Samsung Android 10 BLE MTU>23 issue

Question

Issue: After setting MTU>23 on Samsung Device (S10e) running Android 10 and writing data to a characteristic the connection times-out and is closed.

Root cause: The reason for the timeout is the phone hardware is not actually sending anything...so the end device does not respond(ack).

How do I know it’s not an end device issue: This problem does not occur with other devices (pixel) and is also doesn’t occur on the S10 if it’s running Android 9.

Details: We are using requestMtu in BluetoothGatt to change the MTU to 185, and then onMtuChanged return mtu value is 185 and status Is GATT_SUCCESS. When we send the longer message which is about 40 bytes, the return value of writeCharacteristic of BluetoothGatt is true, but the onCharacteristicWrite callback will give us a 133 status code, which is not GATT_SUCCESS. Then the devices will disconnect.

Why do I need a larger MTU: What we expect is to be able to use a bigger MTU so that we can send data to end-device to provision Wi-Fi credentials. The end device was built to only accept the credentials all at one time(assumes MTU>23 wouldn’t be an issue. So we have field devices that customers can’t use.

Question: Is there any ideas or workaround that we can try?

Phone info: Samsung s10e Os: Android 10 Android security patch: March 1, 2020

End-Device info: ESP32

Answer 1

I'm not using a Samsung device, but I had the same issue with a Sunmi device and apparently the problem is attempting to send the full MTU or any value close to that. If I negotiate 512 and attempt to send 512 I get a 133 status followed by a disconnection, but if I negotiate 512 and send 500 bytes the whole thing runs fine. Right now I'm using this formula, but I have only tested it for an MTU negotiated value of 512:

private int getMtu() {
    return Math.max(GattClient.DEFAULT_MTU, (int)(this.mtu * .99));
}

The idea is that you'll always send at least 20 bytes, since that works fine even if the MTU is not negotiated. Other than that, I send only 99% of the MTU. I'm not getting the 133 status anymore.