Rate avalanche: Effects on the performance of multi-rate 802.11 wireless networks

Originally designed to deal with the hidden node problem, the Request-to-Send/Clear-to-Send (RTS/CTS) exchange is often turned off in most infrastructure-based 802.11 networks with the belief that the benefit it brings might not even be able to pay off its transmission overhead. 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. 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: high collision rates not only lead to retransmissions but also drive the nodes to switch to lower date rates; the retransmissions and the longer channel occupation caused by lower rates will further deteriorate the channel contention, which yields more collisions. This vicious circle could significantly degrade the network performance even 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 effectively suppress the rate avalanche effect. Various scenarios/conditions are extensively examined to study the impact of RTS/CTS on the network performance. Our study provides some important insights about using the RTS/CTS exchange in multi-rate 802.11 WLANs.