## 3 Network Architecture and Topology
### Network Architecture Classification
#### 3.5.3 Heterogeneous Networks  
Heterogeneous systems combine several wireless technologies to facilitate the development of more effective applications. Since every wireless technology offers distinctive advantages and disadvantages, a heterogeneous system seeks to focus on the advantages of a particular technology to compensate the drawbacks of another technology also employed in the final system. For example, WSNs have their main weakness in their constrained use of the scarce available energy. However, this is a minor issue in VANETs. On the contrary, achieving high technology penetration rates in VANETs in order to boost performance is not straightforward, but the installation of WSN nodes on selected roads is a simpler task. One can take advantage of the composition of heterogeneous technologies, which results in the devices of each technology arranged in their own tier, assigning sensing and distribution tasks to either of the tiers. Two different types of heterogeneous applications can be distinguished. The first are WSN-centric applications, in which a road WSN deployment is complemented by a VANET [12, 38] or a cellular network [11, 15]. Those applications are quite similar to those described in the preceding sections, but they differ from them in that either VANETs or cellular networks perform the data distribution tasks. This may suppose alleviating static WSN nodes from the burden of long range data forwarding and thus preserving energy. In the case of VANETs, they offer the possibility of propagating information gathered by the WSNs either to other vehicles which are approaching to the sensed area or to distant WSN nodes, possibly detached with the originating nodes. Therefore, vehicles may know the existence of dangerous conditions in advance, as shown in Figure 1 above. The same can be achieved by using cellular networks. However, information follows a different path in order to arrive to smartphones in the vehicles or, thanks to the wide coverage of cellular networks, to distant and centralized TMCs [15]. In this case the distribution tier is not only formed by the cellular network itself, but also by a set of WSN nodes which forward data from the sensing nodes to the nearest base station [11], as illustrated in Figure 4.  
The other category of heterogeneous applications is the VANET-centric applications. In this category the WSNs are used to improve the performance of existing VANETs [13, 33]. They assume that VANETs are established but that they are split into different isolated partitions, which consequently cannot share information. WSNs can solve this problem in an analogous way as RSUs do, _i.e._, storing information from a partition and delivering it to subsequent partitions when vehicles arrive at the WSN location (as depicted in Figure 5). However, RSUs are scarce and expensive equipment. In contrast, simple WSN islands, ranging from a single node to a barrier of them, are a cheaper solution for those locations where RSUs are not already available or cost-effective. Applications which involve the use of VANET communication entail the interconnection of two different wireless network technologies. Consequently, they require gateway devices equipped with two wireless interfaces in order to enable the intercommunication between both networks (e.g., portable PCs with WiFi [49] and 802.15.4 [50] or the recently available NEC LinkBird-MX [51] with 802.11p [52] and 802.15.4 interfaces). These are more expensive and power consuming devices, which motivates several saving procedures. Clustering devices are the most common. At the road WSN deployment, assigning gateway functionalities to cluster heads decreases power consumption and costs in the rest of cluster members. Similarly, arranging vehicles into clusters also saves energy, since gateways only need to communicate with the leading vehicle of each cluster which can, in turn, disseminate information within the cluster.