Efficient congestion control in VANET for safety messaging

The prime goal of Vehicular Ad hoc Network (VANET) is to provide life safety on the road. To achieve this, vehicles make use of two types of safety messages i) Periodic safety messages (beacons): to exchange status information e.g. position, speed etc. ii) Event-driven messages: broadcast in case of an emergency situations e.g. accidents, hard-braking etc. As both type of messages use same Control Channel, in dense traffic periodic beacons may consume the entire channel bandwidth resulting in a saturated/congested channel. In a congested channel event-driven messages may not be able to access the channel at all, thus providing no safety. Researchers have proposed different strategies to overcome this problem. Most of these strategies rely on the concept of a limiting beacon bandwidth usage level below a specific threshold and thus implicitly reserving a chunk of bandwidth for high priority even-driven messages. In the proposed strategies either transmission rate or power control techniques are used to control congestion. In this paper we evaluate such techniques and highlight following major drawbacks first: using only power control techniques do not satisfy requirements of envisioned beacon-dependent safety applications, second: methods used for measuring channel usage level in transmission rate control technique may not be as effective under real world conditions. We also present a conceptual view of a congestion control scheme using transmission rate and transmission power simultaneously for optimal results.