## 6 Communications
### Routing
This is also the case in WSN-based ITS applications in spite of the fact that there are many applications dealing with top-down or peer to peer routing [16, 37] or considering them as future work, as will be explained later. This data propagation scheme allows for simple routing, either linear routing between Cluster Heads [37], possibly using cluster members as intermediary nodes [26], or constructing a routing tree between them [30, 37] in which each node forwards every message to its parent. In both of the approaches data fusion tasks, if needed, are performed by the cluster heads. A top-down routing of information between a base station and other nodes is useful in order to issue commands to the latter ones (e.g., turning on warning systems, taking a photograph, _etc._). Multicast is an easy option if a routing tree exists, making each parent transmit to all its child nodes. Transmission to specific nodes, in turn, has been handled according to different ways. For example in [37] it has been accomplished by means of a source routing scheme, which forces a more powerful base station to manage a routing path to each destination but avoids intermediary nodes such as cluster heads from managing routing tables. In S3 [16] a level based static addressing scheme has been used to simplify routing, which in addition enables peer to peer and table-less routing. It is based on the construction of a tree and the address allocation to each node according to its position in the tree. This is currently used in the S3 system in order to allow that sensor nodes measuring speed trigger the capture of a photograph by a nearby camera-equipped sensor node without the need of previously routing to the base station, since the address of the camera node provides all the required routing information. Other works focus on the auto-configuration of the network. For instance, DGS [15] uses dynamic addressing for this purpose, automatically assigning 16-bit network address during the construction of the routing tree. In [26], incremental growth of the network is provided by means of an auto-discovery mechanism based on local broadcasts. In this work nodes deployed at different roads may communicate thanks to a hierarchy of controllers/cluster heads which act as simple routers. The discovery takes place iteratively: each node, regardless of its level in the hierarchy, discovers its closest neighbors in both directions as well as its preceding controller in the hierarchy. The routing mechanism allows sending messages to other nodes on the same road or to the preceding controller if the destination is not on the same road. This controller may, in turn, route the message to its presiding, adjacent or subordinate controllers in order to reach the destination of the message. Finally, the last routing scheme covers the location-based routing protocols, which use position information to route data to the desired regions. This scheme is quite frequent in VANET by means of the so-called geocast protocols. To this regards there are WSN-based proposals that, rather than implementing geographical routing on the WSN side, add geographic information to the data they gather and rely on VANET to propagate them to the appropriate region [40]. Other proposals in which vehicles extract information from the WSN using a polling mechanism (e.g., street parking availability checking) rely on location-based protocols on the WSN side [31, 93] since, due to the mobility of vehicles, the response may need to be delivered to a different location from the one where the data was originally requested. In these protocols once the requested data has been obtained by the WSN, the response is routed according to the position, speed and direction of the vehicle. In [31] each node forwarding the response selects the next hop according to the current time and expected destination of the vehicle, which periodically updates its mobility information.