Understanding the channel busy ratio metrics for decentralized congestion control in VANETs

Most of the emerging applications for road safety and traffic management rely on the frequent exchange of awareness messages among vehicles. Unfortunately, the 802.11 protocol poorly behaves under congested scenarios and cannot guarantee the reliability and timeliness demands of massively transmitted broadcast messages, leading to the severe degradation of safety. Recently, there has been a consensus from academia, automotive industries and standardization bodies in adapting transmission parameters (e.g., rate, transmission power) according to the channel load status. Although the objective of controlling channel load can be met with local load measurements only, the participation and fairness principles require the dissemination and sharing of load information among vehicles. In this paper, we aim to shed light on the dynamics of the channel busy ratio (CBR) metrics (locally measured and shared over one/two hops), commonly used in the literature and at the basis of the Decentralized Congestion Control (DCC) standard, under different density and mobility settings.