Flexible RTS and Concurrent Transmission Schemes for Reducing Exposed Node Problem in 802.11 WLANs


Author(s): N. Ramesh Babu, Dr. N. Geethanjali

Hidden and exposed nodes are the problems that occur in IEEE 802.11 wireless local area networks. The hidden node problem is mitigated in such network using RTS/CTS mechanism provided by IEEE 802.11. Since then the IEEE 802.11 standard has been improved several times. The recent standard 802.11n supports multi-rate transmission. However, the problem with this is that there is significant difference between data rate and control frame rate and their coverage. Due to this, the RTS/CTS method which is meant for reducing hidden node problem actually causes exposed node problem. An exposed node is the node which is located outside the transmission range of receiver node but within the transmission range of sender node. When such nodes receive RTS from sender nodes, they have to wait until CTS and ACK from the receiver. Thus the performance of the network is deteriorated in terms of throughput. Adjusting RTS/CTS transmission rates and optimizing throughput is the area less explored in IEEE 802.11 networks. In this paper, we proposed two methods of reducing exposed nodes. First method explores RTS/CTS dynamics in terms of adjusting RTS transmission range to optimize the network throughput besides reducing exposed nodes. The second method lets nodes to identify themselves as exposed nodes based on the sequence of packets and opportunistically schedule concurrent transmissions. Empirical results through NS2 simulations revealed that the proposed methods are capable of increasing throughput by reducing exposed nodes effectively