## 3 X-NETAD: The Idea  
### The Big Picture  
The goal of the X-NETAD project was to design and implement an innovative traffic alerts dissemination system based on a cross-network (cellular and WiFi) software application, installable on dual-interface (UMTS and WiFi) smartphones [11]. An illustrative representation of the envisioned cross-network traffic alert dissemination system is shown in Figure 1. The key idea of the X-NETAD approach is that of leveraging on the spontaneous formation of WiFi local VANETs, with direct connections between neighboring vehicles, in order to disseminate traffic alerts and GPS location in the VANETs very quickly and inexpensively. It is important to remark that X-NETAD has been conceived of for a unidirectional exchange of information from a centralized data center to mobile vehicles, but it does not allow collecting information from the nodes. (A distributed detection application, based on a similar architecture, is proposed in [25].)  
The X-NETAD system has been designed in such a way that, at any given time, only a small percentage (e.g., 10%) of the vehicles (_i.e._, the smartphones inside the vehicles) are assumed to be directly connected to the traffic alerts dissemination system through an Internet connection provided by a cellular network. More precisely, the X-NETAD architecture is based on three types of entities: (i) _Primary Users_ (PUs); (ii) _Secondary Users_ (SUs); and (iii) an _vehicular ad hoc networks (VANETs)_ (AS). The AS is in charge of maintaining, filtering and distributing the traffic information, collecting it from a series of third-party providers--in the specific case of the X-NETAD project, the Israeli company, Cellular, had this role. The AS supports the following minimum set of requirements and operations:  
* to provide suitable access interfaces on the Internet;
* to provide a secure communication channel with the PUs;
* to possess authentication, authorization and accounting capabilities;
* to support a basic set of operations to allow the PUs to register to the service and to obtain localized traffic information (of a given geographical area or route) according to a pull-based distribution model.  
Figure 1: Illustrative Cross-Network Effective Traffic Alert Dissemination (X-NETAD) traffic alerts dissemination scheme.  
PUs are privileged users that periodically retrieve the information of interest from the AS, through a 3G (in general, cellular) Internet connection. The SUs obtain information from the PUs via Vehicle 2 Vehicle (V2V) communications according to a push content distribution model (e.g., the PUs periodically disseminate it without any explicit query from the SUs), but they cannot receive the information directly from the AS. To this end, a very efficient broadcast technique for information dissemination in VANETs, denoted as Irresponsible Forwarding (IF), has recently been proposed [26, 27]. IF assigns to each vehicle an optimized rebroadcast probability, which takes into account the surrounding vehicle spatial density and can be easily tuned; this guarantees fast information dissemination, even in the presence of high vehicular traffic. According to theoretical and simulative analysis presented in [27], the IF protocol and the related Silencing IF protocol perform better than flooding in terms of latency and efficiency and slightly worse in terms of reliability. Furthermore, the advantages of IF with respect to flooding become more evident as the network congestion increases, as in the case of high average vehicular densities or high network loads. The IF protocol does not require nodes to exchange any additional message and, therefore, could easily replace the legacy flooding protocol that is still used by many Car-2-Car Communications Consortium (C2C-CC) [28] related projects. For instance, the geo-broadcasting protocols defined within the GeoNet project (e.g., GeoBroadcast and TopoBroadcast) rely on the flooding protocol for multihop broadcast communications [29]. We provide additional details on the IF protocol in Subsection 3.3.