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Deep analysis of invalid handoffs in WLANs based on network-client collaborative framework

As 802.11 network has become an important infrastructure for the Mobile Internet, the performance of WLAN handoff is critical to the quality of user's experience. Although it has shown that there are a large number of invalid handoffs in large-scale 802.11 networks with dense AP, the reason and seriousness still remain unclear. In this paper, we propose HandoffAnalyser, a client-network collaborative framework, to deeply measure and analyze the handoffs. Then, we give a clear explanation of invalid handoffs by defining two patterns termed Loose Ping-Pong (LPP) and Strict Ping-Pong (SPP) based on association history. The experimental results of real campus WLAN with about 2,700 APs and 220,000 mobile devices show that 22.1% and 31.3% of handoffs are SPP and LPP respectively. We even observe that several clients experience more than 280 LPP and 220 SPP per day respectively. Such a large number of ping-pong will seriously affect the performance of both client and network. We carry out in-depth analysis and find that ping-pong has relationship to some wireless metrics such as channel utilization, network configuration such as AP with Multi-SSID, and hardware defect such as poor 5 GHz chip. These findings and conclusions can help us to better optimize the WLAN.