Recent advances in wireless mesh networks (WMNs) have overcome the drawbacks of traditional wired networks and wireless ad hoc networks. WMNs will play a leading role in the next generation of networks, and the question of how to provide seamless mobility management for WMNs is the driving force behind the research. The inherent characteristics of WMNs, such as relatively static backbones and highly mobile clients, require new mobility management solutions to be designed and implemented.
In this paper, a hybrid routing protocol for forwarding packets is proposed: this involves both link layer routing and network layer routing. Based on the hybrid routing protocol, a mobility management scheme for WMNs is presented. Both intra-domain and inter-domain mobility management have been designed to support seamless roaming in WiFi-based WMNs. During intra-domain handoff, gratuitous ARP messages are used to provide new routing information, thus avoiding re-routing and location update. For inter-domain handoff, redundant tunnels are removed in order to minimize forwarding latency. Comprehensive simulation results illustrate that our scheme has low packet latency, low packet loss ratio and short handoff latency. As a result, real-time applications over 802.11 WMNs such as VoIP can be supported.
Wireless mesh networks (WMNs) are an innovative network technology that has emerged in recent years. Compared to original wireless networks, WMNs have many advantages: dynamic self-organization, self-configuration, high scalability, low cost, and easy maintenance. These characteristics allow WMNs to play an important role in the next generation of wireless networks.
A comprehensive survey of WMNs is given in [2]. In summary, two types of nodes are involved in a WMN: mesh routers and mesh clients. Mesh routers are responsible for routing and maintaining the network topology. Mesh routers, also called access routers (ARs), provide a connection to the network for mesh clients. Certain special mesh routers also perform gateway functions. Mesh clients include mobile devices, such as laptops, PDAs and sensors. An example of a wireless mesh network is shown in Fig. 1.
Generally, there are three types of WMN architecture: infrastructure/backbone WMNs, client WMNs, and hybrid WMNs [2]. In infrastructure/backbone WMNs, mesh routers construct a backbone, and mesh clients can connect to each other by communicating with the mesh routers. Some mesh routers work as gateways that provide a connection to the Internet. The mobility of mesh clients is much greater than that of mesh routers; thus, a backbone composed of mesh routers is almost static. In client WMNs, there are no mesh routers in the network, and mesh clients provide routing, bridging and gateway functions by themselves. This architecture is similar to that of a conventional wireless ad hoc network. In hybrid WMNs, mesh clients can communicate through both mesh routers and mesh clients. Infrastructure/backbone WMNs are the most popular type of architecture, and all of the WMNs discussed in this paper will be of this type.
The performance of WMNs is affected significantly by how the network manages the movements of mesh clients. Therefore, mobility management is one of the most important problems of WMNs. Although many existing mobility management solutions for conventional wireless networks can be applied to mesh networks, new mobility management solutions should be designed and implemented specifically for WMNs, considering their differences.
There are two kinds of roaming: inter-domain roaming, which refers to movement across different domains, and intra-domain roaming, which means movement among different access routers in the same domain. Accordingly, mobility management requires both inter-domain and intra-domain mobility management. This paper proposes an innovative scheme to provide both intra-domain and inter-domain mobility management within WMNs. The proposed solution uses a hybrid routing protocol, which integrates the network layer routing and link layer routing to forward packets and achieves easier handoff. For intra-domain handoff, our scheme avoids location update in the centralized location server, while also decreasing the time for re-routing after the handoff. In addition, our scheme can provide inter-domain handoff with low overhead by minimizing redundant tunnels. It provides seamless handoff with high scalability for real-time applications such as VoIP.
The remainder of this paper is organized as follows. Section 2 presents typical mobility management solutions for WMNs. The hybrid routing protocol is presented in Section 3, and our mobility management scheme is proposed in Section 4. Section 5 describes the simulation set-up and discusses the performance results. Finally, Section 6 summarizes our work and concludes this paper.
In this paper, existing mobility management solutions for WMNs have been reviewed. A hybrid routing algorithm, which routes with both a MAC address at the link layer and an IP address at the network layer, is introduced. With this routing algorithm, during the intra-domain handoff process, location updates in the centralized location server are avoided, and re-routing after the handoff is not required. Moreover, the inter-domain mobility management scheme is also realized in WMNs. According to the experiment results, seamless handoff can be achieved with a lower overhead cost, and with the packet latency and loss ratio remaining at a lower level in both intra-domain roaming and inter-domain roaming. Our scheme thus can be used to support seamless real-time applications. The issue of security and the question of how to balance security and performance would be an interesting direction for future research.
