1. Introduction
The rapid development in microelectronics and wireless communication technologies makes the deployment of large scale sensor networks possible. The communication among sensors is typically driven by events. In many applications (e.g., intrusion detection, disaster alarming, etc.), events occur rather infrequently. For such scenarios, sensors spend a considerable fraction of lifetime to monitor the environment, during which little communication is needed and sensors are said to be in the “monitoring state”. Once events are detected, sensors may need to leave the “monitoring state” and actively communicate with each other.