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IEEE 802.11 wireless networks employ the so-called RTS/CTS mechanism in order to avoid data packet collisions. The main design assumption is that all the nodes in the vicinity of a sender and a receiver will hear the RTS or CTS packets, and defer their transmission appropriately. This assumption happens to not hold, in general, even under perfect operating conditions. Often, neighboring nodes are "masked" by other ongoing transmissions nearby and, hence, are unable to receive the RTS or CTS packets correctly. We refer to such nodes as masked nodes. In this paper, we describe the masked node problem and show scenarios leading to data packet collisions. We evaluate the impact of masked nodes through mathematical analysis and real experiments on a small IEEE 802.11 ad hoc network. The analytical and experimental data closely match and reveal that the presence of a masked node in a network can result in an order of magnitude increase in data packet loss compared to a network without masked nodes. These results are further validated by extensive simulations on a large-scale network, which show that masked nodes also significantly affect delay and throughput performance. Therefore, masked nodes severely limit the effectiveness of the RTS/CTS mechanism in preventing performance degradation in wireless LANs.
Date of Publication: Sept.-Oct. 2005