Android bluetooth low energy (ble) writeCharacteristic delay callback

Question

I am implementing a application on Android using BLE Api (SDK 18), and I have a issue that the transfer data process is delay very slow. This is my log.

03-1

Answer 1

You'll have to implement you're own flow control, don't start a write until the OnCharacteristicWrite is sent for the previous write

Answer 2

We had the same problem. It occurs after several writes and goes away when you reconnect the Gatt. What we do is measure the delay between writeCharacteristic() and onCharacteristicWrite(). If it is above a certain threshold (1500ms), we disconnect() and close() the gatt server and reconnect. After that it is usually fine for a while.

When doing this it might become necessary to disable the whole bluetooth adapter and reenable it. Otherwise the first reconnect to the gatt server fails and it takes about 15 seconds until it is reconnected. Sometimes it never reconnects.

When disabling the adapter you might run into the problem that service discovery does not return in onServicesDiscovered(). We have currently no solution for that.

Answer 3

This seems to be a problem for me as well. If you delay the calls in the debugger you can confirm that there is a delay when writing to BLE radios. I will be implementing a queue in my application to handle the latency.

Here is what I do for queueing up commands:

public void sendWriteCommandToConnectedMachine(byte[] commandByte) {
    if(commandByte.length > 20)
        dissectAndSendCommandBytes(commandByte);
    else
        queueCommand(commandByte); //TODO - need to figure out if service is valid or not
}

private void queueCommand(byte[] command) {
    mCommandQueue.add(command);

    if(!mWaitingCommandResponse)
        dequeCommand();
}

Hereis what I do for dequeuing BLE commands

private void dequeCommand() {
    if(mCommandQueue.size() > 0) {
        byte[] command = mCommandQueue.get(0);
        mCommandQueue.remove(0);
        sendWriteCommand(command);
    }
}

here is my characteristic write method

@Override
    public void onCharacteristicWrite(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic, int status) {
        if(status != BluetoothGatt.GATT_SUCCESS)
            logMachineResponse(characteristic, status);     

        mWaitingCommandResponse = false;
        dequeCommand();
    }

Answer 4

Performance of a BLE link is highly dependent on the connection interval used, and if your connection interval is high, the performance you see may not be that unreasonable. By the Core Specification, the connection interval can be between 7.5 ms and 4 s, so there's quite some flexibility.

If it's possible for you, I'd recommend you to try changing the Peripheral you're talking to to use a shorter connection interval, which should improve performance. You may have use in taking a look at this page, explaining BLE throughput, and this page, explaining connection parameters.