# Managing Emergency Situations in VANET Through Heterogeneous Technologies Cooperation
## 3 Architecture and Communication Protocol
### Proposed Architecture  
The proposed architecture is composed of three layers in order to make the system more scalable, efficient and able to respond faster to the dynamics that may happen in the real environment. This may help to reach our goal by reducing reaction time for humans and machines. The high level is composed of the _High Road Traffic Manager_ (HRTM). It is instantiated on the cloud and cooperates with the _Low Road Traffic Manager_ (LRTM), which is instantiated on edge layer. These two layers are responsible of coordinating traffic conditions by exploiting monitored data. These data are collected from observations on the field by exploiting _RoadSide Units_ (RSUs), vehicles and road devices. In particular, HRTM bringsinformation from LRTM and it elaborates them for achieving continuous data about road, area status and events that may change traffic load. These information are used by HRTM to optimize vehicle flows at global level by exploiting field devices such as traffic lights and smart road signs. In this way, HRTM can globally optimize traffic loads and reduce journey time. It may also dynamically change the city zone to _restricted_ or _unrestricted_ by allowing or not traffic flows in some areas. LRTM periodically communicates with local RSUs for collecting data about roads and traffic levels. LRTM is responsible of a direct interface with the covered RSUs by querying them or sending broadcasted alert messages in the network. It has other tasks to accomplish but they will be discussed deeply in the next sections. HRTM and LRTM are active actors in the optimizations of vehicle distribution inside the covered area. HRTM is responsible of the global optimization by dynamic rearranging weights that are used by vehicles to evaluate their paths. LRTM, instead, is responsible to local optimization and it communicates alerts when dangerous events occur. Both HRTM and LRTM exploit RSUs to disseminate information to reach vehicles.  
In case of congestion or dangerous situation, each vehicle can re-evaluate its own itinerary based on new path computation with updated cost and constraints made by HRTM. The messaging system among vehicles and infrastructure is made exploiting the _Vehicle-To-Infrastructure_ (V2I) link. Message format is based on the use of _Wave Short Message Protocol_ (WSMP) [39; 40; 41], which allows _Vehicle-To-Everything_ (V2X) communications [42]. This link is used by RSUs also to communicate the messaging coming from higher architecture level such as LRTM or HRTM. V2V communication link is also used by vehicles to disseminate information as happens in common VANETs architectures. Instead, regarding the Traffic lights and road signs they are controlled directly by LRTM exploiting wireless link by taking advantage of _Machine to Machine_ (M2M) communication protocol to make more reliable and scalable networking, avoiding resource wastages. Regarding the core network, wired connections are used because of the delay constraints. In fact, it is important to have low delays, as well as possible, to make the system response time faster. As depicted in Figure 1 the end system layer is composed of heterogeneous devices.  
Figure 1: The proposed heterogeneous and scalable architecture used in _Vehicular Ad-hoc Network_ (VANET) environment. Here three layers are responsible of the network management and services such as data dissemination, data collection, data storage and data elaboration.