A cognitive radio determines a frequency channel of a spectrum, determines from a locally stored association of individual channels to either of at least two categories (e.g., in use and backup, other is a third category) a selected category for the determined frequency channel. From the selected category is determined a time at which to sense the determined frequency channel. The determined frequency channel is then sensed at the determined time and results of the sensing may be transmitted. Timers may be associated with each of the categories and adjusted based on density of cognitive nodes. A timer for the in use category is shortest, a timer for the other category is longest, and a timer for the backup category is between shortest and longest.