nRF9160 RRC Mode Transition

[mrks115]

Hi there,

I'm reaching out regarding the nRF9160 and RRC Mode.

We have devices in motion (for example, during highway travel) that attempt to determine their position (via GNSS and MultiCell) while in transit.

For both tasks, it's essential that the devices transition to RRC idle mode (currently, we wait with a timeout of 45s for RRC connected → idle) before initiating position determination. However, particularly during travel, it often happens that we don't transition to RRC idle before our timeout.

My initial suspicion was that this might be due to cell changes prompted by the change in position. However, upon examining the logs, it became clear that no cell change had occurred.

1. What other reasons could prevent the UE from transitioning to RRC idle?
   1. Is there any influence we can exert over this process? (besides RAI with NB-IoT)
2. Since we have no control over the RRC mode transition, what timeout does Nordic recommend?
3. Have I overlooked anything else?

Thank you very much.

Best regards,
Markus

[timfrau]

The article's thorough and comprehensive approach is deeply appreciated. Solar

[Hakon]

Hello,

What other reasons could prevent the UE from transitioning to RRC idle?

1. Is there any influence we can exert over this process? (besides RAI with NB-IoT)

I am not aware of any other functionalities besides RAI that can reduce the time to go into RRC idle. Have you checked that it's enabled properly and that it's supported by the network?

Since we have no control over the RRC mode transition, what timeout does Nordic recommend?

You need to check what RRC inactivity timer is configured by the network;

"The network dictates the RRC Inactivity Timer period, and the UE cannot negotiate it."

Please refer to this document for more information.

Have I overlooked anything else?

I don't think so.

[mrks115]

Hi Hakon,

thanks for your reply.

I have one lingering question:

1. What other reasons could prevent the UE from transitioning to RRC idle?
But I guess the application either way has no chance to get notified about ongoing stuff which prevents going to idle.

You need to check what RRC inactivity timer is configured by the network

Yes, I know, but there is no way to know which timer is configured, right? Since our device is roaming international obtaining this information from all operators seems impractical.

RRC timer [...] up to 60 seconds

In the document you referenced, it mentions the above timer. Could you clarify the origin of this 60-second duration? Is it specified by the 3GPP?

I'm aware that AS-RAI pertains to Rel. 14. While I'm familiar with RAI for NB-IoT in Rel. 13, is there a corresponding feature for LTE-M + Rel. 13?

Sorry for all the questions. I think I've got most of the theory down, but I just want to double-check everything to make sure I'm on the right track.

Best regards
Markus

[Hakon]

mrks115 said:

Yes, I know, but there is no way to know which timer is configured, right? Since our device is roaming international obtaining this information from all operators seems impractical.

It's not possible to read this from the modem. Perhaps in the future there will be some at cmd for reading this, but not now.

mrks115 said:

Is it specified by the 3GPP?

Most probably it's specified there. I can't give you more information because I didn't write this guide.

mrks115 said:

I'm aware that AS-RAI pertains to Rel. 14. While I'm familiar with RAI for NB-IoT in Rel. 13, is there a corresponding feature for LTE-M + Rel. 13?

There are two types of RAI:

• Control Plane RAI (CP-RAI) - Used for control plane data in NB-IoT.

• Access Stratum RAI (AS-RAI) - Used for both control plane and user plane data in LTE-M and NB-IoT.

Is this what you meant?