Dropped packets with bt_nus_send

[clockis]

Greetings,

I am using the peripheral_uart Bluetooth nrf sample as a base for my custom application.

I have a ble_write_thread that just receives data on a message queue and calls the bt_nus_send function the same as the sample. And I also have a thread that produces the data and loads it to the ble message queue for transmission.

Recently because of a new requirement we need to send a lot more data via BLE than previously( to the central which is a mobile phone with the nRF Connect app). This has created issues with the integrity of the data packets we are sending. When the device is close to the edge of the Bluetooth range the device starts dropping packets.

As other members have previously informed me on the Zephyr discord this could be because while trying to resend the data (because of a poor connection on the edge of the range) new data are being queued in the BLE stack's buffers and the older ones that have still not been sent are overwritten thus they are never sent ( and appear as if they were dropped ) This only happens when the device is on the edge of the Bluetooth range & the issue is that the BLE stack is still trying to resend older packets.

I have increased the CONFIG_BT_BUF_ACL_TX_COUNT=200 & CONFIG_BT_BUF_ACL_TX_SIZE=251 (which is the maximum if I am not mistaken). I have also decreased the Supervision Timeout to the minimum (100 ms) so that the device disconnects for any transmission that takes more than 100 ms but this also seems not to solve the problem. I have also decreased the rate with which we queue new data to the ble_write_thread.

Is there something I am missing on the BLE operation or anything else I could do to eliminate this issue?

Thank you in advance!

Best regards,

Stavros

[Karl Ylvisaker]

Hello Stavros,

The best approach to handle this will depend on your applications requirements and constraints, could you elaborate on the requirements to your data transfer?
How much data will you generate, and how quickly will you do so, that you will be sending to the phone?

In any case, the best way to approach this is to implement a local error handling after the call to queue the data for sending over the NUS service, so that you can handle it in the case that the data fails to queue due to the buffer being full.
To handle this you could for example implement a ring-buffer, which you feed into the NUS service.
Alternatively, you could handle the data that fails to queue in another way - for instance, if it is not important for your application that every packet of data makes it across the link you could discard this data just as well. I mention both these approaches since I do not know enough about your application to know which one fits your use-case.

Best regards,
Karl

[clockis]

Hello Karl,

If you require any additional information or clarifications or if you have any updates on this please let me know.

Also please note that I am using a ring buffer as I am using the Zephyr Message Queue(which is implemented as a ring buffer) to pass the generated data from the producer thread to the consumer(ble_write_thread).

Kind regards,

Stavros

[Susheel Nuguru]

I think handling your message queue incorrectly was the issue here based on the code snippet you provided. I am glad that it is solved now. Please mark my reply as verified so that others from this forum who have similar issue can be benefited.. 

[clockis]

Yes, it was indeed the handling of the Message Queue, thank you very much for your support and feedback!