Impact of decentralized congestion control on contention-based forwarding in VANETs

Vehicular ad hoc networks based on IEEE 802.11 OCB suffer from channel congestion, which will be critical for safety applications when the channel becomes saturated. Therefore, decentralized congestion control (DCC) is needed that keeps the channel load under a pre-defined threshold. In the European system for vehicular communication, a DCC-gatekeeper has been inserted into the protocol stack that reduces the data traffic generated or forwarded by the vehicle. This DCC-gatekeeper affects the functionality of the contention-based forwarding (CBF) and degrades its performance. In this paper we evaluate the performance of CBF under congestion-free and saturated channel conditions for several protocol variants: DCF, EDCA and DCC-gatekeeper. The results show that the standard DCC-gatekeeper hampers the functionality of CBF even for small node densities. We enhance the DCC-gatekeeper-based approach to overcome this issue and evaluate the protocol variants in terms of reliability, latency and channel load in a freeway scenario with realistic trace-based mobility.