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

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

Hakon — Wed, 10 Jun 2026 09:44:41 GMT

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.

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

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.

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

Hakon — Wed, 10 Jun 2026 08:30:17 GMT

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

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

felan — Tue, 09 Jun 2026 23:32:02 GMT

Hi Hakon ,

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

Thanks

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

Hakon — Fri, 22 May 2026 11:40:55 GMT

Ok, I will ask the modem team to comment on this.

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

felan — Thu, 21 May 2026 02:12:08 GMT

Hi Hakon,

Thanks for checking with the modem team.

I have done a few addition test and the behaviour still appears to be present even when I reduce the MQTT keepalive interval to 30-60 seconds.

The main observation is that the device does not always receive the MQTT command when it is published from the broker while the device is in RRC Idle. Instead, the command appears to be received only when the device later sends its own uplink packet (MQTT keepalive / mqtt_live() or telemetry).

So the issue may not be that my previous keepalive interval was longer than an operator NAT timeout. Even with shorter keepalive intervals, the behaviour still looks uplink-triggered.

Current runtime PDN information:

PDP type: IPv4

MTU: 1430

IMSI prefix: 90128

The device receives a private IPv4 address, and the IMSI prefix appears to be the Vodafone global IoT range. Therefore, I agree that operator-side NAT/firewall/IoT-core behaviour may be involved.

What I am trying to evaluate is whether MQTT over TLS on LTE-M can realistically support low-latency downlink commands, ideally in the 5–10 second range, while keeping monthly data consumption as low as possible.

Experimentally, using a very short MQTT keepalive (10-15 seconds) improves responsiveness, but the TLS/TCP overhead makes that approach unsuitable for the target data budget. With 30–60 second keepalive, the command latency still appears to be bounded by the next client-originated uplink rather than by immediate network paging, note this is an intermittent behaviour but easily reproducible.

Could the modem team please clarify:

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

I have not yet had time to capture a modem trace, but I will provide one as soon as I can capture a clean reproduction.

Thanks for your assistance on this.

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

Hakon — Wed, 20 May 2026 07:22:39 GMT

Modem team;

Quite common DRX paging cycle is indeed 1.28 seconds which matches the peak seen in the PPK2 plot.

Very strange that MT paging wouldn’t work in normal DRX mode and especially MQTT+TCP based connection. Which carrier is used and is it an MVNO SIM? It’s quite common that with MVNO SIM the IP NAT route is kept open in the cellular IP network from the server towards the device only ~60 seconds at best, i.e. IPv4 based traffic towards the device doens’t have any route to the device anymore after this timeout.

Can you clarify whether the MQTT publish works for a period of time after last connection for some time, e.g. 60 seconds and then publish starts to fail?

MNO SIMs normally can maintain the IPv4 NAT routes much longer.

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

felan — Mon, 18 May 2026 22:27:17 GMT

Hi Hakon,

Thanks for your prompt response.

I will collect a modem trace and attach it to this thread, In the mean time I look forward to getting feedback from the modem team on the outlined behaviour.

Cheers,

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

Hakon — Mon, 18 May 2026 12:36:09 GMT

Hello,

I have forwarded your questions to the modem team. Would you be able to provide a modem trace, to speed up the debugging?