# Adaptive Multi-Channel Clustering in IEEE 802.11s Wireless Mesh Networks +
## 4 The CHaChA Clustering Approach
### Initial Clustering and Channel Assignment
Since 802.11s already implements mesh routing at the link layer, multi-hop communication of applications is also possible with link-local IPv6 addresses, which can be derived directly from the MAC addresses of the network interfaces [36]. For this reason, we do not discuss further concepts for the IP configuration of the nodes' primary and secondary interfaces. In phase 3, i.e., before cluster joining of the CFNs and subsequent channel selection, the CH announcement messages carry only the mesh ID of the cluster, which corresponds to the MAC address of the respective CH. After the PHASE_4_DELAY waiting period, the MCH initiates the transition to phase 4. **Phase 4-CFN Cluster Joining:** All CFNs determine the closest CH in terms of ALM path cost, join its cluster, and move to the node role CM. Figure 4d shows the intermediate result at the end of phase 4 in the example network. After expiry of the PHASE_5_DELAY waiting time, the MCH initiates the transition to phase 5. CFNs prioritize CHs in their neighborhood when choosing their cluster. Moreover, if the MCH is among them, it is chosen preferentially over other neighboring CHs. This strategy implies advantages for administrative applications, such as decentralized status monitoring of all nodes. Here, the MCH can serve as a synchronization point in a central position, to which a regular transmission of information from the other CHs takes place, thus providing a global network view. This eliminates the need to exchange information from those nodes that already belong to the cluster of the MCH. To join a cluster, a CFN sends a JOIN unicast message to the corresponding CH, which then adds it to its CM participant list. Analogous to the JOIN message, CHaChA defines a LEAVE message and associated handshake messages for negotiating cluster joins and leaves (JOIN/LEAVE request and response, respectively). In combination, these messages implement a coordinated switching (roaming) of CMs between clusters, which is discussed as part of further concepts in Section 4.4. **Phase 5-CH Channel Selection:** In this phase, the CHs select their cluster channels for the secondary interface. This is done by sequentially picking items from a predefined pool of pairwise non-overlapping channels in the 2.4 and 5 GHz frequency bands. For the smallest channel bandwidth of 20 MHz, this results in up to three orthogonal channels in the 2.4 GHz band and 19 channels in the 5 GHz band according to European regulations. Despite higher achievable link data rates of the optional channel bandwidths of 40, 80, and 160 MHz introduced with 802.11n and 802.11 ac, these are deliberately avoided in the CHaChA concept. The use of a large number of narrow channels instead of fewer wide channels facilitates the formation of clusters with independent collision domains, whose data transmissions can take place in parallel, especially with regard to densely populated mesh topologies [12]. In practice, technical and regulatory restrictions may reduce the number of usable channels. In this context, the transmit filters of real WLAN transceivers are generally not able to completely limit the generated signal power to the respective channel bandwidth (e.g., 20 MHz). Therefore, despite maintaining a center frequency spacing of one channel bandwidth, depending on the intensity of the signal power, so-called Adjacent Channel Interference (ACI) between neighboring cells may occur [37; 38]. To effectively avoid this, their frequency separation must be increased or the transmit power reduced. In addition, parts of the 5 GHz band are shared with radar technology and WLAN devices are required to detect and avoid radar signals, which might be intolerable for certain application scenarios. Similarly, there may be differences in frequency band support between different WLAN devices that must be considered when choosing a cluster channel. In this work, we specify the channel pool under the simplifying assumption of a homogeneous network of nodes with the same hardware capabilities.