2.1 IEEE 802.11s Architecture

Wireless local area networks (WLANs) have become very popular in recent years. IEEE 802.11 is the standard for WLAN. This specification defines a physical layer and an Ethernet like MAC layer for wireless links [1]. IEEE 802.11 consists of mobile stations (STAs) and access points (APs). A mobile station is a networks device with a wireless networks interface card. APs are acting as bridge by providing connectivity to stations. APs are connected to each other through wired links.

IEEE 802.11 provides a cost effective and simple way for wireless networking. However, the problem is the wired connection between the APs. The wired links increases complexity and deployment cost in many situations. Therefore, it is desirable to connect the APs via wireless links as well and create a WLAN Mesh. In WMNs, APs turn into mesh access points (MAPs). Mobile stations are sometimes referred as mesh clients. The new IEEE 802.11s standard for WMNs introduces a third class of nodes called mesh points (MPs) [2]. MPs and MAPs support WLAN mesh services, allowing them to forward packets on behalf of other nodes to extend the wireless transmission range. Mesh clients can associate with MAPs but not with MPs. Mesh portals (MPPs) are MAPs which provides connectivity to other networks thus acting as a gateway for Mesh Networks.

2.2 Infrastructure of Wireless Mesh Networks

Infrastructure WMNs uses MPs and MAPs as backbone for the clients. Availability of gateway like MPPs provides connectivity to external networks such as Internet. The clients connect to the MAPs via standard 802.11, but do not forward packets [3]. This is the most used architecture nowadays. Access Points are also used for increasing coverage area. Figure 2.1 shows typical structure of this type of networks.

Fig. 2.1
figure 1

Basic infrastructure WMNs

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2.2.1 Client Wireless Mesh Networks

In this type of architecture, client nodes constitute the actual networks to perform routing and configuration functionalities as well as providing end user applications to customers. As an example let us consider a collection of MPs connected to each other, they can communicate to each other within networks and also forward data on behalf of other. In Client WMNs, a packet destined to a node in the networks, hops through multiple nodes to reach the destination [4]. Client WMNs are usually formed using one type of radios on devices. Moreover, the requirements on end-user devices is increased when compared to infrastructure meshing, since, in Client WMNs, the end-users must perform additional functions such as routing and self-configuration.

2.2.2 Hybrid Wireless Mesh Networks

This architecture is the combination of infrastructure and client meshing. Mesh clients can access the networks through MPs as well as directly meshing with other mesh clients. While the infrastructure provides connectivity to other networks such as the Internet, Wi-Fi, WiMax, cellular, and sensor networks; the routing capabilities of clients provide improved connectivity and coverage inside the WMN. The hybrid architecture will be the most applicable case in our opinion. IEEE 802.11s is the typical scenario of Hybrid WMNs, we have clients (Ordinary stations), which uses Mesh Access Points (MAPs) for accessing networks and we have independent nodes (MPs) which can directly access the networks [5]. All these entities are based on 802.11-based radio technology. The typical structure of hybrid WMNs are shown in Fig. 2.2.

Fig. 2.2
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Hybrid WMNs

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Due to different system architecture mesh networks have different requirement to the physical layer, the MAC mechanism and the routing protocol than legacy IEEE 802.11 LANs [1]. The changes made in above layer for WMN are discussed in next section. In 802.11s which employ multi-hop capabilities, data can be routed along an alternate path to avoid interference. Also if a node requires a large amount of resources like in the case of single hop access points where bottlenecks are created, the networks can dynamically route traffic through other networks nodes hence avoiding the congested node.

In the WMN where routing is done in layer 3 cooperation is only done between stations that are mesh enabled. Hence stations without mesh capabilities do not join in the networks. The 802.11s provides an extension to the MAC frame header where APs could be able to connect to each other wirelessly establishing peer to peer wireless links creating a wireless distribution system (WDS) or a Mesh Basic Service Set (MBSS) [5]. This would create a backhaul infrastructure with no need of all the APs being connected to an 802.3 networks and therefore merging the lines between infrastructure and client devices in some usage scenarios. The extensions in the MAC header of 802.11s frame makes it possible for addresses of legacy stations to be reached in a WMN. Also provided for in the standard is a robust and transparent data layer which supports all higher protocol layers. This was necessary so that the new devices could be compatible with the existing ones hence the only change being in the MAC layer header frame. IEEE 802.11s aligns with the 802.11i amendment which concerns security and also 802.11e which concerns quality of service (QOS) [6].