SenML

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SenML

Introduction on the SenML data representation

SenML is the default data format for device data in KPN Things. Currently KPN Things only supports SenML in JSON. We plan to support SenML in CBOR in the future.
Introduction
A SenML message is officially called a SenML pack. One SenML pack consists of one or more SenML records. One SenML record expresses a single measurement. That way a SenML pack expresses a set of measurements
The first record in a SenML pack always contains base information. Base information expresses from which device the measurements are from (base name) and the moment the measurements were taken (base time). 
There are different type of SenML records:
Number record - for communicating a number measurement
Boolean record - for communicating an on/off measurement
String record - for communicating a string measurement
Data record - for communicating raw data measurement
Below you find a non-exhaustive example of a SenML pack:
SenML example (explanation below)
[
  {"bn": "urn:dev:DEVEUI:0123456789ABCDEF:", "bt": 1.58565075E9},
  {"n": "temperature", "v": 20.5, "u": "Cel"},
  {"n": "batteryVoltage", "v": 3.6, "u": "V", "t": 10},
  {"n": "active", "vb": true},
  {"n": "modus", "vs": "Active"},
  {"n": "image", "vd": "aGFsbG9vb29vISE="}
]
Line nr.
Explanation
2
First line with base attributes, stating measurement source and timestamp.
3
Number record
4
Number record with relative time
5
Boolean record
6
String record
7
Data record
Attributes
This section explains the attributes you can use to express your measurements in SenML
Base name - bn - required in pack
The base name of a SenML pack states which device the measurements are from. It should contain a unique resource name. The parts of the base name, which are colon separated, are as following:
urn stands for unique resource name.
dev means we are identifying a device.
DEVEUI is the type of device identifier used to identify the device.
01234567890ABCDEF is the DevEUI, the device identifier.
In the example above we identify a LoRa device by its DEVEUI. If we would have an M2M device, we would identify it by its IMEI and the base name would be as following: urn:dev:IMEI:0123456789012345:.
A base name always ends with a colon :!
Base time - bt - optional in pack
The timestamp of the measurement in UNIX timestamp format. If KPN Things receives a measurement without base time, the time of receiving the measurement is taken as base time. SenML being sent by KPN Things will always contain a base time.
Name - n - required in record
Name of the individual measurement.
 The
 (..) name MUST consist only of characters out of the set "A"
 to "Z", "a" to "z", and "0" to "9", as well as "-", ":", ".", "/",
 and "_"; furthermore, it MUST start with a character out of the set
 "A" to "Z", "a" to "z", or "0" to "9".
- The SenML spec, RFC8428 section 4.5.1​
When designing your SenML measurement format for M2M and Internet devices, you should take into account our Common measurements list. By using as much name and unit pairs from that list, you achieve higher compatibility with current and future data processing features.
Unit - u - optional in record
Unit of the measurement. Write your u-values in camel case, so start with a lower case character.
Time - t - optional in record
The timestamp of an individual record relative to the base time in seconds.
Example
You have a SenML pack with "bt": 1600185600 (2020-09-15T16:00:00+00:00) and the pack contains a measurement with "t": -60.
The timestamp of that measurement is then 2020-09-15T15:59:00+00:00, so one minute earlier then the base time.
Number value - v - required in number record
Value of the number measurement.
Boolean value - vb - required in boolean record
Boolean value for a boolean measurement. Should be true or false.
String value - vs - required in string record
String value for a string measurement. 
Data value - vd - required in data record
Data value for a data measurement. Should be a base64 encoded string.
And more...
There are more attributes in the SenML specification, but these values are not used by KPN Things. You can read about them in the Sensor Measurements Lists specification:
RFC 8428 - Sensor Measurement Lists (SenML)
RFC 8428 - Sensor Measurement Lists (SenML)
Client libraries
For easier integration with KPN Things we have created client libraries in C and Python for you to use in your devices.
Library Reference
Client libraries for SenML in JSON and CBOR format
ThingsML, a SenML extension with extra compression
To make SenML more suitable for communication over LoRa, we have introduced ThingsML. ThingsML is a valid extension of SenML that introduces measurement indices. We have listed several common measurements, allowing you to only send the measurement index number instead of sending the complete name and unit of the measurement. Learn more about ThingsML.