Carrier sense lets the transmitter know if the channel near itself is clear. However, for one transmitter's wireless signal to be successfully received, the channel around the receiver must be clear—the transmitter's channel doesn't matter. The receiver's channel must be clear to prevent interference from multiple signals at the same time. However, the transmitter can successfully transmit with another signal in the air, because the two signals will pass through each other without harming the transmitter's signal.
So why does 802.11 require the transmitter to listen before sending? There is no way for the receiver to inform the transmitter of its channel conditions without itself transmitting. In networks that are physically very small—well under the range of Wi-Fi transmissions—the transmitter's own carrier sensing can be a good proxy for the receiver's state. Clearly, if the transmitter and receiver are immediately next to each other, the transmitter and receiver pretty much see the same channel. But as they separate, they experience different channel conditions. Far enough away, and the transmitter has no ability to sense if a third device is transmitting to or by the receiver at the same time. This is called the hidden node problem.
Figure 1 shows two transmitters and a receiver in between the two. The receiver can hear each transmitter equally, and if both transmitters are sending at the same time, the receiver will not be able to make out the two different signals and will receive interference only. Each transmitter will perform carrier sense to ensure that the channel around it is clear, but it won't matter, because the other transmitter is out of range. Hidden node problems generally appear this way, where the interfering transmitters are on the other side of the receiver, away from the transmitter in question.
Figure 1: Hidden Nodes: The receiver can hear both transmitters equally, but neither transmitter can hear the other
802.11 uses RTS/CTS as a partial solution. As mentioned when discussing the 802.11 protocol itself, a transmitter will first send an RTS, requesting from the receiver a clear channel for the entire length of the transmission. By itself, the RTS does not do anything for the transmitter or receiver, because the data frame that should have been sent would have the same effect, of silencing all other devices around the sender. However, what matters is what the receiver does. The CTS it sends will silence the devices on the far side from the sender, using the duration value and virtual carrier sense to cause those devices to not send, even though they cannot detect the following real data frame (seeFigure 2).
This is only a partial solution, as the RTSs themselves can get lost because of hidden nodes. The advantage of the RTS, however, is that it is usually somewhat shorter than the data frame or frames following. For the RTS/CTS protocol to be the most effective against hidden nodes, the RTS and CTS must go out at the lowest data rate. However, many devices send the RTSs at far higher rates. This is done mostly to just take advantage of RTSs determining whether the receiver is in range, and not to avoid hidden nodes.
Furthermore, the RTS/CTS protocol has a very high overhead, as many data packets could be sent in the time it takes for an RTS/CTS transmission to complete.
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