Wireless Networking

We now will utilize Tock's network capabilities to connect our board to a Thread network.

Before starting to develop this app, please remove any existing apps from the board using the following tockloader command:

$ tockloader erase-apps

Importantly, be sure you flash the correct kernel image by running

$ make install

in the Tock boards/tutorials/nrf52840dk-thread-tutorial directory.

Background

IEEE 802.15.4

To facilitate wireless communication, Tock provides an IEEE 802.15.4 network stack. IEEE 802.15.4 (hence forth abbreviated 15.4) is a physical (PHY) and media access control (MAC) specification that is purpose-built for low-rate wireless personal area networks.

Notable examples of popular wireless network technologies utilizing 15.4 include:

• Thread
• Zigbee
• 6LoWPAN
• ISA100.11a

Tock exposes 15.4 functionality to userspace through a series of command syscalls. Within the kernel, a 15.4 capsule and a 15.4 radio driver serve to virtualize radio resources across other kernel endpoints and applications. To provide platform agnostic 15.4 logic, Tock prescribes a 15.4 radio Hardware Interface Layer (HIL) that must be implemented for each 15.4 radio supported by Tock.

Thread

Thread networking is a low-power and low-latency wireless mesh networking protocol built using a 15.4, 6LoWPAN, and UDP network stack. Notably, each Thread node possess a globally addressable IPv6 address given Thread's adoption of 6LoWPAN (an IPv6 compression scheme). Although we will not exhaustively describe Thread here, we will provide a brief overview and pointers to more in-depth resources that further describe Thread.

Thread devices fit into two broadly generalized device types: routers and children. Routers often possess a non-constrained power supply (i.e., "plugged in") while children are often power constrained battery devices. Children form a star topology around their respective parent router while routers maintain a mesh network amongst routers. This division of responsibilities allows for the robustness and self-healing capabilities a mesh network provides while not being prohibitive to power constrained devices.

Further resources on Thread networking can be found here.

Tock and OpenThread

OpenThread is an open source implementation of the Thread standard. This implementation is the "de facto" Thread implementation.

For a given platform to support OpenThread, the platform must provide:

• IEEE 802.15.4 radio
• Random Number Generator
• Alarm
• Nonvolatile Storage

These functionalities are provided to OpenThread using OpenThread's platform abstraction layer (PAL) that a given platform implements as the "glue" between the OpenThread stack and the platform's hardware.

OpenThread is a popular network stack supported by other embedded platforms (e.g. Zephyr). Typically, the OpenThread PAL is exposed either directly to hardware or links directly to the kernel. Tock faces a unique design challenge in supporting OpenThread as the Tock kernel's threat model explicitly bans external dependencies.

Subsequently, Tock provides an OpenThread port that runs as an application. This provides the added benefit that a bug in OpenThread will not cause the entire system to crash and that a faulting OpenThread app can be recovered and restarted by the Tock kernel without affecting other apps. The libtock-c OpenThread port can be found in the libtock-c/libopenthread directory for further details. libopenthread directly checks out the upstream OpenThread repository and as such possesses the entire set of OpenThread APIs.

Now that we have a primer for OpenThread let's continue our tutorial!

• If you are completing the Temperature Sensor Tutorial click here!
• If you are completing the Encrypted Sensor Data Tutorial click here!