A WiFi Network MAC Layer Handoff Mechanism Based On Cooperative Channel Scan Technology

IEEE802.11 series standard based WiFi technology has been widely used in the wireless local area network(WLAN) access services.The major operators in a number of cities across the country planed to set upsome Wireless City projects, so the WLAN and other mobile communication networks GPRS, 3G, LTE and other network integration to provide wireless Internet access services for the city in anytime and anywhere.Asthe signal transmission power of Ap is limited, and coverage also is limited. Mobile terminals(MT) need to handoff between multiple APs.However, the handoffing delay will significantly affect the quality of real time services such as VoIP.In this thesis, we mainly focus on the problem of handoffing delayin large-scale WiFi networks.In order to reduce the hanoff delay and improve the service quality with MT in crowed user enviroment.By introducing an assistant node to scan the available channels, so that the terminal connection reliability and real-time performance under the multi-APs hanoff environment will be improved.In this paper, the main research work is shown as follows:(1) By analyzing the MT in MAC layer handoff process, influence factors of time delay, and with some reseach on the delay of the most time-consuming scanning phase, we proposed a special mechanism of cooperative scanning technologyand described its process.(2) Before MT launched MAC layer handoff process, we designed a scanning process whichtriggered by the AP signal strength threshold.Foruncertainty of actual available assitant scanning node, the distance priority basedassitant node selection algorithm was proposed;(3) Meanwhile, for the scanning process of adjacent channel interference problems, we put forward a selective parallel channel assisted scanning algorithm.Combined with distributed detecting and assitant nodes’ feedbackAP informations collecting from MT, the aglorithm can efficiently collecting informations for handoff.(4) In order to further improve the handoff speed of MT in the history APs, weproposed a list of recent AP cache scheme. The scheme can effectively avoid the shorter time intervals of repeated scanning times, reducing scan delay.By deploy a large number of mobile nodes in NS2 simulation software and introduction of the auxiliary channel scanning algorithm, WiFi network handoffingwere tested.Experimental results show that the proposed hanoffing scheme can effectively reduce the handoff latency and improve the quality of services such as VoIP, video streaming and other real-time applications.