My implementation was “inspired” by the myDevices C/C++ sample code. The first step was to allocate a buffer to store the byte encoded values. I pre allocated the buffer to try and reduce the impacts of garbage collection. The code uses a manually incremented index into the buffer for performance reasons, plus the inconsistent support of System.Collections.Generic and Language Integrated Query(LINQ) on my three embedded platforms. The maximum length message that can be sent is limited by coding rate, duty cycle and bandwidth of the LoRa channel.
public Encoder(byte bufferSize)
{
if ((bufferSize < BufferSizeMinimum) || ( bufferSize > BufferSizeMaximum))
{
throw new ArgumentException($"BufferSize must be between {BufferSizeMinimum} and {BufferSizeMaximum}", "bufferSize");
}
buffer = new byte[bufferSize];
}
For a simple data types like a digital input a single byte (True or False ) is used. The channel parameter is included so that multiple values of the same data type can be included in a message.
public void DigitalInputAdd(byte channel, bool value)
{
if ((index + DigitalInputSize) > buffer.Length)
{
throw new ApplicationException("DigitalInputAdd insufficent buffer capacity");
}
buffer[index++] = channel;
buffer[index++] = (byte)DataType.DigitalInput;
// I know this is fugly but it works on all platforms
if (value)
{
buffer[index++] = 1;
}
else
{
buffer[index++] = 0;
}
}
For more complex data types like a Global Positioning System(GPS) location (Latitude, Longitude and Altitude) the values are converted to 32bit signed integers and only 3 of the 4 bytes are used.
In part 1 & part 2 I had been ignoring the payload_fields property of the Payload class. The documentation indicates that payload_fields property is populated when an uplink message is Decoded.
I used JSON2Csharp to generate C# classes which would deserialise the above uplink message.
// Third version of classes for unpacking HTTP payload
public class Gps1V3
{
public int altitude { get; set; }
public double latitude { get; set; }
public double longitude { get; set; }
}
public class PayloadFieldsV3
{
public double analog_in_1 { get; set; }
public int digital_in_1 { get; set; }
public Gps1V3 gps_1 { get; set; }
public int luminosity_1 { get; set; }
public double temperature_1 { get; set; }
}
public class GatewayV3
{
public string gtw_id { get; set; }
public ulong timestamp { get; set; }
public DateTime time { get; set; }
public int channel { get; set; }
public int rssi { get; set; }
public double snr { get; set; }
public int rf_chain { get; set; }
public double latitude { get; set; }
public double longitude { get; set; }
public int altitude { get; set; }
}
public class MetadataV3
{
public string time { get; set; }
public double frequency { get; set; }
public string modulation { get; set; }
public string data_rate { get; set; }
public string coding_rate { get; set; }
public List<GatewayV3> gateways { get; set; }
}
public class PayloadV3
{
public string app_id { get; set; }
public string dev_id { get; set; }
public string hardware_serial { get; set; }
public int port { get; set; }
public int counter { get; set; }
public bool is_retry { get; set; }
public string payload_raw { get; set; }
public PayloadFieldsV3 payload_fields { get; set; }
public MetadataV3 metadata { get; set; }
public string downlink_url { get; set; }
}
I added a new to controller to my application which used the generated classes to deserialise the body of the POST from the TTN Application Integration.
[Route("[controller]")]
[ApiController]
public class ClassSerialisationV3Fields : ControllerBase
{
private static readonly ILog log = log4net.LogManager.GetLogger(System.Reflection.MethodBase.GetCurrentMethod().DeclaringType);
public string Index()
{
return "move along nothing to see";
}
[HttpPost]
public IActionResult Post([FromBody] PayloadV3 payload)
{
// Check that the post data is good
if (!this.ModelState.IsValid)
{
log.WarnFormat("ClassSerialisationV3Fields validation failed {0}", this.ModelState.Messages());
return this.BadRequest(this.ModelState);
}
log.Info($"DevEUI:{payload.hardware_serial} Payload Base64:{payload.payload_raw} analog_in_1:{payload.payload_fields.analog_in_1} digital_in_1:{payload.payload_fields.digital_in_1} gps_1:{payload.payload_fields.gps_1.latitude},{payload.payload_fields.gps_1.longitude},{payload.payload_fields.gps_1.altitude} luminosity_1:{payload.payload_fields.luminosity_1} temperature_1:{payload.payload_fields.temperature_1}");
return this.Ok();
}
}
I then updated the TTN application integration to send messages to my new endpoint. In the body of the Application Insights events I could see the devEUI, port, and the payload fields had been extracted from the message.
This arrangement was pretty nasty and sort of worked but in the “real world” would not have been viable. I would need to generate lots of custom classes for each application taking into account the channel numbers (e,g, analog_in_1,analog_in_2) and datatypes used.
I also explored which datatypes were supported by the TTN decoder, after some experimentation (Aug 2019) it looks like only the LPPV1 ones are.
AnalogInput
AnalogOutput
DigitalInput
DigitalOutput
GPS
Accelerometer
Gyrometer
Luminosity
Presence
BarometricPressure
RelativeHumidity
Temperature
What I need is a more flexible way to stored and decode payload_fields property..
