IEEE 802.11 RTS/CTS
RTS/CTS (Request to Send / Clear to Send) is the optional mechanism used by the 802.11 wireless networking protocol to reduce frame collisions introduced by the hidden terminal problem. Originally the protocol fixed the exposed terminal problem as well, but modern RTS/CTS includes ACKs and does not solve the exposed terminal problem.
Protocol Description
A node wishing to send data initiates the process by sending a Request to Send frame (RTS). The destination node replies with a Clear To Send frame (CTS). Any other node receiving the RTS or CTS frame should refrain from sending data for a given time (solving the hidden node problem). The amount of time the node should wait before trying to get access to the medium is included in both the RTS and the CTS frame. This protocol was designed under the assumption that all nodes have the same transmission range.
RTS/CTS is an additional method to implement virtual carrier sensing in Carrier sense multiple access with collision avoidance (CSMA/CA). By default, 802.11 relies on physical carrier sensing only which is known to suffer from the hidden terminal problem.
RTS/CTS packet size threshold is 0-2347 octets. Typically, sending RTS frames is turned off by default (threshold ≥ 2347). If the packet size the node wants to transmit is larger than the threshold, the RTS/CTS handshake gets triggered. If the packet size is equal to or less than threshold the data frame gets sent immediately.