Research On Transmission Performance Over 802.11-based Wireless Mesh Networks

With the 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. In WLANs, terminals access Internet through Access Point (AP) which connects to wired networks directly. Since WLAN 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 sill 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. Deployed in mesh topology, Internet access is provided by wireless multi-hop relay in WMNs. A WMN is composed of Mesh Routers, Mesh Clients, and Mesh Gateways. Mesh Gateways are the WMN-and-Internet joints; Mesh Routers with fixed locations and stable power supplies constitute the backbone of WMN; Mesh Clients access WMNs by connecting to Mesh Routers, they can be routers also in some special scenarios. It has been noticed by former researches that the performance achieved does not satisfied the requirement of users with existent MAC, routing and transport protocols. So it is very necessary to modify these protocols to enhance the transmission performance in WMNs.In this dissertation, the background and research status of WMNs are introduced firstly. As a new wireless access network, there are many problems worthy to be studied and considered in WMNs within each layer. As an Internet access network, various traffic flows should be supported in WMNs, especially the reliable data transfer which occupies approximately 80% of Internet traffics currently, as well as real-time multimedia traffic which rapidly increases along with the development of multimedia technologies.In order to measure the transmission performance in IEEE 802.11-based WMN accurately, Network Simulator tools and test bed are used to investigate the performance of TCP flows and real-time multimedia flows in WMNs. The experiment scenarios can be classified into 4 categories: single TCP flow, multiple contention TCP flows, single multimedia flow and multimedia flow with background traffic. Transmission performance such as throughput, delay and jitter are recorded and analyzed. The experiment results reveal the problems of TCP and real-time multimedia transmission in WMNs.Based on the simulation and experimentation results, IEEE 802.11 MAC and TCP used in WMNs are analyzed in Chapter 4. MAC layer and TCP state transitions are modeled with Markov chain. The contrasts between the theoretical results and simulation results validate that the analytical model describes the influence factors of transmission performance in WMNs accurately. From the analysis, it can be found that physical layer characteristics and MAC layer protocol are the essential influence factors for poor TCP performance in WMNs. Therefore, to enhance transmission performance in IEEE 802.11-based WMNs, two MAC layer approaches are proposed in the following two chapters.High Bit Error Rate (BER) due to noise and multi-path is one of the most important reasons for packet loss in WMNs. BER changes dynamically for the environmental variety. A Dynamic Fragmentation and Aggregation (DEA) mechanism is proposed in Chapter 5. Nodes calculate the optimum frame length based on channel states, and frames in the queue are fragmented or aggregated to form the optimal size frames before sending. Local optimization in each hop leads to global optimization. Channel utilization is optimized and packet loss due to BER is reduced simultaneously with DEA. So the overhead of packet retransmission is reduced, and transmission performance is enhanced in WMNs. Simulation results validate that DEA is an efficient approach to improve transmission performance in WMNs.Since WMNs bear various traffics, and the transmission performance requirement such as bandwidth and delay of these traffics are different, an Adaptive Contention Window Adjustment method (ACWA) is proposed in Chapter 6 to satisfy transmission performance requirement of different services. ACWA is a cross layer solution which differentiates packets priorities by coordination of network layer and MAC layer. Packets with different priorities are put into different MAC queues, and the contention windows are adjusted based on channel and queue states. Therefore, ACWA provides QoS assurance for various services. Furthermore, local congestion in WMNs can also be partially avoided with ACWA. Simulation results show that ACWA is an efficient transmission performance assurance approach in WMNs.Works in this dissertation are focused on transmission performance enhancement in IEEE 802.11-based WMNs. More challenges will exist in WiMax, UWB and hybrid protocols WMNs.