How to transfer images via Bluetooth (LE) to a desktop application

Question

We are currently trying to implement the transmission of images from a mobile device (in this case an IPhone) to a desktop application. We tried already the Bluetooth Serial plugin which works fine for Android but does not list any devices when scanning for our desktop application.

To cover iOS support (AFAIK iOS only supports BluetoothLE), we reimplemented our desktop application to use BluetoothLE and behave like a peripheral. Also we altered our Ionic application to use BLE plugin.

Now BluetoothLE only supports the transmission of packages with the size of 20 Byte whilst our image is about 500kb big. So we could obviously split our image into chunks and transmit it with the following function (taken from this gist):

function writeLargeData(buffer) {
    console.log('writeLargeData', buffer.byteLength, 'bytes in',MAX_DATA_SEND_SIZE, 'byte chunks.');
    var chunkCount = Math.ceil(buffer.byteLength / MAX_DATA_SEND_SIZE);
    var chunkTotal = chunkCount;
    var index = 0;
    var startTime = new Date();

    var transferComplete = function () {
        console.log("Transfer Complete");
    }

    var sendChunk = function () {
        if (!chunkCount) {
            transferComplete();
            return; // so we don't send an empty buffer
        }

        console.log('Sending data chunk', chunkCount + '.');

        var chunk = buffer.slice(index, index + MAX_DATA_SEND_SIZE);
        index += MAX_DATA_SEND_SIZE;
        chunkCount--;

        ble.write(
            device_id, 
            service_uuid, 
            characteristic_uuid, 
            chunk, 
            sendChunk,         // success callback - call sendChunk() (recursive)
            function(reason) { // error callback
                console.log('Write failed ' + reason);
            }
        )
    }
    // send the first chunk
    sendChunk();
}

Still this would mean for us that we would have to launch about 25k transmissions which I assume will take a long time to complete. Now I wonder why is that the data transmission via Bluetooth is that handicapped.

Answer 1

If you want to try out L2CAP your could modify your Central desktop app somehow like this:

private let characteristicUUID = CBUUID(string: CBUUIDL2CAPPSMCharacteristicString)
...

Then advertize and publish a L2CAP channel:

let service = CBMutableService(type: peripheralUUID, primary: true)

let properties: CBCharacteristicProperties = [.read, .indicate]
let permissions: CBAttributePermissions = [.readable]

let characteristic = CBMutableCharacteristic(type: characteristicUUID, properties: properties, value: nil, permissions: permissions)
self.characteristic = characteristic
service.characteristics = [characteristic]

self.manager.add(service)
self.manager.publishL2CAPChannel(withEncryption: false)
let data = [CBAdvertisementDataLocalNameKey : "Peripherial-42", CBAdvertisementDataServiceUUIDsKey: [peripheralUUID]] as [String : Any]
self.manager.startAdvertising(data)

In your

func peripheralManager(_ peripheral: CBPeripheralManager, central: CBCentral, didSubscribeTo characteristic: CBCharacteristic) {

respective your

func peripheralManager(_ peripheral: CBPeripheralManager, didPublishL2CAPChannel PSM: CBL2CAPPSM, error: Error?) {

offer the PSM value (= kind of socket handle (UInt16), for Bluetooth stream connections):

let data = withUnsafeBytes(of: PSM) { Data($0) }
if let characteristic = self.characteristic {
    characteristic.value = data
    self.manager.updateValue(data, for: characteristic, onSubscribedCentrals: self.subscribedCentrals)
}

finally in

func peripheralManager(_ peripheral: CBPeripheralManager, didOpen channel: CBL2CAPChannel?, error: Error?) 

open an input stream:

channel.inputStream.delegate = self
channel.inputStream.schedule(in: RunLoop.current, forMode: .default)
channel.inputStream.open()

where the delegate could look something like this:

func stream(_ aStream: Stream, handle eventCode: Stream.Event) {
    switch eventCode {
    case Stream.Event.hasBytesAvailable:
        if let stream = aStream as? InputStream {
             ...
            //buffer is some UnsafeMutablePointer<UInt8>
            let read = stream.read(buffer, maxLength: capacity)
            print("\(read) bytes read")
        }
    case ...
}

iOS app with Central Role

Assuming you have something like that in your iOS code:

func sendImage(imageData: Data) {
    self.manager = CBCentralManager(delegate: self, queue: nil)
    self.imageData = imageData
    self.bytesToWrite = imageData.count
    NSLog("start")
}

then you can modify your peripheral on your iOS client to work with the L2Cap channel like this:

func peripheral(_ peripheral: CBPeripheral, didUpdateValueFor characteristic: CBCharacteristic, error: Error?) {
...
    if let characteristicValue = characteristic.value {
        let psm = characteristicValue.withUnsafeBytes {
            $0.load(as: UInt16.self)
        }
        print("using psm \(psm) for l2cap channel!")
        peripheral.openL2CAPChannel(psm)
    }   
}

and as soon as you are notified of the opened channel, open the output stream on it:

func peripheral(_ peripheral: CBPeripheral, didOpen channel: CBL2CAPChannel?, error: Error?) 
 ...
    channel.outputStream.delegate = self.streamDelegate
    channel.outputStream.schedule(in: RunLoop.current, forMode: .default)
    channel.outputStream.open()

Your supplied stream delegate might look like this:

func stream(_ aStream: Stream, handle eventCode: Stream.Event) {
    switch eventCode {
    case Stream.Event.hasSpaceAvailable:
        if let stream = aStream as? OutputStream, let imageData = self.imageData {
            if self.bytesToWrite > 0 {
                let bytesWritten = imageData.withUnsafeBytes {
                    stream.write(
                        $0.advanced(by: totalBytes),
                        maxLength: self.bytesToWrite
                    )
                }
                self.bytesToWrite -= bytesWritten
                self.totalBytes += bytesWritten
                print("\(bytesWritten) bytes written, \(bytesToWrite) remain")
            } else {
                NSLog("finished")
            }
        }
    case ...

There is a cool WWDC video from 2017, What's New in Core Bluetooth, see here https://developer.apple.com/videos/play/wwdc2017/712/

At around 14:45 it starts to discuss how L2Cap channels are working.

At 28:47, the Get the Most out of Core Bluetooth topic starts, in which performance-related things are discussed in detail. That's probably exactly what you're interested in.

Finally, at 37:59 you will see various possible throughputs in kbps. Based on the data shown on the slide, the maximum possible speed with L2CAP + EDL (Extended Data Length) + 15ms interval is 394 kbps.