Node Roles and Types

Forwarding roles

OT Node Roles

In a Thread network, nodes are split into two forwarding roles:

Router

A Router is a node that:

  • forwards packets for network devices
  • provides secure commissioning services for devices trying to join the network
  • keeps its transceiver enabled at all times

End Device

An End Device (ED) is a node that:

  • communicates primarily with a single Router
  • does not forward packets for other network devices
  • can disable its transceiver to reduce power

Device types

Furthermore, nodes comprise a number of types.

OT Device Taxonomy

Full Thread Device

A Full Thread Device (FTD) always has its radio on, subscribes to the all-routers multicast address, and maintains IPv6 address mappings. There are three types of FTDs:

  • Router
  • Router Eligible End Device (REED) — can be promoted to a Router
  • Full End Device (FED) — cannot be promoted to a Router

An FTD can operate as a Router (Parent) or an End Device (Child).

Minimal Thread Device

A Minimal Thread Device does not subscribe to multicast traffic and forwards all messages to its Parent. There are two types of MTDs:

  • Minimal End Device (MED) — transceiver always on, does not need to poll for messages from its parent
  • Sleepy End Device (SED) — normally disabled, wakes on occasion to poll for messages from its parent

An MTD can only operate as an End Device (Child).

Upgrading and downgrading

When a REED is the only node in reach of a new End Device wishing to join the Thread network, it can upgrade itself and operate as a Router:

OT End Device to Router

Conversely, when a Router has no children, it can downgrade itself and operate as an End Device:

OT Router to End Device

Other roles and types

Thread Leader

OT Leader and Border Router

The Thread Leader is a Router that is responsible for managing the set of Routers in a Thread network. It is dynamically self-elected for fault tolerance, and aggregates and distributes network-wide configuration information.

Border Router

A Border Router is a device that can forward information between a Thread network and a non-Thread network (for example, Wi-Fi). It also configures a Thread network for external connectivity.

Any device may serve as a Border Router.

Partitions

OT Partitions

A Thread network might be composed of partitions. This occurs when a group of Thread devices can no longer communicate with another group of Thread devices. Each partition logically operates as a distinct Thread network with its own Leader, Router ID assignments, and network data, while retaining the same security credentials for all devices across all partitions.

Partitions in a Thread network do not have wireless connectivity between each other, and if partitions regain connectivity, they automatically merge into a single partition.

Note that the use of "Thread network" in this primer assumes a single partition. Where necessary, key concepts and examples are clarified with the term "partition." Partitions are covered in-depth later in this primer.

Device limits

There are limits to the number of device types a single Thread network supports.

Role Limit
Leader 1
Router 32
End Device 511 per Router

Thread tries to keep the number of Routers between 16 and 23. If a REED attaches as an End Device and the number of Routers in the network is below 16, it automatically promotes itself to a Router.

Recap

What you learned:

  • A Thread device is either a Router (Parent) or an End Device (Child)
  • A Thread device is either a Full Thread Device (maintains IPv6 address mappings) or a Minimal Thread Device (forwards all messages to its Parent)
  • A Router Eligible End Device can promote itself to a Router, and vice versa
  • Every Thread network partition has a Leader to manage Routers
  • A Border Router is used to connect Thread and non-Thread networks
  • A Thread network might be composed of multiple partitions