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Network Architecture

Wireless mesh networks (WMN) are an emerging technology. In comparison to the traditional wired networks that have been used for a long time, there is no accepted definition of WMNs and, thus, no common practice for designing such networks. Unfortunately, there is no one and only one definition or architecture of WMNs and, thus, no common practice for designing such networks. On the one hand, there exist WMN definitions regarding MANETs as the simplest variant of WMNs [6]. On the other hand, WMNs are considered to be special MANETs [2]. In order to face the ambiguity of the term “wireless mesh network” a common definition of WMNs is presented here. The Figure below depicts a hierarchical and layered architecture that integrates various approaches and, thus, helps to identify the main parts of a WMN. This view is more general than that usually presented insofar as that other approaches often leave out some layers, e.g., they consider only clients without routing functionality [1].

WMN Architecture

On the top level of the Figure are the backbone mesh gateways connected to the Internet by wire, indicated by solid lines. They provide wireless Internet access (dashed lines) to the second level entities, the so-called backbone mesh routers. These wireless routers form the core by building the wireless, meshed backbone of the WMN. On the lowest level, there are the mobile user devices, the mesh clients. As the Figure shows, these clients are subdivided into two groups. On the left hand side, there are routing mesh clients that also communicate among each other in a multi-hop fashion. They form a MANET with gateways that are not directly connected to the Internet, but to the backbone mesh gateways. On the right hand side, there are non-routing mesh clients, which connect to mesh routers in the same way as conventional clients associate to wireless access points.

The architecture outlined above needs further discussion. First of all, the mesh gateways are specific mesh routers that have a wired, high-speed connection to the Internet. These wired connections are considered not to be part of the WMN. Thus, the WMN itself is fully wireless.

The mesh routers and gateways are installed at certain fixed positions. They establish a permanent infrastructure. However, new routers and gateways can easily be added, since the communication is wireless. Thus, the infrastructure and therefore the network topology is not completely static but has low dynamic character. Mesh routers and mesh gateways together establish a wireless multi-hop network that serves as a backbone. It routes traffic hop-by-hop from a mesh client to a backbone mesh that can forward it to the external network, and vice versa. This way of communicating is a ma jor difference to conventional wireless access points. These provide only gateway or bridge functionality. In addition, a mesh router has multi-hop routing capabilities. Furthermore, the hierarchy achieved by the distinction between clients and routers promotes the utilization of multiple radios, separating the traffic in the backbone from that of the clients. Routing and configuration tasks are assigned to mesh routers in order to unburden mesh clients that are probably power-constrained because of their inherent mobility.

Due to the mobility of the mesh clients the wireless mesh network has a spontaneous and dynamic character. Mesh clients can leave the WMN at any time, and new clients arrive that want to join the WMN. The Figure introduces two groups of clients, in which the non-routing mesh clients are confined to direct communication with mesh routers only. They do not participate in the routing process of the WMN and use the mesh routers similar to conventional wireless clients communicating with their access point. The routing mesh clients are able to connect not only to mesh routers but also to other routing mesh clients. Since they participate in routing, these clients build up a sub-network on their own, which can also be considered as a MANET.