Increasing the MTU Android fails to write characteristic

[Tony]

I have a device based on the Nordic nRF52832. I am using softdevice s132. I am using Nordic nRF5_SDK_15.3.0_59ac345. My Android device I am testing with is running Android 9.1 and has a 4.2 BLE hardware in it. It is a samsung Galaxy Tab tablet.

My firmware works fine if I have the NRF_SDH_BLE_GATT_MAX_MTU_SIZE set to 255 or lower. If I set it to > 255 (even 256), then when I try to write to my characteristic from an Android app I am writing the OS returns an error of "Failed to write characteristic - 128".

I am trying to determine if this is an error on the app side or the firmware side.

I did read one post on this forum about it not being a good idea to increase the MTU above 255. Can you clarify whether that is a hard limit of some kind?

The other interesting thing is that my firmware receives ALL of the bytes I send. I have the MTU currently set to 488 and I receive all 488 bytes on the firmware device even though the Android app is giving the write error 128.

Can this be explained? 

[Simonr]

Hi

I believe that the Android device won't accept higher MTU values than 255 in this case, which is why you see this error I believe. It is indeed not recommended to set the MTU size higher than 255, seeing as that's how big the DLE (Data Length Extention) feature allows the payload of each packet to be. You can see more on how to achieve maximum throughput in this blog post (keep in mind that this is for Bluetooth 5, so some features won't be available in a BLE 4.2 connection).

So, while 255 is not a hard limit, it's just the highest supported MTU size at the time being, and trying to send more data in each packet will lead to errors and eventual packet loss between devices I believe.

Best regards,

Simon