RTS/CTS

Request to Send/Clear to Send (RTS/CTS) is a traffic management technique which helps minimize overlapping transmissions in a crowded environment. The process is similar to raising your hand in a busy classroom. The client will transmit an RTS frame to request permission to speak. Then the AP will transmit a CTS frame to grant permission to speak. The client will then proceed transmitting the message. Other classroom participants can see this activity and understand that they should not start speaking when somebody else has been called on and received a CTS frame.

While this may seem like a lot of overhead and wasted bandwidth, you can actually increase total throughput in a noisy environment because the number of retransmissions will be reduced considerably. After all, if everybody is shouting at once, nobody can hear anything.

However, using RTS/CTS (and all those extra frames) in a room with a small number of clients may actually result in lower performance than if you weren't using RTS/CTS at all. Going back to our classroom example, if there is only one (or a small number) of students in the room, why go through all the trouble of forcing people to raise their hands?

The other situation where RTS/CTS is very useful is in the case of a hidden node (Figure 2.8). Let's say that you have two clients and each can see the AP, but not each other. Using RTS/CTS will help ensure that the clients don't attempt to unknowingly speak at the same time.

Note that RTS/CTS is an opt-in optional mechanism and its use is not required to be compliant with the 802.11 specification.