# Adaptive Multi-Channel Clustering in IEEE 802.11s Wireless Mesh Networks +
## 6 Experimental Validation
### Assessment of Online Cluster Adaptation
#### 6.3.1 Cluster Balancing through Roaming  
In a first set of experiments, the subsequent cluster balancing by autonomous roaming of CMs was investigated. As described in Sections 4.4.3 and 4.4.4, CMs make their roaming decisions based on the current cluster information received via periodic CH broadcast \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Topology** & **Estimated** & **Measured** & **TCP Data** & **\# TCP** & **UDP Data** & **\# UDP** \\  & **Duration [s]** & **Duration [s]** & **Size [kB]** & **Packets** & **Size [kB]** & **Packets** \\ \hline \hline
2 \(\times\) 2-node grid (P2) & 50 & 49.3 & 131.3 & 1131 & 44.2 & 374 \\ \hline
3 \(\times\) 3-node grid (P2) & 50 & 50.5 & 472.8 & 4069 & 105.2 & 835 \\ \hline
4 \(\times\) 4-node grid (P2) & 50 & 52.9 & 1156.6 & 9952 & 191.5 & 1498 \\ \hline
5 \(\times\) 5-node grid (P2) & 50 & 55.6 & 1929.2 & 16,695 & 597.2 & 4133 \\ \hline
5 \(\times\) 5-node grid (P1) & 130 & 135.5 & 1113.7 & 9699 & 627.8 & 5248 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Comparison of clustering duration and communication overhead (# indicates “number of”). Reprinted from ref. [18]. Figure 19: Clustering duration depending on grid size/node count (parameterization P2; data points with standard deviation). Reprinted from ref. [18]. messages. Thereby, only CMs at the cluster edge, which are not connecting their CH to other members of their cluster, are eligible for roaming. To determine this, from phase 7 onward, all CMs exchange NH2 CH unicast messages periodically with their neighbors. These contain information about which neighbor serves as next hop on the mesh path to the CH, so that each CM can check its suitability for a cluster change. In all experiments, the NH2 CH broadcast interval was chosen to be identical to the broadcast period of the CH messages (2 s for parameterization P2). Since our testbed devices were equipped with only one WLAN adapter, only NH2 CH information of the primary interface was exchanged. In this setup, it is theoretically possible that the path to the own CH could also pass through nodes of a neighboring cluster. However, in the example studied, all CMs were in direct proximity to their CH, so that paths were always created without intermediate nodes. Consequently, all CMs at the edge to other clusters were able to perform roaming operations. According to the balancing concept (see Section 4.4.4), CMs at the cluster edge only initiate a roaming request if the size of their cluster is at least 2 nodes above that of a neighboring cluster. In the course of handshaking with old and new CH, simultaneous migrations (within a CH_PERIOD) of multiple CMs from the same source to the same destination cluster are prevented to avoid avalanche effects. Figure 20 shows the course of an experiment in the \(5\times 5\)-node grid, in which the cluster constellation formed most frequently with parameterizations P1 and P2 was given as initial configuration. The five CHs are each marked with a black square, with the MCH in the center of the grid not highlighted individually. As in the offline visualization, all members of a cluster are shown in the same color. Immediately after initialization of the constellation in \"second 0\", the red and yellow clusters were each two nodes larger than the purple and green clusters. Only the central blue cluster already corresponded to the targeted average size of five nodes. In second 3 of the experiment, it became apparent that one CM of the red cluster had switched to the green cluster. Independently, two CMs of the yellow cluster had simultaneously switched to the green and purple clusters, respectively. This was allowed since they were different target clusters, but now resulted in an excess CM in the green cluster. Subsequently, the CM that had previously changed to the green cluster roamed back to the yellow cluster, completing the balancing in second 6. In this example, the initial constellation was already close to the balancing target and only a few roaming steps were needed to align the cluster sizes.