# Adaptive Multi-Channel Clustering in IEEE 802.11s Wireless Mesh Networks +
## 4 The CHaChA Clustering Approach
### Online Cluster Adaptation
#### 4.4.2 Node Isolation and Fault Detection  
After successful completion of the clustering phase flow, nodes are either in the CM or CH role (see state diagram in Figure 5). By checking the status of their mesh interfaces, CMs and CHs can detect their own isolation as well as mutual disconnects. Possible causes for this are manifold and include, for example, dynamic obstacles, weather effects, relocation of devices in the course of network maintenance, or even device failures due to hardware defects or lack of power supply. In addition, CMs may not be able to reach their CH through the secondary interface after initial clustering. Cluster joining in phase 4 already follows the strategy to select the closest CH in terms of ALM. However, due to the simultaneous and individual decision-making of all nodes, it is not guaranteed that they always form coherent groups. Therefore, after configuring the secondary interfaces of the nodes (setting the channel and mesh ID in phase 6), connectivity problems may occur within the clusters. Although the primary interface, statically configured to the common base channel, guarantees the highest possible connectivity, isolation on the cluster channel must still be handled by the mechanisms described below. In all the situations mentioned, there is a loss of link and path information at the 802.11s MAC layer. Here, different constellations exist for the status of the primary interface on the base channel and the secondary interface on the cluster channel, which are handled differently depending on the node role. In Figure 5, the interface condition is highlighted in the corresponding states (green--no connection problems, red--connection failure, blue--arbitrary status). **CM-side handling:** From the perspective of a CM, there are no connectivity problems if it reaches its CH as the anchor point of the cluster via both interfaces. This is guaranteed as long as 802.11s path information to the CH is available, while also assuming that at least one link exists per mesh interface. Since CHs use the proactive HWMP mode both on the base channel and within their cluster, path information to them is periodically updated on all nodes. In the state without connectivity problems, there is the possibility of coordinated switching (so-called roaming) between neighboring clusters with the goal of cluster balancing. These mechanisms are described in detail in Sections 4.4.3 and 4.4.4. If a CM permanently loses the connection to its CH on the cluster or base channel, detected by exceeding a timeout (CONN_TIMEOUT) in the state without path information to the CH, it jumps back to phase 0 as CFN. Here, the mechanisms for reintegration into the network or repeated cluster formation take effect (see Section 4.4.1). Even with a mesh connection in place, CMs can detect a functional failure of their CH by missing CH broadcast messages at the application layer. CHs send these messages as periodic heartbeats and to announce their cluster information. Absence of the messages signals the loss of the CH role, e.g., as a result of software failures above the WLAN MAC layer. In this case, the affected CMs also switch back to phase 0 as CFNs. **CH-side handling:** A CH makes the assumption of error-free operation as long as both of its mesh interfaces are connected. In this state, at least one link each exists to the base and cluster channels, and the CH's main task is to respond to possible changes in its cluster. Join and leave requests of CMs, e.g., as a result of installing new nodes or in the course of roaming, require an update of the cluster information, which the CH announces by means of CH messages in the network. Similarly, the failure of an associated CM is detected if no path information is available to it, despite the proactive HWMP mode on the cluster channel. A CH detects its isolation by exceeding a timeout (CONN_TIMEOUT) in the state without link information.