## 3 Network Architecture and Topology
### Sensing Subsystem
The  
Figure 3: Sensing topologies and examples: **(a)** String topology, overtaking assistance, **(b)** mesh topology, parking lot, **(c)** star topology, speed detection. typical actions of CHs are the execution of routing tasks to other clusters or subsystems [26], activation of sleeping nodes upon the detection of a vehicle [12] or the communication with vehicles through additional wireless interfaces [12]. \(\bullet\) **Star Topology**  
In the star topology a few sensor nodes are set around a sink. Its advantages are simplicity and the avoidance of routing schemes, which helps to preserve energy. Two differentiated cases can be considered depending on whether transmissions are performed in a unidirectional [37] or bidirectional way [15]. Unidirectional communications are assumed to deliver information from sensor nodes towards a sink for data analysis. In spite of potentially offering less functionality, they enable additional power savings since sensor nodes are not the destination of any message and, therefore, they do not need to periodically power up their radio and wait for incoming messages. Consequently, these are the most energy-efficient cases of all, especially the former. However, the placement of nodes is limited to the area of coverage of the sink. \(\bullet\) **Barrier Topology**  
The barrier topology deploys several nodes transversely, one per lane, across the road (see an example in Figure 4). It can be considered as a particular case of star topology, which inherits all of its characteristics. Since it is a very simple topology, its functionality is restricted. However, it allows to obtain the number of vehicles passing a determined point of the road and it is therefore quite useful in applications which need an estimation of the traffic load. This topology may be employed in a clustered fashion [37]. Each of the sensor nodes at every lane does a simpler function, detecting vehicles and reporting to the cluster head at the roadside (therefore likely to employ only unidirectional communications). Apart from receiving notifications from sensor nodes, the latter manages communications with vehicles as well as with other neighbor clusters through the Distribution subsystem. Figure 4: Distribution subsystem based on the availability of cellular networks. * **Disconnected Nodes (with Mobile Sinks)**  
A common requisite of all the previous topologies is that there must be connectivity between any static node in the deployment and at least one of its neighbors, which limits the maximum separation between nodes and forces the emplacement of nodes in areas where other neighbor nodes are situated. Using vehicles as mobile sinks can overcome this limitation since disconnected static nodes can send and receive information while a vehicle is nearby. Therefore, they can be placed arbitrarily. This offers several advantages including scalability, since the system may be easily extended by installing a single node at the desired position; robustness, since the malfunction of one node does not affect its neighbors; and finally, as a consequence of not having a maximum separation between nodes, the possibility of covering larger areas at lower cost (at the expense of a low resolution). In contrast, this scheme has some drawbacks, such as requiring that all static nodes be provided with extended capabilities (they all have to sense and communicate with vehicles) or restricting the events about which a node can inform to those happening in its local sensing area, which are transmitted to vehicles when they are close to the phenomenon. The latter can be partially overcome by using communication standards which support transmissions at a longer range than typical WSNs protocols. However, in order to increase the anticipation with which vehicles receive information, the use of a vehicular distribution tier is required [38].