## 3 Network Architecture and Topology
### Sensing Subsystem  
The Sensing subsystem is composed of all the devices in charge of acquiring relevant information mainly relative to traffic and road states. In a WSN-based ITS application, not necessarily all the devices use WSN technology, allowing a distribution of tasks among devices using different technologies. However, as regards the acquisition of data, WSNs are the prevailing technology of choice. Consequently, the implantation of the sensing subsystem consists of the deployment of one or several WSNs throughout the observation area (roads or parking lots), which detect vehicles through their sensors and optionally communicate wirelessly with them. After observing how different applications have dealt with the deployment of these WSNs, it was noticed that they follow some basic topological patterns which determine important properties of the system such as lifetime, costs and functionality. WSN nodes are divided into groups following a similar outline, and the deployment consists of a composition, typically homogeneous, of these groups of nodes. Therefore, they can be considered as the building blocks of the sensing subsystem. It should be noted that, in addition, the nodes forming these blocks may propagate information within them, but this should not be confused with the Distribution subsystem. Data propagation in the Sensing subsystem is restricted to local areas, aimed at extracting information from the block/group (towards a sink node) or enabling collaborative processing with nearby nodes. If data dissemination through larger areas is required, for example to transmit information to distant TMCs or to communicate with neighbor blocks, a distribution subsystem is also required. Below, the different topologies which were identified, shown in Figure 3, are introduced:  
* **Mesh Topology** A mesh topology presents a generic case used in applications concerning many WSN domains. As a result, there are many communication protocols available which are intended for the creation of self-forming and self-healing (fault-tolerant) networks, performing many-to-one and one-to-many  
Figure 2: Reference architecture for WSN-based ITS applications. communications. Consequently, this topology is adequate for applications which need to deploy their nodes in a grid layout (e.g., parking lots [28]), not necessarily regular, and which deliver their data to a central sink for decision making. However, this topology complicates the development of collaborative applications since the network self-forming mechanisms do not allow to control how nodes establish links between them. Linear or string based topologies arrange static nodes in a row parallel to the road, leading to an important reduction in the complexity of the routing protocols, since every node only has to decide in which direction to forward. This implies the use of a 1-dimensional geo-routing policy with simple addressing schemes which enables point-to-point communications between nodes, thus simplifying the development of collaborative applications. For example, a node detecting a vehicle can start the collaboration with neighbor nodes just by indicating that it intends to share the event with nodes up to a determined hop count. Two different alternatives fall into this group of topologies, the uniform string topology [36] and the cluster string one [11]. The difference between them lies in the allocation of tasks among the nodes. In a uniform string topology all the nodes have similar hardware resources and perform comparable tasks regarding vehicle detection and routing. In a clustered string, nodes are grouped into clusters including a more powerful Cluster Head (CH) and several constrained cluster nodes. Through this arrangement it becomes possible for those tasks requiring more powerful hardware or higher power consumption to be accomplished by a small subset of the nodes with additional capabilities; therefore allowing a reduction in costs and prolonging the lifetime of the rest of the network.