# Coordination of Congestion and Awareness Control in Vehicular Networks
## 1 Introduction
Instead, it receives as input their configuration requests, and selects a configuration of the \(p_{1}\) and \(p_{2}\) parameters that is suitable to both protocols. The integrated methodology depends on the specific details and implementations of the protocols. This is not the case of the coordinated methodology since it does not require the modification of the protocols it tries coordinating. COMPASS can hence coordinate the operation of a varying number of protocols without the need for a new re-design every time an additional protocol needs to be taken  
into account or be removed from the stack. COMPASS can be implemented within the transversal Management layer defined by ETSI [4] or the CALM Management entity defined by ISO TC204 [17]. From the Management layer, COMPASS has access to all the metrics collected by the different layers (e.g., the channel load or the number of neighboring vehicles) and can configure parameters at different layers (e.g., the transmission power and packet transmission frequency). The advantages of COMPASS compared with existing solutions can be summarized as:  
* COMPASS solves conflicts between protocols that operate independently over the same communication parameters. Such conflicts can occur, for example, when two different protocols request contradictory changes to a given transmission parameter. * COMPASS can solve these conflicts by coordinating the operation of different protocols operating over the same communication parameters. Such coordination does not require the integration of these protocols or their re-design, which facilitates the future evolution of vehicular networks and the coexistence with legacy solutions. * COMPASS is not restricted to a set or number of protocols. COMPASS can coordinate the operation of a varying number of protocols (acting over the same communication parameters), and this number can change over time. This again increases the impact of COMPASS and the future evolution of vehicular networks. * COMPASS can achieve similar performance levels to that achieved by integrating the conflicting protocols, with the added benefits of an easier evolution of vehicular networks and the coexistence with legacy solutions. * COMPASS has been designed so that it can be easily integrated into the transversal Management layers of the ITS communications architecture defined by different standardization organizations such as ETSI or ISO. * COMPASS has a very low computational complexity which facilitates its real-world implementation and possible impact on standardization. The COMPASS methodology is analyzed in this study considering the IEEE 802.11p standard. IEEE 802.11p is the first standard developed for vehicular networks, and is the basis of DSRC in the US and ITS-G5 in Europe. However, COMPASS is not specific to a particular standard. In fact, the proposed methodology is generic and could be used to coordinate interacting protocols over alternative radio access technologies. In particular, COMPASS could also be used to address congestion control with C-V2X or LTE-V mode 4 standard that was published at the end of 2017 under 3 GPP Release 14 for cellular-based V2V communications without infrastructure support [18]. C-V2X Mode 4 does not specify a particular congestion control algorithm, but defines the related metrics and possible mechanisms to reduce the channel congestion. ETSI and 3 GPP are currently analyzing whether the current DCC solution could be valid for C-V2X Mode 4. This paper is structured as follows. Section 2 presents the congestion and awareness control protocols used in this study as a reference, as well as a protocol that integrates congestion and  
Figure 3: Integration and coordination of protocols operating over the same communication parameters. awareness control. Section 3 presents the proposed COMPASS methodology for the cross-layer coordination of multiple vehicular protocols operating over the same communication parameters.