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We develop a novel approach for conserving energy in battery-powered communication devices. There are two salient aspects to this approach. First, the battery-powered devices move through multiple, progressively deeper, sleep states in a predictable manner. Nodes in deeper sleep states consume lower energy while asleep, but incur a longer delay and higher energy cost to awaken. Second, the nodes are woken up on demand through a paging signal. To awaken nodes that are in deep sleep, the paging signal has to be decoded using very low power circuits such as those used in radio frequency tags. To accommodate this need, in a manner that scales well with the number of nodes, the number of distinct paging signals has to be much less than the number of possible nodes. This is accomplished through a group-based wakeup scheme, which initially awakens the targeted node along with a number of other similarly disposed nodes that subsequently return to their original sleep state. Tradeoffs among energy consumption, delay and overhead are presented; comparisons with other protocols show the potential for 16% to 50% improvement in energy consumption.