Collision-Aware Rate Adaptation in multi-rate WLANs: Design and implementation

Many rate adaptation algorithms have been proposed for IEEE 802.11 Wireless LAN devices and most of them operate in an open-loop manner, i.e., the transmitter adapts its transmission rate without using the feedback from the receiver. A key problem with such transmitter-based rate adaptation schemes is that they do not consider the collision effect. Accordingly, they often result in severe throughput degradation when many transmission failures are due to frame collisions. In this paper, we present a transmitter-based rate adaptation scheme, called CARA (Collision-Aware Rate Adaptation), and its MadWifi-based implementation. The key idea of CARA is that the transmitter combines adaptively the RTS/CTS (Request-to-Send/Clear-to-Send) exchange with the CCA (Clear Channel Assessment) functionality in order to differentiate frame collisions from transmission failures due to channel errors. The effectiveness of CARA schemes is evaluated via extensive ns-2 simulations and testbed experimentations.