# Adaptive Multi-Channel Clustering in IEEE 802.11s Wireless Mesh Networks +
## 3 State of the Art and Related Works
However, depending on the availability of practical interfaces for information exchange, there may be greater challenges with regard to implementing the methods and integrating them into existing systems. Figure 2: Classification of channel selection approaches acc. to [PERSON] et al. [12] (attributes of own work CHaChA highlighted in blue). Reprinted from ref. [18]. The focus of our own work CHaChA lies on a group-based channel selection in the form of a clustering approach, which simultaneously pursues the goal of application distribution and more efficient utilization of the frequency spectrum. The decentralized, semi-dynamic, and peer-oriented approach is implemented at the application layer and integrates local connection information of the 802.11s link layer according to the cross-layer principle without making any changes to the network protocol stack. Another important objective was to implement a prototype and prove its practicality. Table 1 contrasts CHaChA with related research papers that also present combined approaches to clustering and channel selection. Older solutions without integration of 802.11s, already discussed in the surveys [12; 13; 14], are listed as well as more recent contributions that consider 802.11s at least in a preliminary or modified form. The approaches are grouped according to the constellation of the following requirements: the consideration of 802.11s, the decentralization of the approach, and its investigation in a real test environment. Most of the earlier work done before 802.11s was specified and therefore does not use its routing mechanisms and connection information. All of the work was also studied only simulatively. Nevertheless, the contributions CCA [30] and ISC [27], whose concepts regarding a dedicated base channel (common channel) as well as the metric \"Highest Connectivity\" were also integrated into CHaChA, deserve special mention. Only the works [9; 10; 11] consider 802.11s in their studies. The graph-theoretic approach CGCA [9] uses centralized clustering depending on a mesh gateway that obtains global network knowledge by communicating with all nodes. CGCA also requires position information of the nodes and excludes topology changes. The distributed approach of [PERSON] et al. [10] declares mesh gateways as cluster heads regardless of their topological fitness. Cluster membership of nodes is determined using hop distance. However, the 802.11s routing mechanism Hybrid Wireless Mesh Protocol (HWMP) uses the Airtime Link Metric (ALM) by default, which takes into account not only the length of a path, but also its average data rate and error probability. The work JRCAP [11] presents a combination of decentralized clustering-based channel selection and a routing protocol. The HWMP and ALM are replaced, so there is no interoperability with default 802.11s. A mesh gateway is required to implement the custom routing protocol. In addition, since no basic connectivity is ensured over a common channel, relay nodes between clusters are required. However, partial concepts of JRCAP for cluster roaming are considered in the context of CHaChA. The approaches [9; 10; 11] were only investigated simulatively above preliminary 802.11s models and do not integrate HWMP information. In contrast to the related works, CHaChA represents a distributed solution that simultaneously preserves interoperability with standard 802.11s and integrates its local connection information directly via existing operating system interfaces. In addition, CHaChA has been studied as a prototype in a real test environment. \begin{table}
\begin{tabular}{l c c c} \hline \hline
**Works** & **Consideration of 802.11s** & **Distributed Approach** & **Real Testbed** \\ \hline CCAS [26], ISC [27], & — & — & — \\ MCI [28], MCCA [29] & — & — & — \\ \hline CCA [30], CoMTaC [31], & — & ✓ & — \\ DCITCA [32], Max-Min-D [33] & — & ✓ & — \\ \hline CGCA [9] & * & — & — \\ \hline [PERSON] et al. [10], JRCAP [11] & * & ✓ & — \\ \hline CHaChA [17] & ✓ & ✓ & ✓ \\ \hline \hline \end{tabular}
\end{table}