LL_LENGTH_REQ & LL_LENGTH_RSP on nRF52840 with different TX and RX size

RE: LL_LENGTH_REQ & LL_LENGTH_RSP on nRF52840 with different TX and RX size

Anton Fedorov — Tue, 20 Feb 2024 13:08:06 GMT

Thank you very much for the confirmation.

RE: LL_LENGTH_REQ & LL_LENGTH_RSP on nRF52840 with different TX and RX size

Hung Bui — Tue, 20 Feb 2024 12:44:20 GMT

Hi Anton,
I'm glad that you found what trigger the peripheral.
Regarding your question, the information I got from the team is that at this moment we don't support asymmetric DLE unfortunately. We will always match the RX time and TX time. We may support it in the future but I don't know when it will be implemented.

RE: LL_LENGTH_REQ & LL_LENGTH_RSP on nRF52840 with different TX and RX size

Anton Fedorov — Mon, 19 Feb 2024 16:27:22 GMT

Unrelated to the topic question -- I've discovered that actual key to force peripheral to start sending was to send LL_CONNECTION_UPDATE_IND, as long as the requested packet length is within accepted by the peripheral parameters (which I can tweak). So I've got the stream I wanted.

That doesn't invalidate the question about LL_LENGTH_REQ -- is it possible to get it the diverse, like other stacks allows to, or not?

RE: LL_LENGTH_REQ & LL_LENGTH_RSP on nRF52840 with different TX and RX size

Anton Fedorov — Mon, 19 Feb 2024 14:16:29 GMT

Here is the traces:

original-to-original-slice.pcapng.gz -- it's the trace of original central to original peripheral
phone-to-original-slice.pcapng.gz -- this is the trace of the phone to original peripheral
nRF-to-original-slice.pcapng.gz -- this is the trace of nRF connect() to original peropheral

I've also patched the firmware of the peripheral to send 251/251 at the connection time, but surprisingly, the phone replies with 27/251 to this packets:

phone-to-patched-slice.pcapng.gz

And nRF same responds with equal TX & RX as to the original:

nRF-to-patched-slice.pcapng.gz

Yes, "Empty PDU" is what I called NOOP, sorry for wrong word usage.

The peripheral is NOT properly GATT-compliant, it actually just sends stream of notifications with no other GATT server functionality, basically that's why I need a central-on-chip where I can get these notifications (android ble stack doesn't give access to raw notification data out of native code unless device gives list of characteristics).

I had an cc2640r2-based central implementation that was working fine, it also connected using 27/251 (same as original central do), but I can't show you the trace because I've burned it out 😃 and nRFs is the only kits i have left atm.

Regardless of these, answer LL_LENGTH_REQ with 251/251 to 27/251 looks like a bug to me, as it should use effectively MIN() between requested of the both sides, isn't it?

Even if I not rely on the default only and add a request for MTU from the central, it looks like this:

Peri=>Cen: LL_LENGTH_REQ(27/251)
Cen=>Peri: LL_LENGTH_RSP(251/251)
Cen=>Peri: LL_LENGTH_REQ(251/251)
Peri=>Cen: LL_LENGTH_RSP(27/251)

Which also seems weird to me.

devzone.nordicsemi.com/.../5670.traces.zip

RE: LL_LENGTH_REQ & LL_LENGTH_RSP on nRF52840 with different TX and RX size

Hung Bui — Mon, 19 Feb 2024 13:16:03 GMT

Please capture a sniffer trace.

Have you made sure you do a write command from the central to the peripheral to enable notification. Without that the peripheral will not send anything to the central.
What is NOOP ? You meant empty packets ?

RE: LL_LENGTH_REQ & LL_LENGTH_RSP on nRF52840 with different TX and RX size

Anton Fedorov — Mon, 19 Feb 2024 13:07:58 GMT

The peripheral doesn't reject the connection, it just doesn't work, failing inside of its firmware either allocating the receive buffers (if it's 251/251) either failing to send non-fragmented notification (27/27).

The original central has:

CONN_IND

LL_LENGTH_REQ(27/251)

LL_LENGTH_RSP(27/251)

NOOP + GATT_NOTIFICATION

phone connection has more stuff going on, but practically the same the gist of it.

The nRF52840 trace is

CONN_IND

LL_LENGTH_REQ(27/251)

LL_LENGTH_RSP(251/251) or 27/27

and after only NOOPs... 😮😞

RE: LL_LENGTH_REQ & LL_LENGTH_RSP on nRF52840 with different TX and RX size

Hung Bui — Mon, 19 Feb 2024 12:59:57 GMT

Hi Anton,
Could you capture a sniffer trace of the communication so we can check if the peripheral reject the connection because of the data length ?

I will need to double check if it's possible to adjust RX length different to TX data length. From what I can see the

bt_conn_le_data_len_update() only allow update the tx length, not the rx.