nRF9151 LTE-M: MQTT downlink not always waking device from RRC Idle to RRC Connected when PSM/eDRX are disabled

[felan]

Hi There, 

I am currently developing an LTE-M application on nrf9151 using ncs v3.2.1, the device is a MQTT client that publishes telemetry periodically every 3 minutes, and it is meant to receive MQTT commands from the broker and act upon them within a few seconds i.e 5 to 10 seconds.  

modem FW : mfw_nrf91x1_2.0.4

Current software configuration: 

- MQTT QoS: QoS 1 for commands/status
- MQTT clean session: enabled
- PSM: disabled
- eDRX: disabled

CONFIG_LTE_LINK_CONTROL=y
CONFIG_LTE_LC_PSM_MODULE=n
CONFIG_LTE_LC_EDRX_MODULE=n
CONFIG_MQTT_CLEAN_SESSION=y
CONFIG_NET_SOCKETS_OFFLOAD=y
CONFIG_LTE_NETWORK_MODE_LTE_M_NBIOT=y

Observed behaviour: 

1. Device attaches to LTE-M and connect to MQTT properly. 
2. Device subcscribes to command topcis succesfully. 
3. While the device is in or near RRC Connected state, MQTT commands are received correctly.
4. After the network releases the device to RRC Idle, I publish a command from the broker.
5. Sometimes the modem transitions RRC Idle -> RRC Connected and the MQTT command is received.
6. Other times, no RRC transition is observed after the broker publish, and the command is not delivered to the application until a later uplink event (MQTT keepalive or telemetry publish).
7. I am not using PSM or eDRX, so my expectation was that the UE should remain reachable in RRC Idle via normal paging/DRX.

Power profiler observation:

I can see regular current peaks of approximately 80 mA every ~1.27 seconds while the device is in the idle state. I assume these are related to LTE-M paging/DRX activity, but I would like confirmation on how to interpret this. Despite these periodic peaks, downlink MQTT traffic does not always appear to page/wake the device into RRC Connected.

The issue is intermittent: sometimes downlink MQTT wakes the modem correctly, sometimes it does not appear to do so until uplink activity occurs.

1. With PSM and eDRX disabled on nRF9151 LTE-M, should downlink TCP/MQTT traffic reliably trigger network paging and cause the UE to transition from RRC Idle to RRC Connected?
2. If the device is in RRC Idle and an MQTT command is published from the broker, but no RRC Idle -> RRC Connected transition is observed, what are the most likely causes?
3. Are the periodic ~80 mA current peaks every ~1.27 seconds likely to be LTE-M idle-mode paging/DRX activity? If yes, does that confirm the modem is monitoring paging occasions correctly?
4.  Are there known issues or recommendations for MQTT over TLS on nRF9151 where downlink data is only received after the next uplink packet, such as an MQTT keepalive or telemetry publish?
5. Is there a recommended MQTT keepalive interval, socket configuration, or LTE configuration for low-latency downlink MQTT on nRF9151 while keeping data usage low?

Thanks for looking into this, your assistance is much appreciatted. 

Cheers, 

[felan]

Hi Hakon ,

Just following up on this , checking whether you have received any feedback from the modem team ?  

Thanks

[Hakon]

I have not received any feedback. I will ask them again.

[Hakon]

1. If PSM and eDRX are disabled, and the UE is in normal RRC Idle DRX, should mobile-terminated TCP data for an existing TLS socket reliably cause paging and RRC Idle -> Connected?

Yes it should. Next question is then whether they can see that nRF91 shows CSCON:1 notification when device is being paged for the downlink data? If there is a transition from CSON: 0 to CSON:1 it means that carrier radio network has indeed received data targetting the device. Modem trace log would show whether anything is actually then sent from the network side when device answers paging and establishes the connection for reception.

We’ve seen cases with eDRX that network does something we call “empty paging”, i.e. radio network triggers the paging (transition from CSCON:0 to CSCON:1) but nothing is sent to the device.

If nothing happens OR “empty paging” happens then this has to be further debugging with they service provider and roaming network provider.

2. Is this behaviour something the nRF modem can influence, or is it mainly controlled by the operator APN / packet core / firewall / NAT configuration?

This is under network operator control. IPv6 might help but i don’t think many MVNOs support that e2e, usually has to be an MNO SIM

3. For a global LTE-M product needing 5–10 second command response with very low data usage i.e 2 MB per month , would Nordic recommend a different approach for the downlink behaviour (SMS will also be limited)? 

SMS is one option that is quite reliable, especially if normal idle drx is being used. All in all MQTT should be reliable but like said, this is something that MVNO/Carrier has to look at and explain why paging is not successful.