Rate avalanche: The performance degradation in multi-rate 802.11 WLANs

The request-to-send/clear-to-send (RTS/CTS) exchange was defined as an optional mechanism in DCF (distributed coordination function) access method in IEEE 802.11 standard to deal with the hidden node problem. However, in most infrastructure-based WLANs, it is turned off with the belief that the benefit it brings might not even be able to pay off the transmission overhead it introduces. While this is often true for networks using fixed transmission rate, our investigation leads to the opposite conclusion when multiple transmission rates are exploited in WLANs. In particular, through extensive simulations using realistic channel propagation and reception models, we found out that in a heavily loaded multi-rate WLAN, a situation that we call rate avalanche often happens if RTS/CTS is turned off. The rate avalanche effect could significantly degrade the network performance even if no hidden node presents. Our investigation also reveals that, in the absence of effective and practical loss-differentiation mechanisms, simply turning on the RTS/CTS could dramatically enhance the network performance in most cases. Various scenarios/conditions are extensively examined to study their impact on the network performance for RTS/CTS on and off respectively. Our study provides some important insights about using the RTS/CTS exchange in mutlirate 802.11 WLANs.