Exposed node problem

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In wireless networks, the exposed node problem or exposed terminal problem occurs when a node is unnecessarily prevented from sending packets to other nodes due to a neighboring transmitter, even though no interference would actually occur. Consider an example of 4 nodes labeled R1, S1, S2, and R2, where the two receivers are out of range from one another, yet the two transmitters in the middle are in range of each other and one of the receivers. Here, if a transmission between S1 and R1 is taking place, node S2 is prevented from transmitting to R2 as it concludes that it will interfere with the transmission by its neighbor S1. In order to increase the throughput, the exposed node S2 should be allowed to transmit in a controlled fashion without interfering with the on-going transmission between S1 and R1.

The solution is the use of the IEEE 802.11 RTS/CTS mechanism. When a node hears an RTS from a neighboring node, but not the corresponding CTS, that node can deduce that it is an exposed node and is permitted to transmit to other neighboring nodes.

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