extended ATT_MTU does not work in long-range mode (coded PHY)

[SergeyT]

Hello
I want to use MTU = 247 bytes.
For 1M PHY, everything is working fine.
When I set the coded PHY, there is a problem.
I get the GATT event NRF_BLE_GATT_EVT_ATT_MTU_UPDATED with att_mtu_effective = 247.
But transmitting 244 bytes of data takes a time equal to 11 connection intervals. From this, I conclude that the actual MTU = 244 / 11 = 23 bytes.

SDK 15.3.0, nRF52840

Best Regards,

Sergey

[Simonr]

Hi Sergey

LE Coded PHY has a significantly different packet format, so you won't be able to increase the MTU when Coded PHY is used. Keep in mind that the Coded PHY data rate is expected to be a maximum of 125kbps, which is why you see the low throughput.

Best regards,

Simon

[SergeyT]

Hi Simon

Correctly, I understand you that for coded PHY the maximum size of MTU is 23 bytes?

Bluetooth Core Specification V5.0, Vol 6, Part B, 2.2 PACKET FORMAT FOR THE LE CODED PHY

From table 2.1 we can see that PDU can take values ​​from 16 to 2056 bits, this gave me hope that I can use the extended MTU. I was wrong?

Best Regards,

Sergey

[Simonr]

Hi

Yes, the max MTU size is 23 bytes according to the S140 SoftDevice release notes. Please see the snippet from these release notes below which describe this:

Best regards,

Simon

[SergeyT]

Thanks for the answer

Best Regards,

Sergey