Sensor Object Library
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README.md

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This repo contains a set of libraries to manipulate sensor Objects.

An sensor Object contains the following conceptual fields:

  • M: MAC address of the device creating the Object
  • T: timestamp of when the Object was created
  • t: type of Object, a number
  • L: the length of the value
  • V: Object value, a opaque string of bytes

We refer to this format as the MTtlv format. It is a generalization of the well-known "Type-Length-Value" (TLV) format.

Installation

Download source: git clone https://github.com/realms-team/sol.git

Code documentation

http://realms-sol.readthedocs.io

Registry

Objects' "types" registry

See registry.

Representations

The SOL Objects are manipulated in groups. Each group of Objects can be represented in binary, JSON or HTTP format.

Binary representation

This representation is used for:

  • sending data in packets
  • writing data to a file

Each group of Objects consists of the following fields:

| SOL header | Objects list |

Some rules:

  • all multi-byte value are encoded "big endian" (a.k.a "network order")
  • when saving Objects in a binary file, each Object MUST be framed using HDLC.

SOL Header

 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| V |T|M|S|Y| L |
+-+-+-+-+-+-+-+-+
  • V: Version of the Object:
    • Only value b00 is defined in this document. Other values for the 2 first bits are reserved and may be defined in later revisions of this document.
  • T: Type of MTtlv Object:
    • 0: single-MTtlv Object
    • 1: multi-MTtlv Object (MTNtlv) this implies the 1st byte next to timestamp is N: number of Objects
  • M: MAC address encoding:
    • 0: no MAC address present
    • 1: 8-byte MAC address present
  • S: timestamp encoding
    • 0: timestamp is a 4-byte Linux epoch in UTC (1-second granularity)
    • 1: timestamp is elided, and recovered from the timestamp field present in the SmartMesh IP header
  • Y: type encoding
    • 0: 1-byte type field
    • 1: 2-byte type field
  • L: length encoding
    • b00: use well-known value. No length field present
    • b01: 1-byte length field present
    • b10: 2-byte length field present
    • b11: elided. The length is recovered from the length of the packet or HDLC frame.

Object List

According to the header flags, the Object list structure can vary.

  • If the T flag is 0 then the message will have the following structure:
    || SOL Header || MT | tlv |
  • If the T flag is 1 then the message will have the following structure:
    || SOL Header || MT | N | tlv | tlv | ...

Example transmission use cases

Example 1. transmitting a single 2-byte temperature sensor reading, taken in the past:

  • [1B] SOL Header
    • V=00 (version 0)
    • T=0 (Type of MTtlv Object)
    • M=0 (no MAC address)
    • S=0 (epoch)
    • Y=0 (1-byte type)
    • L=b00 (well-known value, no length field)
  • [--] MAC: elided
  • [4B] Timestamp: 0x........
  • [1B] type=b.. (temperature)
  • [--] length: elided
  • [2B] value: 0x....

Total 8 bytes.

Example 2. transmitting a single 2-byte temperature sensor reading, taken just now:

  • [1B] SOL Header
    • V=00 (version 0)
    • T=0 (Type of MTtlv Object)
    • M=0 (no MAC address)
    • S=1 (elided)
    • Y=0 (1-byte type)
    • L=b00 (well-known value, no length field)
  • [3B] sensor reading 1
    • [--] MAC: elided
    • [--] Timestamp: elided
    • [1B] type=b.. (temperature)
    • [--] length: elided
    • [2B] value: 0x....

Total: 4 bytes.

Example 3. Transmitting 3 sensor readings from 3 different sensors with well-known length, taken at the same time in the past:

  • [1B] SOL Header
    • V=00 (version 0)
    • T=1 (Type of MTtlv Object)
    • M=0 (no MAC address)
    • S=0 (epoch)
    • Y=0 (1-byte type)
    • L=b00 (well-known value, no length field)
  • [--] MAC: elided
  • [4B] Timestamp: 0x........
  • [1B] Number of Objects = 3
  • [3B] sensor reading 1
    • [--] MAC: elided
    • [--] Timestamp: elided
    • [1B] type=b.. (temperature)
    • [--] length: elided
    • [2B] value: 0x....
  • [3B] sensor reading 2
    • [--] MAC: elided
    • [--] Timestamp: elided
    • [1B] type=b.. (RH)
    • [--] length: elided
    • [2B] value: 0x....
  • [3B] sensor reading 3
    • [--] MAC: elided
    • [--] Timestamp: elided
    • [1B] type=b.. (solar)
    • [--] length: elided
    • [2B] value: 0x....

Total: 15 bytes.

Rules for saving to a binary file

The assumption is that a binary file is stored on some hard/flash drive with orders of magnitude more space than a packet. The driving design choice are hence made to allow:

  • simple parsing
  • recoverable file in case parts of it get corrupted.

The following rules hence apply when saving to a binary file:

  • sensor Object chaining is NOT allowed (except on Neomote SD card level)
  • each sensor Object MUST be framed using HDLC framing (RFC1662)
  • the length field MUST be elided, and the L bit in the start header set to b11

JSON representation

A JSON representation is used:

  • when the Objects are stored in a database
  • when manipulating Objects

We use clean indentation for easier readability in these examples. An efficient implementation SHOULD represent the entire JSON string on a single line.

The following is the general format of a JSON representation of sensor Objects:

{
   "mac":       "00-17-0d-00-00-12-34-56",
   "timestamp": 12345678890,
   "type":      39,
   "value":     {
       'temperature': 0x0a33,
   },
}
  • "mac"
    • represented exactly like in the example above, lowercase hex bytes (exactly 2 characters per byte), separated by -.
  • "timestamp"
    • an integer representing the epoch
  • "type"
    • an integer, per the registry above
  • "value"
    • a dictionary of values

HTTP representation

This representation is used for minimal communication overhead (when data transists).

{
   "v": 0,
   "o": [
       ew0KICAgIm1hYyI6ICAg,
       ICAgICIwMC0xNy0wZC0w,
       ...
       1NiIsDQogICAidGltZXw,
   ]
}
  • v: the version of the representation. Only version 0 is defined in this specification. Other values SHOULD NOT be used. Future revisions of this document MIGHT define further versions.
  • o: an array of representations. Each representation is a string representing the binary representation of one or more sensor Objects.
    • the string MUST be a Base64 encoding of the binary representation of exactly one sensor Objects.