# Adaptive Multi-Channel Clustering in IEEE 802.11s Wireless Mesh Networks +
## 6 Experimental Validation  
### Testbed Setup  
We evaluated our CHaChA prototype in a practical testbed, comprising 25 Intel Galileo single-board computers. The testbed is arranged in a regular \(5\times 5\)-node grid setup, offering a network diameter of four hops and a symmetric center position (node 13). Figure 10 shows the grid geometry and node numbering. Based on our miniaturization approach Mini-Mesh [42], using RF attenuators and a fixed 802.11 data rate and TX power combination, we achieve a substantial reduction of transmission range. As a result, only grid neighbors communicate directly, whereas multi-hop paths are established between more faraway nodes by the 802.11s routing protocol HWMP. This allows us to create indoor multi-hop setups in a scale of approximately 1:560. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Type (Opcode)** & **UC/BC** & **Payload of an Example Message** \\ \hline CENT & BC & “CENT\(|0.0021\)” \\ \hline NC & UC & “NC\(|8\)” \\ \hline PCH & UC & “PCH” \\ \hline WNPR & UC & “WNPR\(|0.1875\)” \\ \hline CH & BC & “CH\(|\)mesh\_id\(|36|00:00:00:00:00:AA|\)...\(|00:00:00:00:00:AF” \\ \hline JOIN & UC & “JOIN” \\ \hline LEAVE & UC & “LEAVE” \\ \hline CHAN\_SEL & UC & “CHAN\_SEL\(|00:00:00:00:A1|36|\)...\(|00:00:00:00:A5|153\)” \\ \hline PHASE\_X & BC & “PHASE\_1” \\ \hline NH2 CH\(|00:00:00:00:00:00:AA” \\ \hline JOIN\_REQ/RESP & UC & “JOIN\_REQ” \(\rightarrow\) “JOIN\_RESP\(|1\)” (ACK) or “JOIN\_RESP\(|0\)” (NACK) \\ LEAVE\_REQ/RESP & “LEAVE\_REQ” \(\rightarrow\) “LEAVE\_RESP\(|1\)” (ACK) or “LEAVE\_RESP\(|0\)” (NACK) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Structure of CHaChA unicast (UC) and broadcast (BC) messages. Reprinted from ref. [18]. In [42], we introduced this scaling approach and validated its applicability via comparative measurements in outdoor setups without RF attenuators. The miniaturized \(5\times 5\)-node grid topology in our lab corresponds to an un-miniaturized wireless mesh backbone covering an area of more than 300,000 m\({}^{2}\). The used topology mimics, e.g., a city-district mesh network where backbone nodes are installed on rooftops or streetlight poles in a grid-like manner. The regular layout of such a grid topology also simplifies expectations in terms of node neighborhoods and resulting cluster constellations. Our device configuration is given in Table 7. Besides an on-board Ethernet interface, attached to a wired control network for test automation, each node is equipped with a dual-antenna 802.11n mPCIe card. All cards operate at a fixed 802.11 data rate and start on a default channel, not used otherwise in our institute building. In [42], we conducted a general performance assessment of our device platform (Intel Galileo Gen. 1, 400 MHz single-core CPU) for the 802.11n HT data rates MCS 0-15 (i.e., from BPSK 1/2 to 64-QAM 5/6). Generating saturated TCP/UDP traffic with MTU-sized data frames, MCS 3 was determined as the highest data rate that could be fully utilized before our devices ran into CPU performance limitations. Being the highest rate sensibly applicable in our current setup, MCS 3 was therefore also chosen for the evaluation of CHaChA. \begin{table}
\begin{tabular}{l l} \hline \hline
**Parameter** & **Value** \\ \hline Device & Intel Galileo Board (Gen. 1) \\ \hline CPU & Quark X1000 (single-core 400 MHz) \\ \hline RAM & 256 MB DDR3 \\ \hline OS & Debian 8 (Linux Kernel v4.9) \\ \hline
802.11 NIC & Compex WLE200 NX 802.11a/b/g/n (mPCIe) \\ \hline NIC Chipset & Atheros AR9280 (ath9k driver) \\ \hline Antennas & \(2\times 5\) dBi dual-band omni-directional \\ \hline Attenuators & \(2\times\) Mini-Circuits VAT-30+ (30 dB) \\ \hline Default Channel & 149 (5745 MHz, HT20, Long GI) \\ \hline TX Rate & HT MCS 3 (16-QAM 1/2, 26 Mbps) \\ \hline TX Power & 14 dBm (25 mW) \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Testbed configuration. Figure 10: Testbed grid setup and node numbering.