# Coordination of Congestion and Awareness Control in Vehicular Networks
## 4 Evaluation  
### Scenario and Simulation Settings  
The proposed COMPASS methodology is evaluated in a highway crossing scenario with 4 road segments and 4 lanes per road segment where vehicles are uniformly distributed in each lane. In the crossing scenario, we evaluate the spatial distribution of the channel load and the application's effectiveness experienced by each vehicle. This scenario can be characterized by the traffic density in vehicles/km/lane, and the length of each of the four road segments (3.5 km). Three different traffic densities have been simulated (50, 75 and 100 vehicles/km/lane). The road topology can influence the performance and operation of the protocols considered in this study. The reliability, stability and effectiveness of congestion, awareness and topology control protocols have often been evaluated in straight highway scenarios where the spatial distribution of vehicles can be considered uniform, especially under free-flow conditions. With such a uniform distribution, all vehicles sense similar channel load levels, and the protocols can more easily converge to a stable solution. However, when vehicles sense different channel load levels, it is more difficult to guarantee the stability and fairness of protocols. In this context, the selected scenario is more challenging than for example a straight highway or T-way scenario. This is the case because highway crossing scenarios generally experience higher concentration levels of vehicles per square kilometer closer to the crossing area. As a result, vehicles close to the crossing area experience significantly higher channel load levels than vehicles located far away from the crossing area since they can sense more vehicles. In the selected scenario, we consider that all vehicles periodically transmit 1-hop broadcast packets on the control channel using IEEE 802.11 p. Each packet has a payload of 250 Bytes, and contains the positioning, speed and basic status information of the transmitting vehicle. All vehicles initially usethe same transmission power (by default \(P_{t}\) = 23 dBm) and a random packet transmission frequency (\(T_{f}\)) between 1 Hz and 10 Hz. Following [25], this study utilizes the Nakagami-\(m\) propagation model with \(m=3\). The radio interface is configured for all vehicles with a carrier sense threshold of \(-90\) dBm and a reception threshold \(-82\) dBm. Awareness control protocols set the communication parameters so that application requirements are satisfied. Each vehicle in the scenario runs a vehicular application that requires that at least \(R\) packets are correctly received per second by all vehicles within a given communication range \(CR\). To decouple this study from any particular application, vehicles set their initial application requirements randomly, with the communication range varying between 50 and 200 m and the packet reception frequency between 1 Hz and 10 Hz. From the initial settings, the application requirements linearly vary during the simulation. \(CBR_{max}\) has been set equal to 0.6 following [22]. The \(CBR\) values computed in this study have been obtained from the aggregation of all packets sensed by each vehicle's radio interface, i.e., with a received signal higher than \(-90\) dBm. COMPASS can adapt its operation to the dynamics of the protocols it coordinates. In fact, COMPASS computes the ICS and reconfigures the communication parameters as soon as the CS of any of the protocols changes. The application requirements and the number of neighbors normally change at a slower pace than the channel load. As result, COMPASS is executed in this study every time a new CBR value is obtained. Following [23], a new CBR value is obtained every 250 ms, which is considered sufficiently high for COMPASS to be able to satisfactorily adapt its operation to the dynamics of the vehicular environment. Table 1 summarizes the communication and simulation parameters. Simulations have been executed to ensure the statistical accuracy of the presented results.