## 2 Node Hardware Components in WVSNs
First of all, the battery capacity is a measure of the total charge that can be extracted from a battery, usually in current time units (e.g., mAh) [10]. Note that the energy consumption is equal to the electric charge times the voltage. The voltage for most of the WVSN platforms is between 1.5 V to 3 V.  
In [10], the power consumption on MICA2 DOT platform is studied, and it is found that the radio power consumption dominates the other two modules (CPU processing and LED lighting). In addition, the battery lifetime for large transmission power (10 dBm) is 45% shorter than for small transmission power (0 dBM). With adaptive power control mechanism, the battery efficiency could increase 52%. In [11], it is also concluded that the adaptive power control could help to prolong the lifetime of the WSN. In [13], the model of battery discharge characteristic in the WSN platform, TelosB, is used to predict the battery lifetime. During the pulse discharge of the battery, the operating voltage varied between an upper and a lower voltage [12]. In [13], the voltage variance of battery is captured in the model to predict the lifetime of the battery more accurately. As indicated above, all the works on the battery discharging behavior are for the sensor nodes in WSNs, and none of these works study power consumption for the camera sensors in WVSNs. To be more specific, the battery discharge behavior for image/video capturing, processing and transmission has yet to be explored. The radio modules of WVSNs could be classified as low rate wireless personal area networks (WPANs) and high rate wireless local area networks (WLANs). IEEE 802.15.4 and Bluetooth are the typical WPAN radio modules in WVSNs. The transmission range of the WPAN radio modules is restricted to 10 to 20 m. Due to their small transmission range, they have lower power consumption. The maximum data rate for the IEEE 802.15.4 and Bluetooth is 250 Kbps and 1 Mbps, respectively. IEEE 802.11b is another popular WLAN radio module in WVSNs. The maximum data rate of the 802.11b could be 11 Mbps and transmission range could reach 100 m. Because of its higher data rate, IEEE 802.11b radio modules in WVSNs are relevant to applications that require real-time video streaming (e.g., vehicle tracking). This higher data rate and longer transmission range comes at a price of higher cost and power consumption. For example, MeshEye [7] adopts the CC2420 radio chip, which is an IEEE 802.15.4-compliant RF transceiver. It consumes 175.9 mW in transmission and 1.8 mW in sleeping mode. On the other hand, Meerkats [3] adopts the IEEE 802.11b module. It consumes 3.5 W in transmission and 49.2 mW in sleeping mode, which is more than one order of magnitude higher than the power consumption in MeshEye [7]. In summary, until now, according to the computational capability and system resource consumption, there are two types of WVSN platform. The first one is the low end WVSN platform that is designed specifically for energy efficiency (e.g., Cyclops [6]). The other is the high end WVSN platform (e.g., Meerkats [3]) that is designed for sophisticated visual data applications where the system resource requirement and energy consumption are about one order of magnitude higher. This offers new research directions and opportunities for designing new WVSN platforms that could perform sophisticated visual data operations in energy efficient ways. In this section, we review the five major hardware components of the sensor node in WVSN, other detailed hardware architecture can be seen in [14]. In Table 1, we summarize the node components for the WVSN platforms.