# Coordination of Congestion and Awareness Control in Vehicular Networks
## 1 Introduction  
Vehicular networks require the exchange of positioning and basic vehicular status information between neighboring nodes. Such exchange is based on the periodic transmission/reception of 1-hop broadcast messages on the so-called control channel [1]. The critical nature of this reference channel has fostered significant efforts in the research and standardization communities to design congestion control schemes that adapt the communication parameters to ensure the scalability and adequate operation of vehicular networks [2; 3]. For example, the ETSI ITS Communications architecture [4] includes a Decentralized Congestion Control (DCC) functionality with several components distributed over different layers of the protocol stack (Figure 1). One key component is DCC_CROSS [5]. DCC_CROSS runs the core of the DCC algorithm and adjusts the communication parameters based on metrics received from the other DCC components using a reactive state-based and linear adaptive approach. The DCC_ACC [6] component measures the local channel load and restricts the access to the channel based on the output from the DCC algorithm. The DCC_NET [7] component disseminates the local DCC parameters to other vehicles. Finally, the DCC_FAC [8] component controls the packet generation rate, packet priorities and re-broadcasts based on the output received from the DCC algorithm. Most congestion control algorithms are based on the dynamic adaptation of the packet transmission frequency and/or power to control the channel load and provide fair and harmonized access to the wireless medium. These communication parameters are also critical for other protocols, and it is therefore necessary to carefully consider their impact. For example, awareness control protocols [9] seek to ensure each vehicle's capacity to detect, and possibly communicate with the relevant vehicles and infrastructure nodes present in their local neighborhood. To this aim, awareness control protocols must adapt the communication parameters to guarantee the necessary communications or awareness range [10; 11]. It is possible that if a congestion control protocol reduces the packet transmission frequency or power to decrease the channel load, the resulting transmission frequency or power does not meet the latency or communications range requirements of a particular application. This scenario is illustrated in the example depicted in Figure 2. The high density of vehicles in the congested road segment would require the use of congestion control protocols to control and limit the channel load. These protocols would, for example, reduce the transmission power or packet transmission frequency to all vehicles in the scenario in order to reduce the channel load and ensure the networks' stability. However, the communication parameters cannot be modified equally to all vehicles since their application requirements are different as a result of the different speeds and distances to neighboring vehicles. Vehicles under free-flow conditions require a communications range sufficiently large to allow for a safe lane-change maneuver for example. Reducing their transmission power would hence negatively affect their capacity to satisfy their application requirements. On the other hand, the communications range for vehicles in the traffic jam direction can be notably reduced as a result of their limited mobility. Vehicles in the traffic jam could hence reduce their transmission power without negatively affecting their application requirements. This example clearly shows the need for an application- and context-centric design of vehicular networks, and a careful coordination of congestion and awareness control. Figure 1: Decentralized Congestion Control (DCC) in the ETSI ITS Communications architecture [6]. Figure 2: Highway scenario with a traffic jam in one direction of driving and free flow conditions in the other direction. Vehicles under the same channel load conditions can have different application requirements.