# Adaptive Multi-Channel Clustering in IEEE 802.11s Wireless Mesh Networks +
## 6 Experimental Validation
### Assessment of Online Cluster Adaptation
#### 6.3.2 Addition and Relocation of Nodes  
Further experiments were dedicated to a network maintenance scenario in which a combination of the mechanisms for dynamic cluster adaptation was triggered. We investigated the addition (subsequent installation) of a node into an existing cluster constellation. First, it required automatic cluster joining of the node according to the concept in Section 4.4.1. This was followed by a (virtual) relocation of the node until its connection to the original CH was lost. Isolation detection (concept in Section 4.4.2) on the node then resulted in joining a new cluster, which in turn caused cluster balancing by roaming of other nodes (concept in Section 4.4.4). In the context of urban mesh backbones, addition and relocation of nodes are expected measures, e.g., to increase coverage or resilience of the network. During our experiment, the new node was permanently placed next to the test environment. Unlike the other devices, its antennas were not equipped with attenuators, allowing a direct connection to any other node. However, we blocked link establishment to all but one of the nodes 1-5 in Figure 21 via Linux WLAN interface configuration. Starting at node 1, the virtual relocation of the additional node along the bottom edge of the network was done by blocking the previous link and allowing the next one, respectively. Analogous to the broadcast messages of the CHs, the additional node sent its NH2 CH messages to the control PC at 2 s intervals both in the mesh network and via Ethernet. Based on the information about which of the nodes 1-5 currently served as next hop to the CH, the position of the \"relocated\" node was updated in the visualization software. Figure 21 shows the results of an experiment execution. The most frequently formed cluster constellation of the \(5\times 5\)-node grid topology again served as initial configuration. Otherwise keeping parameterization P2, the check for existing clusters in phase 0 was now activated on the additional node. After initialization of the grid, the delayed program start of the additional node was preceded by an automatic balancing process between the green and yellow clusters. However, the balancing between the red and violet clusters, which was also pending, had not yet taken place. When starting, the additional node already had a connection to the network via node 1. After evaluating the incoming CH announcements  
Figure 21: Addition and virtual relocation of a node (**a**: added node joined purple cluster, **b.c**: virtual relocation, **d**: node joined green cluster, **e.f**: virtual relocation and balancing). Reprinted from ref. [18]. in phase 0, it chose to join the purple cluster as its CH had the smallest ALM distance. This created an initially balanced cluster constellation. Second 0 in Figure 21 shows the time when the additional node had successfully joined the network, and its position and cluster membership were represented in the visualization according to the received CH and NH2 CH messages. From this point on, the control PC performed the sequential \"relocation\" of the node. Switching the connection to the next of the nodes 1-5 in each case was done script-based in intervals of 30 s.  
In second 50, i.e., 20 s after link handover to node 2, the position change appeared in the visualization. This time delay is partly due to the parameterization of the prototype and the 802.11s link layer, and partly due to the delivery probability of 802.11s management and control frames. According to the CHaChA concept, CHs use the proactive mode of the 802.11s routing protocol HWMP, so that all nodes always maintain path information to them. The sending interval of the proactive PREQ frames for the path update is configurable via the 802.11s parameter HWMProotInterval. For the purposes of this work, its default value of 5 s was kept (see [1, 47] for an overview of 802.11s parameters).