RE: Maximum ATT MTU for BLE NUS

krnd — Sat, 14 Apr 2018 18:23:15 GMT

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`).

RE: Maximum ATT MTU for BLE NUS

run_ar — Fri, 22 Dec 2017 12:24:39 GMT

247 was probably used since this is the maximum packet length of 255 - headers - 1byte because of some old issue(254 max length). 248 should be fine with the nRF52840. In fact the ATT MTU size is not really depending on the Packet size. It is possible to have longer att mtu's than the maximum pdu size.