IEEE 802.11 RTS/CTS
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RTS/CTS (Request to Send / Clear To Send) is the mechanism used by the 802.11 wireless networking protocol to reduce frame collisions introduced by the hidden terminal problem and exposed node 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 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. Any other node receiving the RTS frame but not the CTS frame is permitted to transmit to other neighboring nodes (solving the exposed node problem). 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 node problem.
RTS/CTS packet size threshold is 0-2347. 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.
Another situation where terminal A and B trying to send C, where C and B are very close while A is far but still in the range, Near/Far Terminal problem will occur. Since power decay propotional to distance square, B drowns out A's signal (at physical layer), so C cannot receive A. Thus power control is required.
External Links
- ["Improving WLAN Performance with RTS/CTS" on wi-fiplanet.com]
- ["How Effective is the IEEE 802.11 RTS/CTS Handshake in Ad Hoc Networks?"]
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