# Coordination of Congestion and Awareness Control in Vehicular Networks
## 3 Methodology for Cross-Layer Coordination  
Solutions such as INTERN are tightly coupled to the specific congestion and awareness control schemes that are being integrated. Such integration can limit the future evolution of a system or standard. For example, the replacement of the congestion or awareness control implementations with more advanced schemes would require the design and implementation of a new integrated solution. The integrated solution would also have to be replaced if the need arises to integrate an additional protocol, for example a topology control protocol. To reduce the restrictions introduced by the integrated methodology, we propose a methodology to coordinate different protocols that operate over the same communication parameters. The proposed methodology does not change the protocols, and can operate independently of the protocols to be coordinated. The proposal is first demonstrated considering the coordination of congestion and awareness control. The flexibility and evolution capabilities of the coordinated proposal are then demonstrated by coordinating congestion, awareness and topology control protocols. The coordination proposal is based on the concept of Configuration Sets (CS), and is referred to as COMPASS (Cross-layer coordination of multiple vehicular protocols). A configuration set is here defined as a set of configurations of communication parameters that satisfies certain requirements that can be protocol-dependent or be imposed by hardware/standards restrictions; e.g., power levels, data rates, packet transmission frequencies, or sensitivity levels are all defined or limited by standards. In general, configuration sets have \(C\) dimensions, with \(C\) being the number of communication parameters under analysis. Formally, the configuration set for a certain communication protocol, \(j\), can be represented as follows:  
\[CS_{j}=\Big{\{}\overset{\rightarrow}{c}=(c_{1},c_{2},\ldots,c_{C})\ \Big{|}\ c_{ij}^{min}\leq c_{i}\leq c_{ij}^{max}\ \forall\ i=1,2,\ldots,C\Big{\}} \tag{7}\]  
where \(c_{i}\) represents the communications parameter \(i\), \(1\leq i\leq C\), and \(c_{ij}^{min}\) and \(c_{ij}^{max}\) represent the minimum and maximum values for \(c_{i}\) according to protocol \(j\). Protocol \(j\) could be a congestion or awareness control protocol, for example. Congestion control protocols like A-DCC establish the maximum packet transmission frequency of each vehicle so that the channel load does not exceed \(CBR_{max}\). As a result, their CS includes all packet transmission frequency values below such maximum. Similarly, an awareness control protocol such as MINT calculates the minimum packet transmission frequency and transmission power needed to satisfy the application requirements. As a consequence, its CS includes all packet transmission frequency values and transmission power levels above such minimum, and below hardware/standard limits. It is important noting that different vehicles can have different CSs since their application requirements and experienced channel load levels vary with their vehicular context. For example, vehicles in scenarios with different traffic densities will experience different channel load levels. Also, vehicles in a traffic jam moving at low speeds will require a lower communications range compared to vehicles moving at free-flow speeds. Once the CS of each protocol has been identified, COMPASS defines the Intersection Configuration Set (ICS) as the intersection of the CSs of all protocols that have to be coordinated. ICS then represents the set of configurations of communication parameters that satisfies the requirements of all protocols to be coordinated. ICS is also context-dependent and might therefore be different for different vehicles. ICS can be expressed as follows when coordinating the operation of \(P\) communication protocols:  
\[ICS=\underset{j=1}{\cap}CS_{j} \tag{8}\]  
Figure 4 illustrates the COMPASS proposal and the concepts of CS and ICS for a vehicle implementing congestion and awareness control protocols.