The Things Network Cayenne LPP Support

Uplink Encoding

In my applications the myDevices Cayenne Low power payload(LPP) uplink messages from my *duino devices are decoded by the built in The Things Network(TTN) decoder. I can also see the nicely formatted values in the device data view.

Downlink Encoding

I could successfully download raw data to the device but I found that manually unpacking it on the device was painful.

Raw data

I really want to send LPP formatted messages to my devices so I could use a standard LPP library. I initially populated the payload fields in the downlink message JSON. The TTN documentation appeared to indicate this was possible.

Download JSON payload format

Initially I tried a more complex data type because I was looking at downloading a location to the device.

Complex data type

I could see nicely formatted values in the device data view but they didn’t arrive at the device. I then tried simpler data type to see if the complex data type was an issue.

Simple Data Types

At this point I asked a few questions on the TTN forums and started to dig into the TTN source code.

Learning Go on demand

I had a look at the TTB Go code and learnt a lot as I figured out how the “baked in “encoder/decoder worked. I haven’t done any Go coding so it took a while to get comfortable with the syntax. The code my look a bit odd as a Pascal formatter was the closest I could get to Go.

In core/handler/cayennelpp/encoder.go there was

func (e *Encoder) Encode(fields map[string]interface{}, fPort uint8) ([]byte, bool, error) and func (d *Decoder) Decode(payload []byte, fPort uint8) (map[string]interface{}, bool, error)

Which was a positive sign…

Then in core/handler/convert_fields.go there are these two functions

> // ConvertFieldsUp converts the payload to fields using the application's payload formatter
> func (h *handler) ConvertFieldsUp(ctx ttnlog.Interface, _ *pb_broker.DeduplicatedUplinkMessage, appUp *types.UplinkMessage, dev *device.Device) error {
> 	// Find Application

and

> // ConvertFieldsDown converts the fields into a payload
> func (h *handler) ConvertFieldsDown(ctx ttnlog.Interface, appDown *types.DownlinkMessage, ttnDown *pb_broker.DownlinkMessage, _ *device.Device) error {

Then further down in the second function is this call

var encoder PayloadEncoder
	switch app.PayloadFormat {
	case application.PayloadFormatCustom:
		encoder = &CustomDownlinkFunctions{
			Encoder: app.CustomEncoder,
			Logger:  functions.Ignore,
		}
	case application.PayloadFormatCayenneLPP:
		encoder = &cayennelpp.Encoder{}
	default:
		return nil
	}var encoder PayloadEncoder
	switch app.PayloadFormat {
	case application.PayloadFormatCustom:
		encoder = &CustomDownlinkFunctions{
			Encoder: app.CustomEncoder,
			Logger:  functions.Ignore,
		}
	case application.PayloadFormatCayenneLPP:
		encoder = &cayennelpp.Encoder{}
	default:
		return nil
	}

Which I think calls

// Encode encodes the fields to CayenneLPP
func (e *Encoder) Encode(fields map[string]interface{}, fPort uint8) ([]byte, bool, error) {
	encoder := protocol.NewEncoder()
	for name, value := range fields {
		key, channel, err := parseName(name)
		if err != nil {
			continue
		}
		switch key {
		case valueKey:
			if val, ok := value.(float64); ok {
				encoder.AddPort(channel, float32(val))
			}
		}
	}
	return encoder.Bytes(), true, nil
}

Then right down at the very bottom of the call stack in keys.go

func parseName(name string) (string, uint8, error) {
	parts := strings.Split(name, "_")
	if len(parts) < 2 {
		return "", 0, errors.New("Invalid name")
	}
	key := strings.Join(parts[:len(parts)-1], "_")
	if key == "" {
		return "", 0, errors.New("Invalid key")
	}
	channel, err := strconv.Atoi(parts[len(parts)-1])
	if err != nil {
		return "", 0, err
	}
	if channel < 0 || channel > 255 {
		return "", 0, errors.New("Invalid range")
	}
	return key, uint8(channel), nil
}

At this point I started to hit the limits of my Go skills but with some trial and error I figured it out…

Executive Summary

The downlink payload values are sent as 2 byte floats with a sign bit, 100 multiplier. The fields have to be named “value_X” where X is is a byte value.

Dictionary<string, object> payloadFields = new Dictionary<string, object>();
payloadFields.Add(“value_0”, 0.0);
//00-00-00
payloadFields.Add(“value_1”, 1.0);
//01-00-64
payloadFields.Add(“value_2”, 2.0);
//02-00-C8
payloadFields.Add(“value_3”, 3.0);
//03-01-2C
payloadFields.Add(“value_4”, 4.0);
//04-01-90

payloadFields.Add(“value_0”, -0.0);
//00-00-00
payloadFields.Add(“value_1”, -1.0);
//01-FF-9C
payloadFields.Add(“value_2”, -2.0);
//02-FF-38
payloadFields.Add(“value_3”, -3.0);
//03-FE-D4
payloadFields.Add(“value_4”, -4.0);
//04-FE-70

I could see these arrive on my TinyCLR plus RAK811 device and could manually unpack them

The stream of bytes can be decoded on an Arduino using the electronic cats library (needs a small modification) with code this

byte data[] = {0xff,0x38} ; // bytes which represent -2 
float value = lpp.getValue( data, 2, 100, 1);
Serial.print("value:");
Serial.println(value);

It is possible to use the “baked” in Cayenne Encoder/Decoder to send payload fields to a device but I’m not certain is this is quite what myDevices/TTN intended.

myDevices Cayenne with MQTTnet

As I’m testing my Message Queue Telemetry Transport(MQTT) LoRa gateway I’m building a proof of concept(PoC) .Net core console application for each IoT platform I would like to support.

