Authentication issue has been mostly ignored to ensure fast handoff in 802.11 Wireless Mesh Network (WMN). With the proliferation of WMNs in recent years for practical deployment, secured fast handoff has drawn much attention to enforce authenticated access while reduce the extra delay caused by enabling authentication operations. In this paper, we present an overview on the state-of-the-art advance in this field and tackle the problem from a practical perspective based on experiments and analysis on our real-world testbed HAWK. We propose a novel fast handoff scheme Network-assisted Radio Signature to eliminate probing delay by taking advantage of the characteristic of the actual dynamic topology about mesh routers in WMN. Moreover, we apply an optimistic authentication mechanism Dual Re-authentication to counteract the authentication delay while providing the secured wireless access. In this manner, we have reduced the end-to-end handoff delay of WMN back again to a level below 50ms to achieve secured handoff and support time-sensitive applications. We describe detailed mechanisms, simulation, implementation and experimental results. To our best knowledge, we are the first to achieve such an optimal performance of secured fast handoff.