# Managing Emergency Situations in VANET Through Heterogeneous Technologies Cooperation
## 3 Architecture and Communication Protocol
### Protocol Communications
#### 3.2.4 High Layer Protocol Messages  
In this section, the messages used by the higher layer of the architecture are proposed. Lower layers have the main goal to collect data from vehicles/environments and locally manage traffic in case of recognized events that may congest traffic. Higher layer, instead, has the main goal to keep under control traffic condition globally. In order to allow this kind of task opportune messages have to be sent along the network for keeping updated data on real traffic status. Here the description of the messages is reported:  
* _Hello Message_ (HM): it is sent between LRTMs and HRTM when a LRTM node starts to work. In order to keep updated the list of active LRTM, this message is sent periodically by exploiting the IHLI interfaces;
* The _Command Message_ (CmdMsg): it is one of the most important messages and its main goal is to inform road smart device to make an actuation such as activate a gate control to deny or allow access in a controlled area or by acting on city street lights to change their timing and so on. It is composed as shown in Figure 8. It is sent towards the IHLI and ILLI interfaces as shown in Figure 2; This message is generated by HRTM and LRTM to actuate traffic policies. * The _Collision Beacon_ (CB): it is utilized by vehicles to inform LRTM about a happened collision. Moreover, the LRTMs uses this messages to inform one-hop LRTMs about this event. This message is then forwarded by LRTMs to covered vehicles as Warning Message as already described in Figure 6. _BC_ message is composed of two main fields: _RSU_ID_ (the ID of the RSU device) and _Lane_ID_ (the ID of the lane segment);
* _Street Status Message_ (StrStStMsg): it is sent between LRTMs and HRTM. This message is used for updating the status of the covered roads and to bring up data to HRTM. _Low Road Traffic Manager_ (LRTM) collects information from lower layers and aggregates data. Periodically these data are reported to the higher layer to update roads status globally at the end of an observation window. The message is composed as shown in Figure 8. Here _Type of Data Field_ can be Inhibit or Update. In case of Inhibit message data fields shall not be taken into consideration, otherwise, \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 or RSU that generates the message \\ _Road\_ID_ & Unsigned int & 2 & unique ID that represents the road globally in the ITS \\ _Lane\_ID_ & Unsigned short & 1 & ID of road lane involved in warning \\ _TOW_ & Enumerate & 1 & Warning or Collision Beacon \\ _Interface origin_ & Enumerate & 1 & [ILLLI, HLI,...] \\ _Optional Data Length_ & Unsigned Short & 1 & Optional Data length in Byte \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Warning Message fields specification. Figure 8: High Layer messages sent on _Inter-Low-Layer Interface_ (ILLI) and _Inter-High-Layer Interface_ (IHLI). if the value is set to Update then data fields shall be composed of six bytes that means three float values. The first value represents the weight of the edge, which is called \(w\), the second one shall be the density value called \(\pi\) and the last one shall be the traveling delay in minutes called \(\delta\). These values will be evaluated by RSU locally as shown in Section 5.3. Table 3 specifies the contents of message. It reports three important values used to perform ITS choices in terms of traffic management. In particular, these parameters are referred to a specific lane of a specific road. The \(road\_1D\) field gives us information about the vertexes of the edge to be considered, instead, the \(lane\_1D\) field gives us details about the order to recall the right edge. In fact, commonly the edge \(e_{i,j}\) may be different of \(e_{j,i}\). Thus, the weight of lane in the global maps is represented by the weight of the edge \(e_{i,j}\) called \(w_{i,j}\) as well as the density factory called \(\pi_{i,j}\) and delay term called \(\delta_{i,j}\).