# Adaptive Multi-Channel Clustering in IEEE 802.11s Wireless Mesh Networks +
## 6 Experimental Validation
### Clustering in Static Topologies
#### 6.2.3 Assessment and Reproducibility Analysis  
First, CHaChA was investigated in a test setup with 25 devices (\(5\times 5\)-node grid) with respect to clustering result, duration, and communication overhead. The focus was on functional testing of the clustering phase sequence and analyzing the reproducibility of its results under controlled laboratory conditions. The \(5\times 5\)-node arrangement was deliberately chosen, representing the largest of the mesh topologies considered in this work. Table 8 shows the used parameterization variant P1 of the time constants and thresholds of the CHaChA algorithm. A generous choice of waiting times and broadcast retransmissions was initially intended to ensure robust clustering. The measurement series comprised 50 clustering runs and all results (time required and amount of data sent) were averaged. Figure 13 shows the cluster constellations created in the measurement series. The nodes belonging to a cluster are shown in the same color according to their cluster channel chosen in phase 5. The node roles MCH, CH, and CM are marked as indicated in the legend. As expected for the static test setup, reproducible clustering results were obtained. Overall, only two different constellations emerged. Moreover, constellation 1 (Figure 13a) emerged particularly frequently with an occurrence rate of 94%, i.e., in 47 out of 50 runs. In the formation of this constellation, the centrally positioned node 13 was chosen as MCH in the competition of phase 0 due to its CENT metric. In phase 1, the nodes of the inner \(3\times 3\) grid obtained the PCH role because they each possessed a maximum NC metric with eight neighboring nodes. Among these nine PCHs, as expected, the four corner nodes 7, 9, 17, and 19 won the WNPR metric comparison in phase 2. Together with the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **Value** \\ \hline CENT\_PERIOD & ms & 500 \\ \hline CENT\_THRESH & \# & 20 \\ \hline NC\_PERIOD & ms & 5000 \\ \hline CH\_PERIOD & ms & 5000 \\ \hline CH\_THRESH & \# & 2 \\ \hline PHASE\_DELAY & ms & 10,000 \\ \hline PHASE\_PERIOD & ms & 500 \\ \hline PHASE\_TRIES & \# & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 8: CHaChA parameterization P1 (# indicates integer value without unit). Reprinted from ref. [18]. MCH already chosen in phase 0, five nodes thus received the CH role. After network-wide CH announcement in phase 3, joining of the CMs in phase 4 based on their ALM cost to the CHs already resulted in largely balanced clusters. As shown in Figure 13a, the central cluster of the MCH reached a size of 5 nodes, consistent with the targeted average cluster size for 25 nodes and 5 clusters. The size of the surrounding clusters each differed from this by one node. The roaming mechanism described in Section 4.4.3 would lead to a balancing of cluster sizes after entering phase 7, with one edge node of the red and purple clusters each moving to the orange and green clusters, respectively. Channel selection in phase 5, starting at node 13 (MCH), was done in order of smallest ALM distance to each of the remaining CHs. Sequentially, nodes 13, 17, 19, 7, and 9 took channels 36, 40, 44, 48, and 158 from the predefined pool of orthogonal WLAN channels in the 5 GHz band. After virtual application of the channel configuration in phase 6, all nodes switched to phase 7, thereby completing the cluster formation. The second cluster constellation (see Figure 13b) was formed in only 3 of 50 runs (occurrence rate of 6%). Its main difference from constellation 1 was that node 8 instead of node 13 won the MCH election. This was caused by deviations in the ALM cost between nodes during phase 0. The ALM is part of the calculation of the CENT metric on which the MCH election is based. The formation of different clusters was then caused by the shifted MCH position, as CMs preferentially join the cluster of the MCH if it is in their neighborhood.