# Managing Emergency Situations in VANET Through Heterogeneous Technologies Cooperation
## 3 Architecture and Communication Protocol
### Protocol Communications
#### 3.2.3 Low Layer Protocol Messages  
On the basis of WSMP protocol, in this work, some messages are introduced to allow cooperation between on-board devices. These messages are introduced for warning advertisement goal and to enhance the pleasure of driving. Collision or high traffic may be the main reasons of congestion. ITS system should be smarter enough to find or forecast a situation like that and realize an opportune policy: it should solve or avoid congestion. Commonly the solution is to close the roads near the involved area but this solution is only temporary because it moves the problem to another area. In fact, there is a high probability that roads will be overloaded. A static solution or pre-programmed policies are not the best choice because they do not take into account the dynamics of the traffic and how the vehicles are moving in the areas. For these reasons, it is important to make a real time monitoring of traffic by acquiring continuously data from vehicles, sensors and devices. It is easy to understand that the amount of data to collect is huge and under this point of view a scalable architecture may help the overall system to enhance the performance. As regards the lower layer, a customized protocol, based on VANET architecture, is able to recognize the events and collect data. The high layer, instead, uses protocol to exchange data about event and to organize data elaboration by distributing tasks among local and global processes. For the low layer, the on-board system collects data by vehicle sensors. These data are locally analyzed for recognizing dangerous events and then they are sent towards the higher layers by exploiting interfaces ILLI and IHLI. These messages are herein reported:  
* _Register Message:_ this message is sent by vehicle towards RSUs once it changes Road_ID. Once it is received by RSU, it registers the vehicle_ID in the reachable car list if and only if the vehicle is passing though a managed Road_Id. RSU sends back a Register Acknowledgment that contains data about managed road; The RSU discards register message if vehicle that has generated message is not passing on managed Road_Id. * _POSition Update_ (POSUP): each node manages its own neighbor list by receiving the periodical POSUP message. It is sent by each vehicle once the position and current speed are acquired by inner sensors. Destination nodes are the neighbors of vehicle and lead RSU. Its dissemination is bounded to one hop and it is composed as shown in Figure 6. When a POSUP message is received by a RSU, it elaborates vehicle position and places it in the related lane queue of the road. In Table 1 it is possible to view its fields specification. * _Warning Message_: It is sent by vehicles in the network when something happens during their journey. For example, when a critical situation is recognized by E-OBU by elaborating sensor data. Here two types of warning messages have been considered: _Warning Beacon_ (WB) and _Collision Beacon_ (CB). Let us describe in detail how these messages are composed. In Figure 6 the WB message is depicted. Beacon is encapsulated in the WSM Data field of WSM packet. Its length is 8 bytes composed of mandatory fields plus other optional rows of 2 bytes length. Their number is signaled in a specified mandatory field (see Table 2). The vehicle elaborates dangerous events as shown in Figure 7. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Field Name** & **Type of Data** & **Length in Byte** & **Description** \\ \hline _Source_ID_ & Unsigned int & 2 & ID of E-OBU that generates the message \\ _Latitude_ & Float & 2 & Latitude of Vehicle \\ _Longitude_ & Float & 2 & Longitude of Vehicle \\ _Road ID_ & Unsigned Int & 2 & Road ID \\ _Lane ID_ & Unsigned Short & 1 & Lane ID \\ _Speed_ & Unsigned Short & 1 & Speed in Kmph \\ _Sequence Number_ & Unsigned Short & 1 & Sequence Number \\ _Padding_ & Unsigned Short & 1 & N.A. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _POSition Update_ (POSUP) Message fields specification. Figure 6: Beacon Low Layer Message Data Structure.