start of the data-encoding page for docs

Author: gigapod

docs/data_encoding.md

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+
+# LoRaWAN Data Encoding
+
+To achieve incredible low-power and long-range performance, a LoRaWAN end node minimizes the size of data packets sent across the LoRa network. While the size of a data packet is adjustable based on specific performance needs and network deployment topology, for maximum flexibility, the SparkFun IoT Node - LoRaWAN firmware uses a packet payload size of 11 bytes. This relativity small payload length is a challenge for a general data-logging application and requires that the firmware develop a unique data packing methodology to meet the general data logging goals of the implementation.
+
+## Value Packing Structure
+
+The IoT Node - LoRaWAN firmware employs a simple tagged format to pack and send sensor data across the LoRaWAN network. The key element to this methodology is the ***tag***, which is defined a "Value Type" for the implementation used for sending data. The Value Type tag is followed by the data value, which is formatted for network transport.
+
+The general structure of a packed value is:
+
+ \[ **Value Type** *{1 byte}* ][**Data Value** *{n bytes - network byte order }*]
+
+
+### Value Type
+
+Key attributes of a "Value Type" tag/code:
+
+* There is a common set of "Value Type" code IDs - each code identifies a value, which is defined as a specific measured value/phenomenology and its associated units.
+* The "Value Type" codes are known by both sender and receiver of a data value
+* The length of a data value is defined by it's Value Type.
+  * Data types supported: *unit8, uint16, unit32, int8, int16, int32, float and double*
+
+### Data Formatting
+
+When a data value is "packed", for multi-byte values, the data is encoded for network endianness (big-endian format).
+
+The following table outlines how a data value is packed, based on it's data type size:
+
+| Data Type | Length (bytes) | Format Used |
+|--|--|--|
+| int8   | 1 | No format applied |
+| int16  | 2 | Network Byte Order (2 bytes) |
+| int32  | 4 | Network Byte Order (4 bytes) |
+| uint8   | 1 | No format applied |
+| uint16  | 2 | Network Byte Order (2 bytes) |
+| uint32  | 4 | Network Byte Order (4 bytes) |
+| float  | 4 | Network Byte Order (4 bytes) |
+| double  | 8 | \[ Network Byte Order (4 bytes) ][ Network Byte Order (4 bytes) ] |
+

mkdocs.yml

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 nav:
   - Getting Started:
     - Introduction: introduction.md
+    - Data Encoding: data_encoding.md
   - Support:
     - Troubleshooting: troubleshooting.md
     - Submit Issues: github/file_issue.md