nRF5340 BLE Transmission Stack Buffer Overflow

[AKV]

Hi,

We are developing a mesh network. Our requirement is the one device can connect to three other devices. Using one connection as a peripheral and up to two connections as a central for mesh connections.

We are using nRF5340 DK and the nRF Connect SDK Version 2.6.1.

I am using the function bt_gatt_write_without_response_cb(). The stack buffer size is set with CONFIG_BT_BUF_ACL_TX_COUNT=30.

The CONFIG_BT_BUF_ACL_TX_COUNT is set to 30, allowing 10 buffers for each connection. I'm using a write complete callback to monitor buffer usage. If the buffer usage for any connection exceeds 10 due to transmission issues, I stop writing to that connection until it is cleared.

If one device connects to two other devices, with one being very close and the other being far away and out of range, the stack buffer becomes full for both connections, even for the nearby device. Why does this issue occur, causing difficulty in sending packets to the nearby device when the other device goes out of range?

[Amanda Hsieh]

Hi, 

If one device connects to two other devices, with one being very close and the other being far away and out of range, the stack buffer becomes full for both connections, even for the nearby device. Why does this issue occur, causing difficulty in sending packets to the nearby device when the other device goes out of range?

When a device is out of range, the BLE stack might still try to resend the data, causing the stack's buffers to fill up. This can prevent new data from being queued for transmission, even for other devices that are within range. 

Regards,
Amanda H.

[AKV]

Hi,

Thanks for the response.

Our project currently runs on the nRF52840 with the nRF5 SDK, and we haven't encountered this issue. We are transitioning to the nRF5340 with the NCS to achieve better performance.

However, this buffer overflow issue is significantly affecting our device's performance. Since this is a fundamental problem, could you address it on your end? Additionally, can we expect a solution in the next NCS release?

[Amanda Hsieh]

(Updated)

Could you provide a sniffer log for the team to investigate the issue? 

--

Hi, 

Sorry for the delay. I am checking with the team and will replay when I collect enough information. 

-Amanda H.

[AKV]

Hi,

Please find the attached sniffer log.

nRF5340_1-2_Connection_StackBufferIssue.pcapng

[Amanda Hsieh]

Hi, 

The team said the trace isn't that helpful sadly, and thinks your assumptions are wrong concerning how CONFIG_BT_BUF_ACL_TX_COUNT work. CONFIG_BT_BUF_ACL_TX_COUNT doesn't give 10 buffers per connection because the buffers are shared between all the connections.

You could try to increase the buffers in the Controller by setting CONFIG_BT_BUF_ACL_RX_COUNT. 

-Amanda H.