802.3 Ethernet frame Figure 6-9: Wireless Distribution System Based on Four-Address Format MAC Frame An ESS mesh is a collection of access points, connected by a WDS, that automatically learns about the changing topology and dynamically re-configures routing paths as stations and access points join, leave and move within the mesh. From the point of view of an individual station and its relationship with a BSS and ESS, an ESS mesh is functionally equivalent to a wired ESS. Two industry alliances emerged during 2005 to promote alternative technical proposals for consideration by TGs; the Wi-Mesh Alliance and SEEMesh (for Simple, Efficient and Extensible Mesh). The main elements of the Wi-Mesh proposal are a mesh coordination function (MCF) and a distributed reservation channel access protocol (DRCA) to operate alongside the HCCA and EDCA protocols (Figure 6-10). Some of the key features of the proposed Wi-Mesh MCF are summarised in Table 6-15. The final ESS mesh specification is likely to include prioritised traffic handling based on 802.11e QoS mechanisms as well as security features and enhancements to the 802.11i standard. The evolution of 802.11 security will be fully described in Chapter 8, but mesh networking introduces some security considerations in addition to those that have been progressively solved for non-mesh WLANs by Wireless LAN Standards Mesh Internetworking Measurement Routing Security Mesh Coordination Function (MCF) HCCA / EDCA 802.11n MAC Distributed Coordination Function (DCF) 802.11a/11b/11g/11j PHY 802.11n PHY 802.11s functions 802.11n functions Figure 6-10: Wi-Mesh Logical Architecture WEP, WPA, WPA2 and 802.11i. In a mesh network additional security methods are needed to identify nodes that are authorised to perform routing functions, in order to ensure a secure link for routing information messages. This will be more complicated to achieve in a mesh, where there will commonly be no centralised authentication server. The work of the 802.11s TG is at an early stage, and ratification of the 