## 3 Network Architecture and Topology
### Distribution Subsystem  
The Distribution subsystem is responsible for exchanging information between the different subsystems of an ITS application. In a tiered architecture such as the one in Figure 2, it is placed in a central position, receiving communication requests from all the other subsystems and serving them accordingly. It is in charge of the transmission of sensed data to the Decision Making subsystem and, conversely, of the transmission of commands from the Decision Making subsystem to the Sensing and Execution subsystems [37]. Similarly, it interconnects the different sensing groups of a network (described above). This results in a scalable network created by the composition of these groups. In this respect, the Distribution subsystem can also interconnect physically isolated groups of nodesor vehicles, thus enabling the interconnection of WSN islands deployed in different parts along the road [14] or of clusters of vehicles which other way would form unconnected VANETs [33]. The Distribution subsystem consumes a significant amount of power owing to the requirements imposed on its devices. They are in charge of forwarding every event reported by every node of the Sensing subsystem, which may occur at a high rate. In addition, it must be done under the time constraints defined by the application. This requires very active nodes in order to guarantee information delivery in a timely manner, jeopardizing energy savings. Therefore, the devices used in this subsystem need a more generous power budget than the sensing nodes, thus making additional power sources necessary. There are different ways of implementing the Distribution subsystem. Although the most obvious is deploying WSNs with external power sources, there are other possibilities. One of them consists of employing vehicular networks to disseminate information. Devices used on the vehicles have no energy constraints since they can be powered by the vehicle's onboard facilities. In addition, the mobility of vehicles, which is a limiting factor for other kind of applications, helps spreading data throughout vehicles and static nodes along the road, and it relieves static nodes from distribution tasks. The simplest scheme entails using direct transmission between vehicles, _i.e._, a one-hop network which enables data sharing from a source vehicle to every vehicle approaching it [38, 39]. Given the technologies used up to now, this usually translates into mobile WLANs to which other vehicles associate. The second alternative is based on using a multi-hop VANET distribution network [12, 40]. This option requires more system resources, but it also facilitates faster data dissemination and scales much better as the number of technologically equipped vehicles grows. Obviously VANETs exhibit important advantages which make them an alternative to be considered for the Distribution system. However, they also have some drawbacks; the main one is the negative impact of low technology penetration rates on the system performance. If the density of equipped vehicles is not high enough, multihop routes cannot be constructed and connectivity between vehicles is very sporadic. Currently the implantation of VANETs is still in its early stages and full sets of equipped vehicles can only be seen in some research works but rarely in real life. Therefore, alternative solutions with higher penetration ratios are still an interesting option. Clear examples are smartphones, whose number is increasing exponentially, with 297 million sales in 2010, 72.1% more than the previous year [41]. They are not a distribution technology in themselves, but rather devices intended for user interaction. However, they are supported by cellular networks which can distribute information. As a result, gateways to cellular networks such as 3G networks can be included in the system design as the distribution technology [11]. A Distribution subsystem based on cellular networks is an attractive option, not only because of its high availability but also because of its deployment cost.