Based on the IEEE 802.15.4-2006 standard, Thread uses the specification to define the Medium Access Controller (MAC) and physical (PHY) layers. It operates at 250 Kbps in the 2.4 GHz band.
From a topology point of view, Thread establishes communications with other devices through a border router (usually a Wi-Fi signal in a household). The rest of the communication is based on 802.15.4 and forms a self-healing mesh. An example of such a topology is shown as follows:
Example Thread network topology containing border routers, Thread routers, lead devices, and eligible IoT devices that can combine in the mesh. Interconnections are variable and self-healing.
The following are the roles of various devices in a Thread architecture.
- Border router: A border router is essentially a gateway. In the home network, this would be a communications crossover from Wi-Fi to Thread and forms the entry point to the internet from a Thread mesh running underneath a border router. Multiple border routers are allowable under the Thread specification.
- Lead device: The lead device manages a registry of assigned router IDs. The lead also controls the requests for Router-eligible End Devices (REED) to be promoted to routers. A leader can also act as a router and have device-end children. The protocol for assignment of router addresses is the Constrained Application Protocol (CoAP). The state information a lead device manages can also be stored in the other thread routers. This allows for self-healing and failover in case the leader loses connectivity.
- Thread routers: Thread routers manage the routing services of the mesh. Thread routers never enter a sleep state but are allowed by the specification to downgrade themselves to become a REED.
- REEDs: A host device that is a REED can become routers or a leader. REEDs are not responsible for routing in the mesh unless they are promoted to a router or leader. REEDs also cannot relay messages or join devices to the mesh. REEDs essentially are endpoints or leaf nodes in the network.
- End devices: Some endpoints cannot become routers. These types of REEDs have two other categories that they can subscribe to: full end devices (FEDs) and minimal end devices (MEDs).
- Sleepy end devices: Host devices that have entered a sleep state communicate only with their associated thread router and cannot relay messages.