## 1 Introduction  
Recent advances of sensor technology have led to the development of micro-sensors with low power and highly integrated functions [1-3]. Each sensor includes sensing unit, processing unit, transmission unit and power unit. Sensors deployed in an application field are organized by themselves to form a virtual organization. The sensing unit measures ambient conditions and transforms them into an electric signal. Then, transmission unit forwards the data to the neighbors. According to multi-hops, the data reaches the base station (sink). The process can be shown briefly as Figure.1. Based on WSN, there are many exciting applications including military applications, environmental applications, health applications, home applications, other commercial applications [1, 5]. In Figure.2, there are two typical applications of WSN. In Figure.2 (a), the special events (such as friendly and enemy forces, equipment and ammunition) in a military application can be monitored. By equipping or embedding equipment and personnel with sensors, vehicle-, weapon-, and troop-status information can be gathered and relayed back to a command center to determine the best course of action. Information from military units in separate regions can also be aggregated to give a global snapshot of all military assets [5]. In Figure.2 (b), an environmental application about forest fire detection is given. Since sensor nodes are randomly and densely deployed in a forest, sensor nodes can relay the warning about fire to the end users taking suitable actions to control the fire spread. In most applications of WSN, it needs to deploy and integrate millions of sensor nodes using radio frequencies systems. Because the sensors may be left unattended for months and even years, effective  
Figure 1: The simple description of WSN [4]. Figure 2: Applications based on WSN [5]. power-aware methods are needed. The sensor nodes will collaborate with each other to perform distributed sensing and overcome obstacles, such as trees and rocks [1]. These make WSN significantly different from others, such as Ad hoc. Comparing with other networks, these unique characteristics of WSN include several aspects listed as follows [3]:  
(1) Energy issue is the most important concern. Sensors usually work at unattended area for several years and it is often impractical to recharge or replace a depleted battery. Among all the operations, transmitting and receiving data account for the major portion of the energy [6]. So, energy-aware routing protocol is desired and can significantly prolong the lifetime of WSN. For WSN applications to have reasonable longevity, an aggressive energy-management policy is mandatory. This is currently the greatest design challenge in any WSN applications. (2) Resources for calculating and storing are extremely limited. In order to reduce the sensor cost, sensors are made of simple and low-cost circuits. So, sensor nodes may not have global identification (ID) because of the large amount of overhead and large number of sensors. For example, the Berkley MICA series Motes have 8 MHz processors, 128K programmable memory, 4K RAM and 512K flash memory for storage [7]. For the SmartDust nodes, it have 8-bit and 4 MHz CPU, 8 KB instruction flash memory, 512 bytes RAM, 512 bytes EEPROM, 3500 bytes space for OS code and 10 Kilobits/second bandwidth [8]. (3) Sensor nodes are densely deployed. Since the cost of sensors is low and the components are unreliable, the sensors are prone to failure, which makes the topology of WSN change frequently. It is necessary to deploy a large number of sensors to avoid the node failure and cover the monitoring area. (4) The position of individual sensors can not be predetermined. To some typical applications, the monitoring fields could be hostile or dangerous and human can not access it, such as the forest fire detection, chemical pollution, battlefields. So, WSN is randomly deployed in inaccessible terrains or disaster relief operations by an airplane or missile [9].