Research On Centralized Control Technology Of Dense Wi-Fi Networks

Wi-Fi, abbreviation of Wireless Fidelity, is a kind of wireless local area network, which is composed of APs (Acess Points) and wireless cards. Data traffic surging is bringing a large pressure to beyond 3G and the early LTE. Wi-Fi network has some better advantages, such as cost, throughput, indoor coverage and free frequency resources. Hence, it is able to be one of the most important forms that carry wireless data. Continuous evolution of 802.11 technologies has made Wi-Fi devices become more and more popular, thus APs are deployed more densely and coverage evolves from dispersive hotspots to massive zones with dense hotspots. Traffic has offloaded by amount of Wi-Fi devices in dense networks, so that cen-tralized control mechanism is necessary. Dense Wi-Fi networks need not just deploying AP devices, but achieving availability, manageability as well as controllability, so as to improve the QoE and optimize the performance. Poor QoE will decrease Wi-Fi utilization rate signif-icantly.In dense Wi-Fi networks, interference between APs is one of the most primary factors leading to performance fluctuations. The stronger interference is, the poorer performance as well as the lower system throughput will be. Network interference is determined by the four main reasons, AP density, interference channels, transmit power and transmission rate. How to adaptively adjust system paramters to get the optimal network performance with a specific AP density is vital significant.In this thesis, a dynamic Wi-Fi channel allocation strategy and interference coordination method are studied, and the centralized control protocol extensions are also proposed. The main research contents and contributions are as follow:Firstly, in view of the existing three kinds of centralized control protocols (CAPWAP, SLAPP, WiCoP), this thesis studies the message, state diagram and transfer, signaling and process respectively. Shortcomings of these three protocols are put forward, too.Secondly, aiming at the dense Wi-Fi network, a dynamic Wi-Fi channel allocation strategy and interference coordination method are presented, which are based on how to effi-ciently reduce or avoid interference between APs in order to optimize the performance. The scheme defines CAF (Channel Assessment Factor) through combining CIF (Channel Inter-ference Factor) with COF (Channel Overlap Factor). The CAF is helpful to establish a crite-rion of channel quality ranking, and allocate the optimal channels. On this basis, the optimi-zation scheme of power control comes up. Power changing doesn’t affect the access of ex-isting users, and takes advantage of setting different optimal transmit power for APs so as to further decrease the interference. In addition, a method of limiting AP transmit rate that re-duce the interference is proposed. By analyzing the simulation results, it shows that the scheme has improved the reasonable allocation of resources such as channels, as well as lowered the internal interference of dense Wi-Fi networks.Lastly, a kind of extended CAPWAP protocol is designed based on the dynamic Wi-Fi channel allocation strategy and interference coordination method. Through modifying and expanding the messages, in-band and out-band signaling are the key issues. In addition, this thesis also proposes a signaling process. Furthermore, the extended protocol is implemented on an open source platform named OpenCAPWAP. It turns out that the extend CAPWAP protocol has feasibility, and meets the demand of centralized control in dense Wi-Fi net-works.