## 2 Fundamentals of Aerial Wireless Sensor Networks for Oceanographic Monitoring.
### Wireless Communications  
Network physical topology and density are entirely application-dependant [17], so before deploying an A-WSN it is necessary to understand the environment in which it will be installed. This implies choosing the most suitable number of nodes and their absolute position inside the area to be monitored. Denser deployments improve data accuracy and provide sensor networks with more energy and better connectivity. However, at the same time a denser infrastructure can negatively affect network performance (data collisions, interferences, _etc._) [18]. Thus, network density, physical topology and communication type all determine the logical choice of topology. Every topology has its own characteristics, which determine whether or not it is more suitable than others in terms of attributes of network functionality such as fault tolerance, connectivity, _etc._ This means that depending on the whole set of requirements for network functionality some topologies may have to be discarded in favour of others. Of the topologies most commonly used for interconnecting nodes in a network, the logical topologies most commonly used for A-WSNs are _Tree_, _Chain_, _Partially Connected_ and _Star_ (indicated as point-to-point in Figure 2). It is important to note that after  
Figure 2: General structure of an A-WSN for oceanographic monitoring. deployment, physical topology may change due to variations in the position, reachability (due to noise, moving obstacles, _etc._), available energy, malfunctioning, and task details of sensor nodes [19]. For wireless communication, the sensor node incorporates a radio module, which is chosen to suit the desired range. Sometimes, in order to increase the range, range extenders for RF transceivers are incorporated, thus providing amplification to improve both output power and LNA (Low Noise Amplification). Another option, where such devices are insufficient to cover the distance, is to include a GSM/GPRS module. For communication between sensor nodes it is possible either to develop communications protocols on the data-linking layer using different medium access mechanisms (such as TDMA, FDMA and CSMA), or else to use different wireless communication standards and technologies (Table 1). Which technology is chosen will depend on the requirements of the A-WSN it is proposed to implement, which in turn will be determined chiefly by the amount of information that has to be sent and whether images are to be sent in real time. Another requirement that has to be considered are the \begin{table}
\begin{tabular}{c c c c c c} \hline
**Technology** & **Standard** & **Description** & **Throughput** & **Range** & **Frequency** \\ \hline WiFi & 802.11a & System of wireless data transmission over & & & 5 GHz \\  & 802.11b/g/n & computational networks. & & & 2.4 GHz \\ \hline WiMAX & IEEE & Standard for data transmission using radio & & & 2–11 GHz \\  & 802.16 & waves. & \textless{}75 Mbps & \textless{}10 km & 3.5 GHz: Europe \\ \hline Bluetooth & IEEE & Industrial specification for WPAN which enables voice and data transmission between different devices by means of a secure, globally free radio link (2.4 GHz). & v. 1.2: 1 Mbps & Class 1: 100 m & \\  & & & & & \\ GSM & & & & & \\  & & & & & \\  & & & & & \\  & & & & & \\ GPRS & GSM extension for unswitched (or packaged) data transmission. & 56–144 Kbps & & Dependent on cellular network service provider & 2.5 GHz \\ \hline  & & & & 20 Kbps: 868 MHz: & & \\  & & & & & \\  & IEEE & Standard defining the physical level and control of medium access of WPANs with & 40 Kbps: 915 MHz: & \textless{}100 m & 868/915 MHz and 2.4 GHz. \\  & & low data transmission rates. & & 250 Kbps: 2.4 GHz: & & \\  & & & & & \\  & & & & & \\ ZigBee & IEEE & Specification of a set of high-level wireless communication protocols for use with low-consumption digital radios, based on WPAN standard IEEE 802.15.4. & 250 Kbps: 2.4 GHz: & & \\ \hline \end{tabular}
\end{table} 
Table 1: Wireless communication technologies.