eDRX: packets sent from server during idle period

[Mark Szente]

Hello,

I have been observing some interesting behavior concerning packets sent from server during eDRX idle period. When the period is set to 20.48s or 40.96s, any packet sent will make the device transition to RCC connected state as soon as the idle period ends. However, packets are only received if they were sent a few seconds before the PTW. I guess that's expected, though, as networks may not keep the packets for more than a few seconds.

However, when using 81.92s, which is my goal, I cannot make the device wake up at all. Even if I spam packets every second, or even 0.5 seconds, it won't ever switch to RRC connected mode, let alone receive any of the packets.

What could be the reason behind this? Essentially, I want to be able to wake up the device from the server. 

I am using Onomondo MVNO and the SIM has static IP address behind a VPN. The actual network is One Hungary, with the PTW being 2.56s. Edit: using LTE-M.

Thanks,
Mark

[Achim Kraus]

My experience with that here in Germany:

I use also 81.92s.

Two providers here deliver the messages, regardless when that have been sent.

One provider requires the message to be sent at most 10s ahead of the end of the idle period. If the message is sent out of that time window I just get a RRC connected event.

In Hungary (21630/LTE-M) I didn't get eDRX with a 1nce SIM. I only tested that with 81.92.

[Mark Szente]

Interesting, thanks! Yes, in Hungary, only 21670 offers eDRX, so I'm going with them. 

I could accept messages not being delivered and only RCC connected event is firing. However what annoys me is that I can't even wake up the device with 81.92 whatsoever. So I guess I have three options here:

- Go with 61.44 (just tested, also works with the above caveats) 
- Have the device send some ping message every n minutes and act on the response if needed
- Use SMS

Obviously, all have their tradeoffs in terms of battery lifetime and/or operational cost. Also this sort of stuff requires actual field testing in each country...

[Mark Szente]

Thanks for the heads-up! Here's the modem trace, hopefully it includes everything. 

trace-2025-06-13T12-37-54.224Z.mtrace

[dejans]

Hi,

Thank you for providing your modem trace.

Which modem firmware version do you use?

Best regards,
Dejan

[Achim Kraus]

OK, the idea "PSM TAU interval" doesn't fit, you already wrote, that you don't use PSM and the trace verifies that.

So, I don't know, why that modem gets a RRC active every 54 minutes.

I guess, it would require to capture that, but for that the modem trace must run for an hour.

[Mark Szente]

Captured a longer trace that contains the event.

 dejans  the modem firmware version is 2.0.2

trace-2025-06-14T15-37-46.053Z.mtrace

[Achim Kraus]

The capture shows a TAU request. Seems that's used even without PSM. Makes sense for me, to stay registered at the mobile network.

The pain will be now, to distinguish the TAU connect from one caused by eDRX "incoming data".

(I just have not that much experience running devices without PSM.) 

[dejans]

Hi Mark,

54 minutes is periodic TAU (T3412 timer expires) and it is expected that UE triggers the connection. UE monitors paging occasions according to paging time window. Signal levels are fine. This could indicate that there is a problem on the network side. It might be that the network does not support data transmissions in 81 seconds eDRX.

Best regards,
Dejan