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In this paper we present the Cognitive Medium Access Prototype (CMAP), an experimental test bed for dynamic spectrum access in WLAN bands. The prototype builds upon the Cognitive Medium Access (CMA) protocol, and ensures coexistence with the WLAN system by adhering to an interference constraint. CMAP demonstrates the feasibility of CMA's design concept, and validates model assumptions made in proposing this protocol. CMAP operates in a time-slotted fashion and performs physical-layer sensing and statistical prediction to coexist with the primary user. Its implementation is based on a general- purpose DSP/FPGA board, which allows for a flexible design. The spectrum sensor, the medium access control, and the cognitive transmitter are discussed in detail. CMAP's performance is evaluated for an experimental coexistence setup, and compared to a blind reference scheme. A significant performance gain, both in terms of increased throughput and reduced interference, is observed.