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This paper studies a fundamental problem, clock synchronization, in IEEE 802.11 ad hoc networks. Clock synchronization is important for frequent hopping spread spectrum (FHSS) to ensure that all stations "hop" at the same time; it is also necessary for FHSS, direct sequence spread spectrum (DSSS) and orthogonal frequency-division multiplexing (OFDM) to perform power management. The synchronization mechanism specified in the IEEE 802.11 standards has a severe scalability problem. Remedies have been proposed to solve the scalability problem, but these solutions either can not handle mobility very well or the protocol is too experimental without solid analysis. In this paper, we analyze the root cause of the scalability problem and propose two protocols with analytical guidance for implementation. The new solutions are distributed, scalable and very adaptive to station mobility. The maximum clock drift is improved from over 4000 μs to under 125 μs and 50 μs respectively for our new protocols.