## 2 Wireless Sensor Networks  
WSN can be categorized in various ways, depending on the network architecture, communication coverage area, and communication protocol. The simplest topology we can imagine is a hierarchic architecture where sensor nodes gather data autonomously, and the network passes these data to one or more base stations, which finally forward the data to a sensor network operating center server. Before designing and installing any system, it is necessary to understand its physical environment and deployment in detail. Another important issue is whether the sensors' locations are known a priori: there are situations in which some of the sensor locations are known, and other circumstances in which sensor locations are unknown and can be estimated from the whole of the other sensor data [6]. In particular, considering the network architecture, we have infrastructure-based network, which have a fixed structure, and infrastructureless networks, which are dynamically formed by cooperating independent wireless devices. If we consider the covered area, briefly we can categorize wireless networks as wireless wide-area Networks (WWANs), wireless metropolitan-area networks (WMANs), wireless local-area networks (WLANs) and wireless personal-area networks (WPANs) [7]. WSN can be classified also considering the communication protocols, as shown in Table 1. \begin{table}
\begin{tabular}{l c c c c c c} \hline \hline
**Criteria** & **WiFi** & **WiMAX** & **3G** & **4G** & **BT** & **ZigBee** \\ \hline
**Max downlink speed** & 100 M & 128 M & 100 M & 1G & 1 M (v1) & 250 k \\
**bps** & & & & & 3 M (v2) & \\  & & & & & 24 M (v3) & \\ \hline
**Max uplink speed** & 100 M & 56 M & 50 M & 500 M & & 250 k \\
**bps** & & & & & & \\ \hline
**Frequency Band** & 2.4 GHz & 2.3 GHz & 900 MHz & 2–8 GHz & 2.4 GHz & 868 MHz \\  & 5 GHz & 2.4 GHz & 1,900 MHz & & & 915 MHz \\  & & 3.3 GHz & 2,100 MHz & & & 2.4 GHz \\  & & 3.5 GHz & 2,600 MHz & & & \\ \hline
**Architecture** & Ad hoc & Infra & Infra & Infra & Ad hoc & Ad hoc \\  & Infra & & & & & \\ \hline
**Data Rates** & High & High & High & Very High & Medium & Low \\ \hline
**Distance** & Medium & High & High & High & Low & Low \\ \hline
**Power Consumption** & Medium & Medium & Medium & Medium & Low & Low \\ \hline
**Cost** & Medium & High & High & High & Low & Low \\ \hline
**Nodes density** & Medium & High & High & High & Low & High \\ \hline
**Maturity** & High & Medium & High & Low & High & Medium \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Communication Technologies for sensor nodes. WSNs possess several characteristics that have challenged many aspects of traditional computer network design, such as the scalability issue caused by the large amount of sensor nodes, the infrastructureless network due to additional mobile sensors, and the stringent resource onboard the sensors. These new features call for a re-design of overall structure of applications and services: in particular the computational complexity distribution on different devices must be investigated. Certainly the large amount of sensor node causes scalability problem. A centralized approach sends data from each sensor to a fusion center (or processing center) where data processing and fusion are carried out. This approach works well when the number of sensor nodes is relatively small and the data file needs to be transferred is not large. However, when the amount of sensor nodes increases considerably, the centralized approach will not scale. A natural solution to this problem can be the distributed approach. Distributed sensor network (DSN) has been studied extensively since early 1980s. [PERSON] _et al_. [8] were among the first to propose the design of DSNs. Since then, several efficient DSN architectures have been presented in the literature, including the hierarchical and committee organization, the flat tree network, the de Bruijn based network, and the multi-agent fusion network (see [9] and references therein).