Based On The Mac Layer And Network Layer Protocol Of The 802.11 Wireless Mesh Network

With development of wireless communication techniques, wireless networks change people's daily life dramatically by providing mobile Internet access. Wireless Local Area Network (WLAN),working as the last-hop connection to Internet, has come into market nowadays. Since WLANs is single-hop network and strongly depends on wired network, so the coverage of WLAN is very small due to the limited transmission range, and the cost of deployment is too expensive. To extend the coverage of wireless access networks and reduce the deployment cost, Wireless Mesh Network (WMN) is introduced. WMNs are multi-hop wireless networks, Mesh Clients access Internet by connecting to Mesh Routers. With the development of mobile computing technologies and the popularity of notebook PCs and PDAs, WMN has become the hot topic of research again. MAC and network layer protocols have great effect on the performance of WMN. MAC and network layer protocols in WMN are examined in this thesis. The focuses of the thesis are multiple path routing protocol, cross-layer design for distributed channel assignment over multi-Radio multi-Channel WMN, power control to improve spatial reuse, and mobile terminal hand-off. The main contributions of the dissertation are follows:(1) Because of the characteristics of wireless mesh network, such as multi-hop transmission, unreliable wireless channel, bandwidth limitation and time variation, traditional routing protocols such as routing information protocol and open shortest path first were not suitable to wireless mesh network. Although traditional ad-hoc routing protocols could be used in wireless mesh network, but its routing protocol must obey different designment rules based on its distinct characteristics. Traditional ad-hoc routing protocols were single path routing protocols, which suffered from low transmission efficiency, increasing delay time and unbalanced network load, even arousing network congestion. Single path routing protocol could not support QoS of wireless mesh network.. Compared with single path routing protocols, multiple path routing protocols had more advantages: efficient use of bandwidth, resolving congestion and burst flow, reducing transmission delay, balancing network load and improving reliability of transmission. But, more paths could bring more search time which will increase the cost of the routing protocols. So, the number of path in multi-path routing had the optimum value Two most broad ly applied and most prominent multi-path routing protocols: dynamic source routing multi-path and ad-hoc on-demand multi-path distance vector routing are studied by computer simulation. The simulation is carried out in ad-hoc network and wireless mesh network. The simulation result proves that wireless mesh network based on multi-path routing protocol have a lower overhead, lower packet loss rate and lower latency, so the wireless mesh network can satisfy wireless broadband access. At the same time, based on the distinct characteristics of topological structure of wireless mesh network, source on-demand routing protocols can improve its performance greatly.(2) Multiple channel technology could improve spatial reuse of network, increase simultaneous communication node pair, and reduce chancel interference. So, multi-channel technology was the most effective approach to improve network capacity. Theoretic analysis showed that, compared with system with a certain bandwidth which was divided into several sub-channels, the capacity of system with two radio interfaces and several channels could reach maximum theoretic values of Kumar formula. Cross-layer feedback mechanism could coordinate each layer of protocol stack by exchanging information along all layers, which could adapt to dynamic property and unreliability of wireless communication environment. Based on above analysis, a efficient distributed channel assignment algorithm using cross-layer approach for multi-radio multi-channel wireless mesh networks is proposed. This scheme seeks to assign distinct channel to each node along the same route, subject to a channel availability constraint. Nodes in two-hop neighborhood which belong to two different routes are assigned distinct channel too. Thus, interference along neighboring nodes which belong to same route and different route can be reduced. The proposed scheme combines channel assignment with reactive routing protocol. This scheme exhibits significantly lower communication, computation complexity than existing channel assignment schemes. Simulation results show that this proposed scheme can improve performance of the multi-radio multi-channel wireless mesh networks greatly.(3) Power control had different function to different wireless communication system. Power control had two main functions: the first one is to reduce energy consumption and prolong life of network, the second one is to improve spatial reuse and network capacity. In the wireless mesh network, mesh routers were offered electric power by special electric circuit, the main function of power control is to reduce interference with neighboring nodes and improve network capacity by improving spatial reuse. In order to resolve the problem that access point in wireless mesh backbone network using the same frequency channel to transmit packets could cause serious signal interference, a new power control MAC protocol was proposed. This protocol could control transmitted power of control frame sent by receiving node. Thus, only those neighboring access points that really affected the forthcoming data reception could receive the control frame, and then delayed their transmission to avoid interference. The proposed protocol can improve spatial reuse of access point. Theoretical analysis shows that, the spatial reuse improvement ratio increases more greatly with the distance between access points become more shorter. Simulation studies shows that compared with IEEE802.11 standard MAC protocol, the proposed protocol can allow more concurrent transmission which can't interfere with each other, and it improve network throughput greatly.(4) IEEE 802.11 standard MAC protocol supports mobile terminal handoff in mobile communication, but it can not support smooth handoff of mobile terminal. When a terminal moves from the coverage of current wireless access point to another, the temporary breakage of communication had effect on the performance of the system. Recently, the handoff algorithms include hard handoff and smooth handoff. The hard handoff had high packet loss rate and the smooth handoff could lower packet loss and reduce handoff latency. Smooth handoff required that packets must be forwarded from old access point to new access point through multi-hop routers over wireless mesh network, this would induce handoff latency and data loss. Considering handoff latency and data loss, a relay-router buffering-and-forwarding scheme is proposed to support smooth handoff in the infrastructure wireless mesh networks. Our proposal is based on buffering packets at intermediate relay routers in the packets forwarding route and neighboring relay routers working in promiscuous mode which can overhear the packet transmission from neighboring relay routers to APs. With the help of relay routers, the forwarding path of buffered packets is shortened greatly. The relay routers selection is based on multipoint relays of OLSR protocol. Simulation results show that this scheme improves the data availability and results in reducing the packet loss significantly.