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In a cognitive radio network (CRN), secondary users (SUs) opportunistically utilize idle licensed spectrum bands. We address the natural questions that arise when the incumbents or primary users (PUs) return to the channel the SUs are using opportunistically. Instead of immediately switching to another idle channel as proposed in almost all existing approaches, the SUs may opt to wait silently in their current channel until the PUs depart. This option would be beneficial to the SUs if the returned PUs stay at the channel only for a short period of time and the SUs' channel-switching incurs a non-negligible overhead. We determine how long the SUs should wait in their current channel before switching to a new idle channel. The SUs should also occasionally sense those (called em out-of-band) channels currently not in use for sensing the availability of spectrum opportunities. We propose an efficient, adaptive spectrum-sensing technique to detect when a busy out-of-band channel becomes idle. We also present a spectrum-management architecture that integrates the SUs' strategies and facilitates fast discovery of spectrum opportunities.