Decentralized congestion control techniques for VANETs

Abstract

Direct vehicle-to-vehicle (V2V) communication must operate reliably and with low latency under all circumstances to allow for imminent collision prevention. However, the amount of data to be exchanged is limited by the physical limitations of the communication channel. In vehicular ad hoc networks (VANETs), this limitation is particularly relevant when vehicle density rises and thus lots of information needs to be exchanged, bringing the communication system into a congested state. Rapidly changing topology, channel characteristics, distributed medium access, and challenging active safety application requirements cause many challenges to the resource management. This chapter reviews these issues to motivate suitable Decentralized Congestion Control (DCC) algorithms and cross-layer coordination mechanisms. In particular, it is shown that performance degradations like packet losses, the reduction of the effective communication range, and packet transmission delays are correlated with the channel load. The strategy of the DCC is to avoid these degradations by limiting the load offered by each vehicle to the radio channel, such that a certain channel load threshold is not significantly exceeded. This mitigates the communication range degradation and keeps the packet latencies moderate. The DCC is already part of specifications in the European Telecommunications Standards Institute (ETSI), e.g. ETSI TS 102 687.