Difficulty in reading data from BLE package on Android

Question

I am designing a sensor system with a sensor and an Android device on the receiving end:

For now, since the sensor is not available I am using another android device as the BLE advertiser. Notice that I do not connect to the device, I just read the advertisement package.

On the transmitter side, I am creating a packet that contains the name of the device, the UUID and the data of the specific UUID:

private void advertise() {
    BluetoothLeAdvertiser advertiser = BluetoothAdapter.getDefaultAdapter().getBluetoothLeAdvertiser();

    AdvertiseSettings settings = new AdvertiseSettings.Builder()
            .setAdvertiseMode( AdvertiseSettings.ADVERTISE_MODE_LOW_LATENCY )
            .setTxPowerLevel( AdvertiseSettings.ADVERTISE_TX_POWER_HIGH )
            .setConnectable(false)
            .build();

    ParcelUuid pUuid = new ParcelUuid( UUID.fromString( getString( R.string.ble_uuid ) ) );
    byte[] mdata = "Hello".getBytes(Charset.forName("UTF-8"));

    AdvertiseData data = new AdvertiseData.Builder()
            .setIncludeDeviceName( true )
            .addServiceUuid( pUuid )
            .addServiceData( pUuid, mdata )
            .build();

    AdvertiseCallback advertisingCallback = new AdvertiseCallback() {
        @Override
        public void onStartSuccess(AdvertiseSettings settingsInEffect) {
            super.onStartSuccess(settingsInEffect);
        }

        @Override
        public void onStartFailure(int errorCode) {
            Log.e( "BLE", "Advertising onStartFailure: " + errorCode );
            super.onStartFailure(errorCode);
        }
    };

This part works well, because the Raw data that I receive using the BLE scanner app, are the ones that I want to send.

The problem is on the receiving side. There what I do is:

private void discover() {
    List<ScanFilter> filters = new ArrayList<ScanFilter>();

    ScanFilter filter = new ScanFilter.Builder()
            .setServiceUuid( new ParcelUuid(UUID.fromString( getString(R.string.ble_uuid ) ) ) )
            .build();
    filters.add( filter );

    ScanSettings settings = new ScanSettings.Builder()
            .setScanMode( ScanSettings.SCAN_MODE_LOW_LATENCY )
            .build();

    mBluetoothLeScanner.startScan(filters, settings, mScanCallback);

    mHandler.postDelayed(new Runnable() {
        @Override
        public void run() {
            mBluetoothLeScanner.stopScan(mScanCallback);
        }
    }, 10000);
}

My scanCallBack function is the following:

private ScanCallback mScanCallback = new ScanCallback() {
        @Override
        public void onScanResult(int callbackType, ScanResult result) {
            super.onScanResult(callbackType, result);
            if( result == null
                    || result.getDevice() == null
                    || TextUtils.isEmpty(result.getDevice().getName()) )
                return;

        StringBuilder builder = new StringBuilder( result.getDevice().getName());
        for(int j = 0; j < result.getScanRecord().getServiceUuids().size(); j++){
            builder.append("\n");
            builder.append("Result no. " + j);
            builder.append("\n");
            builder.append(result.getScanRecord().getServiceUuids().get(j));
            builder.append("\n");
            builder.append("Service data: ");
            builder.append("\n");
            builder.append(result.getScanRecord().getServiceData(result.getScanRecord().getServiceUuids().get(j)));
        }
        mText.setText(builder.toString());
    }

This code always returns null, at the point where I try to read my data.

Overall, I can read the device's name and the UUID of the device, but not the data.

Please give me some wisdom on the matter.

Answer 1

I found out that android uses 16-bit UUIDS, meaning that it translates a 128-bit UUID in the following way:

This the original 128-bit UUID: 01122334-4556-6778-899A-ABBCCDDEEFF0.

The data are sent using this UUID.

Once this UUID is received from android, android only keeps 2334 and dumps the rest.

Then it creates a new 128-bit UUID, in a default way:

0000XXXX-0000-1000-8000-00805f9b34fb.

Where XXXX is my 16-bit UUID 2334.

So in order to access my data I need to look for the 128-bit UUID that android gave to my data, in this case:

00002334-0000-1000-8000-00805f9b34fb.

So in my case it was very practical to send my data on the 128-bit UUID that android assigned to my own.

I am not sure why this is happening, but now it works perfectly.