This PoC was to confirm that I could connect to the myDevices Cayenne MQTT API and format the topics and payloads correctly. The myDevices team have built many platform specific libraries that wrap the MQTT platform APIs to make integration for first timers easier (which is great). Though, as an experienced Bring Your Own Device(BYOD) client developer, I did find myself looking at the C/C++ code to figure out how to implement parts of my .Net test client.

The myDevices screen designer had “widgets” which generated commands for devices so I extended the test client implementation to see this worked.

The MQTT broker, username, password, client ID, channel number and optional subscription channel number are command line options.

class Program
{
	private static IMqttClient mqttClient = null;
	private static IMqttClientOptions mqttOptions = null;
	private static string server;
	private static string username;
	private static string password;
	private static string clientId;
	private static string channelData;
	private static string channelSubscribe;

	static void Main(string[] args)
	{
		MqttFactory factory = new MqttFactory();
		mqttClient = factory.CreateMqttClient();

		if ((args.Length != 5) && (args.Length != 6))
		{
			Console.WriteLine("[MQTT Server] [UserName] [Password] [ClientID] [Channel]");
			Console.WriteLine("[MQTT Server] [UserName] [Password] [ClientID] [ChannelData] [ChannelSubscribe]");
			Console.WriteLine("Press <enter> to exit");
			Console.ReadLine();
			return;
		}

		server = args[0];
		username = args[1];
		password = args[2];
		clientId = args[3];
		channelData = args[4];

		if (args.Length == 5)
		{
			Console.WriteLine($"MQTT Server:{server} Username:{username} ClientID:{clientId} ChannelData:{channelData}");
		}

		if (args.Length == 6)
		{
			channelSubscribe = args[5];
			Console.WriteLine($"MQTT Server:{server} Username:{username} ClientID:{clientId} ChannelData:{channelData} ChannelSubscribe:{channelSubscribe}");
		}

		mqttOptions = new MqttClientOptionsBuilder()
			.WithTcpServer(server)
			.WithCredentials(username, password)
			.WithClientId(clientId)
			.WithTls()
			.Build();

		mqttClient.ConnectAsync(mqttOptions).Wait();

		if (args.Length == 6)
		{
			string topic = $"v1/{username}/things/{clientId}/cmd/{channelSubscribe}";

			Console.WriteLine($"Subscribe Topic:{topic}");
			mqttClient.SubscribeAsync(topic).Wait();
			// mqttClient.SubscribeAsync(topic, global::MQTTnet.Protocol.MqttQualityOfServiceLevel.AtLeastOnce).Wait(); 
			// Thought this might help with subscription but it didn't, looks like ACK might be broken in MQTTnet
			mqttClient.ApplicationMessageReceived += MqttClient_ApplicationMessageReceived;
		}
		mqttClient.Disconnected += MqttClient_Disconnected;

		string topicTemperatureData = $"v1/{username}/things/{clientId}/data/{channelData}";

		Console.WriteLine();

		while (true)
		{
			string value = "22." + DateTime.UtcNow.Millisecond.ToString();
			Console.WriteLine($"Publish Topic {topicTemperatureData}  Value {value}");

			var message = new MqttApplicationMessageBuilder()
				.WithTopic(topicTemperatureData)
				.WithPayload(value)
				.WithQualityOfServiceLevel(global::MQTTnet.Protocol.MqttQualityOfServiceLevel.AtLeastOnce)
				//.WithQualityOfServiceLevel(MQTTnet.Protocol.MqttQualityOfServiceLevel.ExactlyOnce) // Causes publish to hang
				.WithRetainFlag()
				.Build();

			Console.WriteLine("PublishAsync start");

			mqttClient.PublishAsync(message).Wait();
			Console.WriteLine("PublishAsync finish");
			Console.WriteLine();

			Thread.Sleep(30100);
		}
	}

	private static void MqttClient_ApplicationMessageReceived(object sender, MqttApplicationMessageReceivedEventArgs e)
	{
		Console.WriteLine($"ApplicationMessageReceived ClientId:{e.ClientId} Topic:{e.ApplicationMessage.Topic} Qos:{e.ApplicationMessage.QualityOfServiceLevel} Payload:{e.ApplicationMessage.ConvertPayloadToString()}");
		Console.WriteLine();
	}

	private static async void MqttClient_Disconnected(object sender, MqttClientDisconnectedEventArgs e)
	{
		Debug.WriteLine("Disconnected");
		await Task.Delay(TimeSpan.FromSeconds(5));

		try
		{
			await mqttClient.ConnectAsync(mqttOptions);
		}
		catch (Exception ex)
		{
			Debug.WriteLine("Reconnect failed {0}", ex.Message);
		}
	}
}

For this PoC I used the MQTTnet package which is available via NuGet. It appeared to be reasonably well supported and has had recent updates. There did appear to be some issues with myDevices Cayenne default quality of service (QoS) and the default QoS used by MQTTnet connections and also the acknowledgement of the receipt of published messages.

myDevices Cayenne .Net Core 2 client
Cayenne UI with graph, button and value widgets

Overall the initial configuration went ok, I found the dragging of widgets onto the overview screen had some issues (maybe the caching of control settings (I found my self refreshing the whole page every so often) and I couldn’t save a custom widget icon at all.

I put a button widget on the overview screen and associated it with a channel publication. The client received a message when the button was pressed

myDevices .Net Core 2 client displaying a received command message

But the button widget was disabled until the overview screen was manually refreshed.

Cayenne UI after button press

I need to revisit how I confirm that the actuator has been set to the desired value and the command completed.

Overall the myDevices Cayenne experience (April 2019) was a bit flaky with basic functionality like the saving of custom widget icons broken, updates of the real-time data viewer didn’t occur or were delayed, and there were other configuration screen update issues.