L2CAP fragmentation with a data length of 251 bytes

RE: L2CAP fragmentation with a data length of 251 bytes

user77782 — Thu, 12 May 2022 11:05:36 GMT

Hi,

Sorry for the late reply.

The problem here is that the client PC has Max Rx time set to 328 microseconds which don't allow our nRF5340 to transmit the 251 bytes in one packet. The suitable time for 251 bytes packet would be 2120us. Our device set such value for the Rx and Tx max time in a response to LL_LENGTH_REQ. But in request from the PC, our device gets that PC max Rx time is 328 microsecond even if max Rx octets size is 251 bytes. For 1MB PHY the 328 microsecond allows our SoC to set packets not longer than 27 bytes because we cannot transmit packets that transmission time would be greater than PC Rx max time. But in a reverse direction from PC to our SoC, it should be possible to send a packet of 251bytes. I hope that this clarifies your issue.

To improve the throughput the client PC needs to request the data length update with Rx time set to 2120us (for 1M PHY). This is only possible when the client PC BLE controller supports such Rx max time. The other solution can be switching to use 2M PHY it will increase throughput in all cases even with the dongle which can handle a packet of 251 bytes. In your case if increasing Rx max time on the PC side is not possible then you can try to use 2M PHY, it gives you faster data transmission in the air and you will send about two times more data in one packet.

-Amanda

RE: L2CAP fragmentation with a data length of 251 bytes

user77782 — Tue, 15 Mar 2022 14:35:33 GMT

Thanks for the tip. We are looking into the issue and will be back if we find anything.

-Amanda

RE: L2CAP fragmentation with a data length of 251 bytes

user114027 — Fri, 11 Mar 2022 10:04:49 GMT

Hi Amanda,

On my first post I already put a trace file. The connection with the PC starts time = 262.381 s. I put you an image of the answer of the PC.

Thanks,

Hichem Ghandri

RE: L2CAP fragmentation with a data length of 251 bytes

user77782 — Fri, 11 Mar 2022 08:21:59 GMT

Hi,

Sounds the nRF52840 dongle can accept and update the data length of 251 bytes instead of the PC. This is decided by the connected central ble. Guess PC ble rejects to update data length to 251. Needs the sniffer log to prove it. Please capture the sniffer starting before the connecting.

Regards,
Amanda