We are excited to introduce the LoRaWAN – FreeRTOS Labs Project, a reference implementation of LoRaWAN connectivity with FreeRTOS. This project is meant to demonstrate how FreeRTOS can simplify the development of IoT applications using LoRa technology. LoRa is a long range and low-power wireless technology operating in the unlicensed spectrum. It is designed for sensors that need to send infrequent, small amounts of data over long distances in a variety of environments. LoRa is the physical layer spread spectrum modulation technique derived from chirp spread spectrum (CSS) wireless communication technology. LoRaWAN is the network system architecture and communication protocol specification for LoRa, developed by the LoRa Alliance.
LoRaWAN Network Architecture
The LoRa Alliance defines the networking layer and the system architecture for the network. A LoRaWAN connectivity flow is depicted in Figure 1, showing the different components that connect end nodes to the cloud and to application servers.
Application server (AS) is responsible for securely handling, managing and interpreting sensor application data. It also feeds the data to drive dashboard UI.
The Join Server (JS) manages over-the-air activation (OTAA) process for end devices to be added to the network. The JS performs the Join procedure (mutual authentication) with end device and notifies the LNS which AS should the end device connect to and performs the session keys derivation. Each JS is identified by a 64-bit globally unique identifier, AppEUI/JoinEUI. The end device derives session keys locally, based on the DevEUI, Join EUI, DevNonce, root keys (AppKey, NwkKey), such that security keys aren’t exchanged over the air. JS communicates to LNS the Network Session Key and to AS the Application Session Key. Individual root keys are securely stored on the end devices, and equivalent matching keys are securely stored on the JS. As such, JS contains the following information for each end-device under its control:
- DevEUI (end-device serial unique identifier)
- Root Keys
- AppKey (application encryption key)
- NwkKey (network encryption key)
- Application Server identifier
- End-Device Service Profile
In Activation by Personalization (ABP) activation procedure, a simplified and less secure commissioning process, the Join procedure is skipped and the IDs and keys are personalized at the time of fabrication. End devices are tied to a specific network/service and and the network identifier (NetID) is part of the device network address such that end devices become immediately functional upon powering up.
LoRaWAN specification defines three types of end devices: Class A, Class B and Class C. Class A device spends most of the time in Idle state (i.e. power save mode). When there are changes to sensor environment the end device is monitoring, the device wakes up and initiates an uplink, transmitting (Tx) the sensor reading to the LNS. The end device then listens for the response from LNS, it opens up to two receive (Rx) windows at specified times (1s and 2s) after the last uplink transmission (Tx). The LNS can send the downlink in either of the Rx windows. Class B extends the Class A Rx functionality by adding scheduled Rx windows for downlink messages from LNS. This is done by using time-synchronized beacons transmitted by the gateway, indicating the end device to periodically open Rx windows. Class C further extends Class A by keeping the receive (Rx) window, always on. Class C devices do not depend on battery power and are always listening for downlink messages, except when they are transmitting (Tx) uplink. As a result, Class C offers lowest latency for communication between the LNS and end-device. For detailed description of LoRaWAN 1.0.3, see here.
LoRaWAN stack in FreeRTOSFreeRTOS offers several benefits for embedded software developers. Read more.
LoRaWAN Stack in FreeRTOS, as shown in Figure 2, uses a LoRaWAN management task to abstract away the timing constraints for a LoRaWAN network and thereby providing a simpler programming model for applications. It separates the concern of processing radio interrupts and MAC layer events from the user application. This allows the application task to be more modular with a sequential flow of execution. Also this allows having well defined helper APIs that can be reused across different applications – for example blocking APIs for joining a network, sending and receiving raw data. There are also helper functions for configuring and retrieving the join credentials, if the device does not support secure provisioning. For the current release, only Class A capabilities are supported. Class B & C will be supported in future releases along with the ability for applications to switch between different classes of operation. Stack also exposes the radio Hardware Abstraction Layer (HAL) which provides applications the ability to control LoRa radio or modem directly. The HAL can be implemented for one or more LoRa radio transceivers or LoRa radio modems.
LoRaWAN stack in FreeRTOS uses the LoRaMac-Node, which is Semtech’s open source implementation of the LoRaWAN end device protocol specification. The stack implements version 1.0.3 of LoRaWAN specification. For more details on LoRaMac-Node visit the Semtech’s documentation. Source code for FreeRTOS support for LoRaWAN is available here.
Developing LoRaWAN IoT Applications
In LoRaWAN communication, both uplink and downlink messages can be either confirmed (an ACK is required from the other party) or unconfirmed (no-ACK required). The following diagrams Figure 3a and 3b shows the sequence of confirmed uplink and downlink. For detailed explanation, refer to Section 18 of LoRaWAN 1.0.3 specification.
The FreeRTOS stack spawns a Class A application task which sends an uplink message periodically following the link fair access policy defined by the LoRaWAN regional parameters. Uplink messages can be send either in confirmed or unconfirmed mode. After a successful uplink, the task waits for any downlink messages or other events from the MAC layer. It writes the frame payload and port received downlink to the console. If MAC layer indicates that there are further downlink messages pending or an uplink needs to be sent for control purposes, then it sends an empty uplink immediately. If a frame loss is detected by MAC layer, then it triggers a re-join procedure to reset the frame counters. Once all downlink data and events are processed, the task goes back to sleep until the next transfer cycle.
All events from MAC layer to application are sent using light weight task notifications. LoRaWAN allows multiple requests for the server to be piggy-backed to an uplink message. The responses to these requests are received by application in order using a queue. A downlink queue exists in case application wants to read multiple payloads received at once, before sending an uplink payload.
Low Power ModeAn important feature of Class A based communication is that application sleeps for most part of its lifecycle thereby consuming less power. Low power mode can be enabled for the demo using FreeRTOS tickless idle feature as described here. Tickless idle mode can be enabled by providing a board specific implementation for
portSUPPRESS_TICKS_AND_SLEEP() macro and setting
configUSE_TICKLESS_IDLEto the appropirate value in
FreeRTOSConfig.h. Enabling tickless mode allows MCU to sleep when the tasks are idle, but be waken up by an interrupt from the radio or other timer events.
|Vendor||MCU||LoRa Radio Shield||IDE|
Gateway hardware needed to build and test FreeRTOS LoRaWAN applications:
The LoRaWAN FreeRTOS Lab project provides a reference implementation of a LoRaWAN node using FreeRTOS to simplify development of low power and long range IoT applications using LoRa technology . You will find the full application source code and a detailed API reference for the LoRaWAN manager in the FreeRTOS LoRaWAN repository. We encourage your contributions to expand the catalog of supported platforms and application examples of this exciting technology.
FreeRTOS is an MIT licensed open source, real-time operating system for microcontrollers that makes small, low-power edge devices easy to program, deploy, secure, connect, and manage. You can get started by downloading source code from FreeRTOS.org or GitHub (https://github.com/freertos/freertos) and can find more information about the FreeRTOS, its libraries and demos on the FreeRTOS User Guide.
- Gaurav Gupta (Sr. Partner Solutions Architect – AWS IoT)
- Paul Butler (Sr. Partner Solutions Architect – AWS IoT)
- Ravishankar Bhagavandas (Software Development Engineer – AWS IoT)