Research On MAC Protocols In Wireless Local Access Network Based Mesh Networks

As a novel organization mode of wireless networks, Wireless Mesh Networks connect mesh points through wireless links to construct a dynamic, self-configured and multi-hop wireless interconnected network. It can enhance robustness, self-configure network, reduce deployment cost and construct heterogeneous networks. In this thesis, the Medium Access Control (MAC) protocol in Wireless Local Access Network based Mesh Networks (WLAN Mesh Networks) will be studied. As a foundation of the research on WLAN Mesh Networks, this thesis firstly focuses on performance optimization of MAC protocol in WLAN.First of all, a novel scheme, Channel Collision Based Adaptive Backoff (CCBAB) scheme, is proposed to enhance the performance of contention based channel access protocol defined by IEEE 802.11e. CCBAB scheme selectively enforces some uncollided traffic streams to reduce their transmission probability and grants collided real time traffic streams larger transmission probability to ensure them to be successfully transmitted in a shorter time. Therefore, CCBAB scheme is resistant towards collisions and delay deterioration at high congestion level.Secondly, a novel MAC protocol, Delegate Polling Hybrid Access (DPHA) protocol, which can significantly enhance WLAN network capacity, is proposed. DPHA designed a delegate polling access scheme, in which each traffic stream delegates Access Points (AP) to execute polling task consecutively so that all traffic streams can transmit in a relay mode. In addition, contention access scheme is adopted as a complement of the delegate polling access scheme to solve the problem caused by wrong polling decisions. The local scheduling algorithm in delegate polling scheme designed a conditional service differentiation mechanism to improve the fairness of non-real time traffics while guaranteeing the delay performance of real time traffics.Thereafter, the performance optimization of MAC protocol in WLAN Mesh Network is studied. Since WLAN Mesh Network is characterized by multi-hop transmission, the single channel based protocol would significantly restrict network capacity due to its channel sharing feature. The Multi-Radio Multi-Channel (MRMC) based protocol can expand network capacity to a large extent by enabling neighboring transmission pairs to operate on different channels and configuring each node with multiple wireless interfaces.Consequently, a channel allocation algorithm for MRMC protocol, the Load and Interference Adaptive MRMC (LIA-MRMC) algorithm, is proposed. It utilizes graph theory to theoretically work out the optimal channel assignment solution which simultaneously preserves network connectivity, reduces interference and balances network load. LIA-MRMC establishes network connectivity graph and conflict graph and models the channel assignment problem as coloring the connectivity graph in order to minimize the average interference of each link in conflict graph. LIA-MRMC proposes a dynamic priority based heuristic algorithm for graph coloring problem. The priority takes into account the traffic load of each node and the flexibility of channel selection while the channel selection scheme takes into account the minimization of interference between links. The theoretical and simulation results demonstrate that LIA-MRMC algorithm can significantly expand network capacity.