Maximum ATT MTU for BLE NUS

[krnd]

Hi,

What is the maximum ATT MTU size for the BLE NUS?

While programming my BLE NUS server application I constantly ran into the problem that my MTU and length updates cause NRF_ERROR_INVALID_PARAM errors. I solved it by setting the MTU size for my connection configuration (ble_cfg_t.conn_cfg.params.gatt_conn_cfg.att_mtu) and the next established connection (nrf_ble_gatt_att_mtu_periph_set()) to 248 (found by trial-and-error and only working in that particular case...).

Where does this magical number come from? Does this mean my max. transmittable data length is 245 (ATT_MTU - OPCODE_LENGTH - HANDLE_LENGTH)?

I also found some information in the specification of another SoftDevice stack (S132, I think...) saying something about a GATT server ATT MTU with the value 247... Is it connected somehow? If yes - why 247?

Environment

• nRF52 (nRF52840)
• SDK 13 (13.1.0)
• SoftDevice S140 (5.0.0-2.alpha)

[krnd]

I figured it out a while ago and want to share...
(I stick to Bluetooth 5 BLE and uncoded PHY - just in case it matters.)

• Maximum PDU size = 257 bytes
      [BLUETOOTH SPECIFICATION Version 5.0 | Vol 6, Part B | 2.1 PACKET FORMAT FOR THE LE UNCODED PHYS]
• Data channel PDU header = 2 bytes
  Data channel PDU MIC (Message Integrity Check) = 4 bytes
      [BLUETOOTH SPECIFICATION Version 5.0 | Vol 6, Part B | 2.4 DATA CHANNEL PDU]
• Basic L2CAP PDU header = 4 bytes
      [BLUETOOTH SPECIFICATION Version 5.0 | Vol 3, Part A | 3 DATA PACKET FORMAT]

Using this we get a max. MTU of 257-2-4-4 = 247 bytes.

For the NUS this results in an max. transmittable data size of 244 bytes (3 bytes NUS header: see `BLE_NUS_MAX_DATA_LEN` inside `ble_nus.h` or `ble_nus_c.h`).