSM has two components, one requested TAU (the sleep period) and the requested active time (active time after TAU).

The lowest period for both these are different for every network provider. In norway, I get this back from AT+CEREG? (enable PSM intervals by issuing AT+CEREG=5 first):

+CEREG: 1,"76C1","014A0305",7,,,"00100001","00000110"

*edit: this was initially parsed wrong, as I read CEREG return Active time as TAU (and vice-versa)* 

Incorrect parsing:

This translates to 1 hour sleep (001 00001 -> GPRS timer 3), and 6 seconds (000 00110 -> GPRS timer 2) of active time after TAU.

Correct parsing:

+CEREG: 1,"76C1","014A0305",7,,,"00100001","00000110"

00100001 is the active time, 001 = minute, 1 * 1 minute is your active time.

00000110 is the TAU (sleep period), 000 = 10 minutes, 10 min * 6 = 1 hour, is the sleep period.

*edit finished*

If you have a larger active time, try to request a lower active time by setting "CONFIG_LTE_PSM_REQ_RAT="00000110"" in your prj.conf file.

This will then request 6 seconds, but note that this is ultimately driven by the network, so be sure to check your returned PSM intervals using AT+CEREG=5 and AT+CEREG?.

 

Kind regards,

Håkon

[thireo]

Hi Håkon

The requested TAU is set to two minutes (0b10000100) and active time is set to 6 seconds (0b0000011). Both have been confirmed and acknowledged by the network. By polling AT+CEREG and by enabling LTE_LINK_CONTROL_LOG_LEVEL_DBG in Zephyr.

The Thingy:91 is located in Denmark and is using a Telia/Telenor NB-IoT SIM-card provided. They have also confirmed that no restrictions are set for PSM on their network.

Best regards,
Andreas

[Håkon Alseth]

Hi Andreas,

 

Are you enabling the GPS? That will also draw 30 to 40 mA (depending on what the CPU is up to)

Could you try the same setup with at_client and see if this also behaves similar?

There are delays in terms of when asset_tracker sends data, especially when GPS is enabled, as the GPS only runs when in PSM or eDRX mode.

 

Kind regards,

Håkon

[thireo]

Hi Håkon

No GPS is not enabled. At least not that I'm aware of. In prj.conf systemmode is set to "CONFIG_LTE_NETWORK_MODE_NBIOT". 
The build generated .config file for zephyr seems to confirm my assumption:
"# CONFIG_LTE_NETWORK_MODE_LTE_M is not set
# CONFIG_LTE_NETWORK_MODE_LTE_M_GPS is not set
CONFIG_LTE_NETWORK_MODE_NBIOT=y
# CONFIG_LTE_NETWORK_MODE_NBIOT_GPS is not set"

Could you give me some pointers as to how I'm supposed to send data via AT commands. I have yet to find any information regarding this, in the datasheets for NRF9160.

Best regards,
Andreas

[Håkon Alseth]

Hi Andreas,

 

thireo said:

Could you give me some pointers as to how I'm supposed to send data via AT commands. I have yet to find any information regarding this, in the datasheets for NRF9160.

You cannot trigger sending or receiving IP data using the AT commands that the modem exposes.

I wanted to see if this happens due to the application triggers sending of data, or if this is a returned active time in PSM that differs.

 

You mention that you have two SIM cards from different vendors (Telia and Telenor). Do both of these behave similar?

 

Kind regards,

Håkon

[thireo]

Hi Håkon

I double checked if GPS is being enabled, by polling the system mode. I can confirm, that it is not being enabled.

Oh okay, I misunderstood you then. I'm still unsure how I should test with the AT_client application. 

I have just tested with both SIM-cards and they show similar results.

Best regards,
Andreas

[Håkon Alseth]

Hi Andreas,

 

In the telenor network here in norway, I cannot get such a low active time as you currently have. It is therefore very important that you check the PSM intervals that are set by the network (not just requested by the nRF).

By setting AT+CFUN=1, then inputting your PSM settings via AT+CPSMS=1,,,"TAU","ACTIVE-time", you should soon get a PSM from the network.

Using AT+CEREG=5 to enable reporting the PSM intervals, and check with "AT+CEREG?"

 

Kind regards,

Håkon

[thireo]

I have just done the following with a Telenor IoT SIM-card and using the AT client sample application.

AT+CFUN=1 - Power on modem
AT+CPSMS=1,,,"00100001","00000001" - request 2 seconds active time and 1 hour Tau.
AT+CEREG=5 - Enabling reporting
AT+CEREG? - Poll for report.

See the attached image for the response.

 

[Håkon Alseth]

Hi Andreas,

 

Thank you for being this patient. I dug deeper into this, and soon figured out that I needed to consult experts.

I have asked the developers and designers internally, and there is a RRC idle + release happening after the tracking area update performed after the T3412 (sleep timer) expires and before the T3324 (active timer) starts. The time you're in this RRC Idle, ie before the RRC release is issued, is a value set by the eNB (network). Any active time will be executed after this. Judging from the plots, it looks like approx. 30 seconds is the specific period in the network you're connected to.

 

Kind regards,

Håkon