# 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
In scenarios with higher node count, network diameter, and dynamics, small deviations of the MCH position from the center are expected and acceptable in a heuristic approach. In addition to the clustering result, the average duration and message overhead of the phase sequence were determined. For the generous choice of time constants and thresholds (see parameterization P1 in Table 8), applying the duration estimation according to Section 6.2.1 resulted in an expected time of 130 s. The mean duration measured in the experiments was 135.5 s (standard deviation 0.9 s), only slightly above the estimate. During this clustering period, an average of 9699 (standard deviation 42) TCP messages with a total data size of 1113.7 kB (standard deviation 4.9 kB) were sent, including all TCP ACK and handshake messages for connection setup and teardown, respectively. In addition, our prototype generated an average of 5248 (standard deviation 152) UDP messages with a total data size of 627.8 kB (standard deviation 20.4 kB). All results include the traffic generated by multi-hop message forwarding and any WLAN retransmissions. Figure 13: Cluster constellations in the \(5\times 5\)-node grid for parameterization P1 (cluster channels (“C#”) highlighted in color; occurrence rate given in %). Reprinted with permission from ref. [17]. © 2018 IEEE. The average total TCP/UDP traffic of 1740 kB, generated over a clustering duration of 135.5 s, can be simplified as overhead traffic with an effective data rate of circa 100 kbit/s. It should be noted that each node in the clustering phase sequence generates only a fraction of the total traffic, and thus this traffic is distributed across all links in the network. The higher message count and traffic share for TCP compared to UDP are on the one hand due to its inherently higher protocol overhead (longer header, acknowledged communication, three-way handshake). On the other hand, in our current prototype implementation, individual TCP connections are established and released for each reliable information exchange. A future optimization could be to keep the connections to neighboring nodes open, since TCP is mainly used to communicate with mesh peers (NC, PCH, and WNPR messages). In comparison, UDP-based broadcasts (CENT, CH, and PHASE messages), including their forwarding in the network, resulted in only slightly more than half the number of messages sent with TCP. This also corresponds to the amount of UDP data generated. However, the number of individual transmissions required to disseminate broadcast information is highly dependent on the network topology and error probability [46]. The UDP traffic share could therefore increase in larger networks. In the CHaChA concept (see Section 4.2), several design decisions were made to keep communication overhead low even in larger mesh topologies. Metrics are exchanged using TCP unicast messages only between neighboring nodes and exclusively during the clustering phase sequence. After initial cluster formation in phase 7, mesh peers only exchange NH2 CH messages, which serve to constrain roaming operations. Roaming of cluster edge nodes may result in occasional transmission of additional TCP messages (LEAVE_REQ/ESP or JOIN_REQ/RESP). In addition, network-wide UDP broadcasts are generated only by CHs, except for CENT messages during MCH election in phase 0. After the initial phase sequence, only CH broadcast messages for the announcement of cluster information are sent periodically. The detection of connection problems is possible on all nodes by merely checking local 802.11s link layer information.