## 2 WSN-Based ITS Applications
### Factors Influencing Application Design  
An ITS application which uses WSNs is affected by several factors which impose or relax the constraints which drive the application design. The most important of them is the limitation of resources of WSN devices. Energy is a scarce resource in WSNs, turning power efficiency into a must. The CPU, in addition, is limited in processing power and its use must be restricted in order to save energy. Something similar happens with wireless communications, offering low rate transfers whose utilization must be reduced in order to improve power efficiency. All of these constraints make WSN a unique technology which requires specialized protocols and algorithms as covered in many different works [5]. In the same way, dedicated software built on top of the popular TinyOS operating system [32], and hardware, for example the well-known sensing platforms TelosB, MicaZ and Mica2 from MEMSIC and their respective sensorboards, which meet the previous constraints, have been developed.  
In spite of experiencing these limitations typical of generic WSNs, an ITS application might not always be subject to them. This is due to the existence of reusable road facilities which provide additional energy availability or processing power. Energy availability is given by the access to power lines in some points of the road where traffic lights or VMS are placed. This does not allow powering all sensor nodes but only a few of them; however it may be enough if those nodes are assigned themost demanding tasks. On the contrary, if power lines are not available, solar panels or other renewable power sources as well as energy scavenging techniques are also interesting options.  
Regarding processing power, it is possible to integrate WSNs into heterogeneous vehicular technology systems, therefore relying complex processing on more powerful devices such as RSUs (Roadside Units). In addition there are other benefits from the integration with other networking technologies, mainly with VANET and WAN (Wide Area Networks). On the one hand VANETs and WSNs can complement each other. Firstly, they handle different data to a large extent. VANETs use the data gathered from sensors onboard vehicles, which allows obtaining their state. WSN in turn monitors the road itself and may get valuable information prior to the arrival of vehicles. A small overlap exists between the information provided, though. However, this increases robustness. Secondly, they are also complementary in how they propagate information. While VANETs promote information dispersion, WSNs favor keeping information in a static location, enabling schemes which combine both. The combination of VANET and WSN has been referred to by some authors as Hybrid Sensor-Vehicular Network (HSVN) [13,14,33]. On the other hand, the integration with WAN technologies enables transporting the acquired information to distant locations, such as Traffic Management Centers, or to passengers in vehicles by means of cellular networks. As it can be seen, WSNs may be only a part of a global solution composed of different complementary technologies.  
The last consideration regards the scenario where the designed system is used. Three generic scenarios may be considered: urban, highway and rural. The choice of the scenario where the application is adopted has an important effect, as it determines the circumstances under which the system operates. Urban scenarios are characterized by the presence of dense grids of streets and intersections as well as medium to high traffic densities. Highways, in turn, present long linear layouts without intersections and peaks of high traffic loads at some points. Finally, rural scenarios have low densities of vehicles and scattered intersections, and unlike the other two scenarios, a low availability of reusable facilities and less frequent maintenance support. Therefore, all of these conditions must be taken into account in the design.