# Adaptive Multi-Channel Clustering in IEEE 802.11s Wireless Mesh Networks +
## 4 The CHaChA Clustering Approach
### Initial Clustering and Channel Assignment
The CHaChA channel selection sequence is started by the MCH, claiming the first channel from the pool. It then adds the value pair of its own MAC address and chosen channel to the payload of a CHAN_SEL unicast message and sends it to the closest CH in termsof ALM path cost. This process is repeated by each subsequent CH until all CHs have claimed a channel. If the channel pool is exhausted before reaching the last CH of the sequence, channels are used multiple times following a classical greedy approach. To avoid interference between neighboring clusters, a CH reuses the channel of the CH in largest ALM distance. The last CH in the sequence sends a final message containing all MAC address/channel value pairs back to the MCH, which now triggers the transition to phase 6 via PHASE_6 messages. Figure 4e shows the intermediate result at the end of phase 5 in the example network. In the current concept, channel selection is based solely on the predefined channel pool, and no external WLAN cells are considered yet. In practice, CHs could also perform an active search for surrounding networks using 802.11 probe-request messages and preferentially select orthogonal channels. In addition, the concept so far focuses on channel selection for secondary interfaces that are used for communication within the mesh network. However, the allocation of additional free interfaces as Access Points (APs) to connect conventional WLAN clients without mesh capability could be done in a similar way. In the simplest case, the selected cluster channels could be shared by a virtual AP interface that is additionally created on the physical secondary interface. Alternatively, based on knowledge of current cluster channels and distance to other clusters, orthogonal channels could preferably be chosen for additional physical AP interfaces. With the end of phase 5, the periodically transmitted CH broadcast messages contain the cluster channel and the list of CMs associated with the cluster, in addition to the cluster's mesh ID. The channel information is needed by CMs to configure their secondary interfaces. In turn, the number and constellation of participants are possible criteria for later cluster balancing. Thus, CMs may preferentially move to underpopulated neighboring clusters in the course of roaming activities (see Section 4.4.4). **Phase 6--Cluster Interface Configuration:** In phase 6, all CMs configure their secondary WLAN interface according to the parameters announced for their respective cluster, enabling communication on the cluster channel. Figure 4f shows the final result after phase 6 in the example network. After configuring their secondary interfaces, all nodes switch directly to phase 7. As on the primary interface, the CHs now activate the proactive HWMP mode on the secondary interface. Unlike on the base channel, however, here they use the mode variant \"proactive PREQ with PREP\", which causes bidirectional path creation. As a result, path information is maintained by CMs and their CH, and node failures can be mutually detected by checking the presence of path entries. **Phase 7--Distributed Cluster Operation:** When reaching phase 7, the clustering procedure is complete. Network services and applications that can be appropriately mapped to different clusters benefit from higher performance as they communicate in parallel on orthogonal channels and in smaller broadcast domains, compared to the base channel. At the same time, unrestrained connectivity between the clusters is always guaranteed via the primary interface. By assigning applications to independent clusters, the load on the base channel can also be reduced, creating additional capacity for inter-cluster data exchange. At the application level, it should be noted that the primary and secondary interfaces of the mesh nodes operate in separate 802.11s networks, configured as different logical subnets with own IP address ranges. The current concept of CHaChA does not dictate how the configuration of these address ranges and the binding of applications should take place.