ATT MTU size

[Sergii]

Hello,

Help me please, I have an issue with ATT MTU size. 

When connecting Android (BLE client) to nrf52 (server) and ATT MTU is 20 bytes in max.

I have changed on ATT MTU on nrf52, but still have 20 bytes.

I see this topic:
https://devzone.nordicsemi.com/f/nordic-q-a/18544/the-maximum-byte-can-received-per-package-used-nordic-uart-android-app/71568#71568

Is it a way to check, does current mobile phone support more 20 bytes payload? 

[Carl Richard]

Hello Sergii!

I would recommend looking at Android documentation and forums for this query, but I did some testing using the nRF Connect application for Android and found that a connection is initialized with an MTU of 23 bytes (with 20 bytes available for data). This happens even if the ATT MTU of the nRF52 device is higher, indicating that the Android device advertises the low MTU of 23 bytes. I could however increase the MTU(to a maximum of 62) of the connection after it was established by requesting a higher MTU. For an Android application the function referenced here should achieve this. 

When it comes to large MTU a phone actually supports you would need to read more about this in the phone/android documentation, but I guess older phones will be more prone to limitations. I used a Samsung Galaxy S9 with Android 10

Hope this helps!

BR
Carl Richard