HCI LE Read ISO Tx Sync does not work for BIS Connection Handles

[Matthias Ringwald]

BGS.pklg

Hi there

While doing the GMAP tests for our Bluetooth stack with the HCI UART bridge of NCS 2.7 with SoftDevice running on nRF5340 Audio DK, we've tried using the HCI LE Read ISO TX Sync command, which worked as expected for Unicast connections / CIS Connection Handles.

When doing the same for Broadcast connections, we receive a HCI Command Disallowed status code. According to the Bluetooth Core v6.0, this should only happen if a) the Controller hasn't sent a single TX packet, or b) the connection handle is invalid. As far as I can tell, neither of these cases match. Please find an HCI packet trace (for Wireshark) to verify.

As NCS 2.8 came out a few days ago, we've had a quick check, but got the same behaviour.

Is there a reason why HCI LE Read ISO TX Sync only works for CIS connections?

Could you consider supporting it?

The HCI LE Read ISO TX Sync is needed to minimize latency on the sending side as well as getting the precise time for sample accurate playback (true-wireless) between sender and receiver, e.g. for 'linked' Bluetooth speakers.

Thanks, 
 Matthias

[Einar Thorsrud]

Hi Matthias,

It looks like SDU sent to controller may be larger than max SDU of the BIG. 154 bytes in the SDU to 75 bytes Max SDU of the BIG. If so, the ISO packets will be recjected.

[Matthias Ringwald]

Hi Einar. Ouch! I'm sorry for missing this. You're totally right: the ISO packet is too large, which leaves the Controller no chance to sent it, hence, the correct response is Command Disallowed as it never gets to send a ISO packet.  Thanks for your swift reply. Cheers, Matthias