Friend Node

What Is a Friend Node?

A Friend Node is a mains-powered Bluetooth Mesh node that provides message-caching services for one or more associated Low Power Nodes (LPNs). The friendship mechanism allows battery-powered nodes to sleep for extended periods while still receiving mesh messages, because the Friend Node stores incoming messages and delivers them when the LPN wakes up and polls.

Friendship Establishment

The friendship is initiated by the LPN through a Friend Request message. The request includes:

• Receive Delay -- the minimum delay between the LPN's poll and the Friend's response (10 -- 255 ms). This gives the LPN time to switch from transmit to receive mode.
• Poll Timeout -- the maximum time between consecutive polls (1 -- 345,600 seconds). If the LPN does not poll within this window, the friendship is terminated.
• Receive Window -- how long the Friend Node must listen for the LPN's poll.

Nearby Friend-capable nodes that hear the request evaluate whether they can fulfill the requirements (sufficient cache, acceptable queue size) and respond with a Friend Offer. The LPN selects the best offer based on RSSI, receive window size, and available cache, then confirms the friendship with a Friend Poll.

Message Caching

Once friendship is established, the Friend Node subscribes to the LPN's unicast address and all group/virtual addresses the LPN is subscribed to. Incoming messages destined for the LPN are stored in the Friend Queue instead of being delivered immediately. The queue has a finite size (implementation-dependent, typically 8 -- 32 messages). When the queue is full, the oldest message is evicted.

Polling Mechanism

The LPN wakes periodically and sends a Friend Poll message. The Friend Node responds with the next cached message (or an empty response if the queue is empty). The LPN continues polling until the Friend responds with the "More Data" flag cleared, indicating the queue is empty. The LPN then returns to sleep.

Power Savings

The friendship mechanism enables dramatic power savings. Without a Friend, a mesh node must keep its radio active to receive advertising-bearer messages continuously. With a Friend, the LPN activates its radio only during polling windows, which can be as infrequent as once every few minutes. A typical LPN polling every 10 seconds with a 100 ms receive window achieves a radio duty cycle of approximately 1%, extending a CR2032 coin-cell battery life to several years.

Security

Friend messages are encrypted at both the network and application layers, just like regular mesh messages. The Friend Node can decrypt the network layer (it shares the NetKey) but cannot read application payloads encrypted with AppKeys it does not possess. The friendship itself is secured through a Friend Security Material derived from the shared NetKey, LPN address, Friend address, and counters exchanged during establishment.

Practical Deployment

Friend Nodes are typically deployed alongside LPNs in a 1:N ratio. A single mains-powered light fixture or smart plug can serve as a Friend to 3 -- 5 nearby sensors. The provisioner should configure friendships based on physical proximity and expected message volume to avoid overloading any single Friend's cache.