Clarification on CSS RTT Accuracy Support and Channel Sounding Security Levels on nRF54L15 Controller

[Shreeyash17]

I am currently using the nRF54L15 DK board along with an external host to develop Channel Sounding functionality. After reviewing related support tickets, I understand that the controller does not support 10ns CSS RTT accuracy, but does support 150ns CSS RTT accuracy.

I have a few questions related to this:

1. What parameters must the host set to ensure that the controller operates with 150ns CSS RTT accuracy and 10ns CSS RTT accuracy?

2. Which parameter(s) determine or distinguish whether the host is setting Channel Sounding Security Level 1, 2, 3, or 4 and for these parameters which values must be assigned for each Channel Sounding Security Level  ?

3. Does the controller generate any event that informs the host which Channel Sounding Security Level is currently being used?

4. Given that the controller does not support 10ns RTT security, if the host attempts to configure parameters that would require 10ns RTT accuracy, what error (e.g., HCI error code) will the controller return to the host?

[Elfving]

Shreeyash17 said:

What is the proper way to request channel sounding Security Level 3 or 4,( considering that  unsupported and controller will give an error)?

The security levels are currently a host-level concept, so you wouldn't be able to set and get a declined security level from the SDC. You just set the features that then imply a certain security level, like 10 ns CS RTT accuracy and CS tones.

Shreeyash17 said:

Are there any other valid Main Mode/Sub Mode combinations(in create config command) or approaches I should use for this purpose?

I believe what you tried, and other combinations should be valid. So I was a bit surprised by the error. Though I believe the host might be part of the problem here, did you use CONFIG_BT_CTLR_SDC_CS_STEP_MODE3?

Regards,

Elfving

[Shreeyash17]

Elfving said:

did you use CONFIG_BT_CTLR_SDC_CS_STEP_MODE3?

yes it is enabled.

Elfving said:

The security levels are currently a host-level concept, so you wouldn't be able to set and get a declined security level from the SDC. You just set the features that then imply a certain security level, like 10 ns CS RTT accuracy and CS tones.

Elfving  Since Channel Sounding Security Level is a host-level concept, I assume the Nordic Host must be sending specific commands to the controller to configure it. I looked through the Nordic Channel Sounding samples but couldn’t find a reference.

Could you please clarify how the Nordic Host sets Security Level 1 and Security Level 2 in practice (i.e., which commands/parameters are used)?

[Elfving]

Could you try without it? I still believe this config should work.

Shreeyash17 said:

Elfving  Since Channel Sounding Security Level is a host-level concept, I assume the Nordic Host must be sending specific commands to the controller to configure it. I looked through the Nordic Channel Sounding samples but couldn’t find a reference.

Well yeah my understanding is that it sets for instance CONFIG_BT_CTLR_SDC_CS_STEP_MODE3, though that is as far as the SDC goes in its understanding of what security level is being used - since it is a host-level concept.

Shreeyash17 said:

Could you please clarify how the Nordic Host sets Security Level 1 and Security Level 2 in practice (i.e., which commands/parameters are used)?

I do not think there is an API for this. You are doing it correctly with this config and setting the procedure modes. 

Regards,

Elfving