Node Roles and Types

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Forwarding roles

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

Mesh Extender

A Mesh Extender 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 Mesh Extender
• does not forward packets for other network devices
• can disable its transceiver to reduce power

Device types

Furthermore, nodes comprise a number of types.

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 two types of FTDs:

• Extender-Capable Device (ECD)
• Full End Device (FED)

An Extender-Capable Device can be in either the Mesh Extender or standby ECD role. In the standby role, it's always an End Device (Child).

A FED is always an End Device (Child).

Minimal Thread Device

A Minimal Thread Device does not subscribe to the all-routers multicast address 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 standby ECD is the only node in reach of a new Thread device wishing to join the Thread network, it can upgrade itself and operate as a Mesh Extender:

Conversely, when a Mesh Extender has no children, it can downgrade itself and operate as a standby ECD:

Other roles and types

Thread Leader

The Thread Leader is a Mesh Extender that is responsible for managing the set of Mesh Extenders 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

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.

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

Recap

What you learned:

• A Thread device is either a Mesh Extender (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 standby ECD can promote itself to a Mesh Extender, and vice versa
• Every Thread network partition has a Leader to manage Mesh Extenders
• A Border Router is used to connect Thread and non-Thread networks
• A Thread network might be composed of multiple partitions

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