Non-IP Modem Codecs for Node.JS

A Node.JS library for decoding and encoding efficient binary blob messages for Non-IP Modem data transport over constrained satellite networks.

NIMO

Non-IP Modem for ORBCOMM protocols is derived from a proprietary codec key file described by XML or JSON. The principle is the first 2 payload bytes comprise a message key describing services (aka SIN) made up of messages (aka MIN). Messages are defined for both uplink (aka Mobile-Originated) and downlink (aka Mobile-Terminated) operations.

Using this approach we can define up to 256 Service(s) each with up to 256 Message(s) in each direction of communication. The intent is that each micro-service encapsulates a set of related remote operations, and the messages represent the individual downlink (command/query) or uplink (response/report) operations.

[!NOTE] When using actual ORBCOMM-protocol based modems (IDP/OGx service) SIN 0-15 are reserved for system use, and SIN 16-127 are pseudo-reserved for ORBCOMM Lua apps.

[!TIP] The key feature of NIMO messages is that data field boundaries do not need to align with byte boundaries.

Each message is defined by a set of attributes:

• name should be unique to the direction in the service
• messageKey (aka MIN) must be unique to the direction in the service
• description is optional
• fields Defines a set of ordered data types in the message payload, where each field includes attributes:
  • name must be unique within fields
  • description is optional to provide additional context
  • type must be one of the supported data types:
    • bool are 1-bit values
    • enum define a set number of bits to index discrete string values
    • int define a set number of bits for signed integer (MSB sign)
    • uint define a set number of bits for unsigned integer
    • string define a set or maximum number of ASCII character bytes
    • data define a set or maximum number of bytes
    • array define a set or maximum number of nested child fields
    • bitmasklist define a set number of bits to index array fields
  • size is used for types: enum, int, uint to represent the number of bits of payload used. It repesents the maximum number of bytes of payload used for string or data. It represents the maximum number of fields used for array or bitmasklist. It is not used for bool.
  • items is a list required for enum or bitmasklist types.
  • fields is a list required for array and bitmasklist types, defining nested fields.
  • optional is optional for any type; flags the field as being present or not, and consumes 1 bit of payload to indicate presence.
  • fixed is optional for variable length fields string, data, array to reserve/pad the maximum data length to a specific payload size.

The XML format was historically uploaded to the Viasat/Inmarsat or ORBCOMM/SkyWave web service API gateways, typically with extension *.idpmsg. Some field types may not be supported on some gateways.

The JSON format provides a condensed version and supports an extended set of field types. The codec library is intended to prefer JSON representation for processing external to the service provider API gateways.