Packet loss during ISO transmission when advertising

[thomas_hexploy]

Hi,

Bug:

With a connected ISO connection between two devices X and Y:

• X acts as a peripheral. It's the ISO server and it is transmiting iso data to Y.
• Y is multirole. It connects to X and establish an ISO connection to receive ISO data.

As long as Y only receives data, the communication is stable (no packet loss). As soon as Y advertises (bt_le_adv_start), Y does not receive all packets.

• Increasing advertising interval, decrease packet loss.
• Decreasing advertising interval, increase packet loss.

Environment:

• OS: Linux
• Toolchain zephyr-sdk-0.16.8
• NRF5340DK
• NCS 2.7 (sdk-nrf v2.7.0 and zephyr v3.6.99-ncs2)

How can solve this issue, which causes too many packet loss ?

Thank you

[yuxuan.cai]

Hi,

The ACL connection interval you are using seems a bit too large. Could you try a smaller value of 60 or 70ms?

Due to clock drifts between devices, the peripheral will perform window widening to ensure it can receive packets from central. Simply put, the larger connection interval it is, the larger window widening it will be. This window widening on ACL connection may block the CIS packet reception.

Cheers,

Yuxuan

[thomas_hexploy]

Hi,

I tried with 60/70ms ACL connection time, I still have the same packet loss.

This is my understanding of the current problem:

Advertising is 4.5ms, so in our case this is the duration of about 3 ISO frames (3 * 1.5ms). It means that 4 retransmissions should be sufficient to cover the collisions. I don't know how much time takes ACL connection, but let's say 1.5ms (feel free to correct me), it would mean 5 retransmissions in the worst case scenario.

So, with 10 RTN and a latency of 50ms, I don't see what can cause packet loss ? I think it should cover every collision.

I don't understand why the worst case scenario (longest duration where ISO frames can't be sent because of collision) isn't known and why a configuration (retransmission and latency) is not possible to remove entirely this packet loss.

We don't have specific requirements on latency and retransmission (as far as it's reasonable), if it allows no packet loss. We are in connected mode (CIS).

Thank you.

[yuxuan.cai]

Hi,

I have tested it a bit locally and managed to reproduce the packet loss you are seeing. It does seem to be an implmentation issue on our side. Although the root cause is not identified yet, it seems to be working when NSE > 2 and FT > 1. NSE and FT are selected by the controller with the provided RTN and max transport latency, and so far the configurations you have tried are, unfortunately, not meeting the two conditions at the same time. The parameter selection algorithm of the controller is not exposed publicly and thus there are some trial & error to do. I am sorry for the inconvinience.

You mentioned RTN=10 and max transport latency=50ms - have you tested this configuration? It had no packet loss locally for me with an advertising interval=1s, ACL connection interval=70ms, max SDU size=120bytes, ISO interval=10ms.

Or alternatively, could you try RTN=5, max transport latency=20ms? This is 48_4_1 in the quality of service configuration, and it had no packet loss for me as well.

Let us know if this is helpful to you!

Cheers,

Yuxuan

[thomas_hexploy]

Hi,

In the following configuration:

• ACL: 70ms
• SDU: 120 bytes
• ISO interval: 10ms
• Advertising: Every second

I tried:

• RTN:10 and latency: 50ms: I still have packet loss
• RTN:5 and latency: 20ms: I still have packet loss

So unfortunately, I see no improvement on my side.

Could you provide me your sample code (that allowed you to have no packet loss) to reproduce it on my end ?

Thank you

[yuxuan.cai]

Hi,

I was using the internal framework for quick testing, and it will take some time before I can convert it into something you could run. Would you mind sharing your codes?

Cheers,

Yuxuan