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Dynamic Spectrum Access can enable a secondary user in a cognitive network to access unused spectrum, or whitespace, found between primary user transmissions in a wireless network. The key design objective for a secondary user access strategy is to "scavenge” the maximum amount of spatio-temporally fragmented whitespace while limiting the amount of disruption caused to the primary users. In this paper, we first measure and analyze the whitespace profiles of an 802.11 network (using ns-2 simulation) and a non-802.11 (CSMA)-based network (developed on TelosB Motes). Then we propose two novel secondary user access strategies, which are based on measurement and statistical modeling of the whitespace as perceived by the secondary users. Afterward, we perform simulation experiments to validate the effectiveness of the proposed access strategies under single and multiple secondary user scenarios, and evaluate their performance numerically using the developed analytical expressions. The results show that the proposed access strategies are able to consistently scavenge between 90 and 96 percent of the available whitespace capacity, while keeping the primary users disruption less than 5 percent.