RTS/CTS (request to send / clear to send) is the optional mechanism used by the

802.11 IEEE 802.11 is part of the IEEE 802 set of local area network (LAN) technical standards, and specifies the set of media access control (MAC) and physical layer (PHY) protocols for implementing wireless local area network (WLAN) computer com ...

wireless networking protocol to reduce frame collisions introduced by the

hidden node problem In wireless networking, the hidden node problem or hidden terminal problem occurs when a node can communicate with a wireless access point (AP), but cannot directly communicate with other nodes that are communicating with that AP. This leads to d ...

. Originally the protocol fixed the

exposed node problem In wireless networks, the exposed node problem occurs when a node is prevented from sending packets to other nodes because of co-channel interference with a neighboring transmitter. Consider an example of four nodes labeled R1, S1, S2, and R2, wh ...

as well, but modern RTS/CTS includes ACKs and does not solve the exposed node problem.

Frame format

The RTS, CTS and ACK frame headers all contain a frame control field (two bytes of metadata flags), duration field, a field for the receiver MAC address, and a

frame check sequence A frame check sequence (FCS) is an error-detecting code added to a frame in a communication protocol. Frames are used to send payload data from a source to a destination. Purpose All frames and the bits, bytes, and fields contained within ...

. Additionally, an RTS frame contains the transmitter's MAC address. This protocol was designed under the assumption that all nodes have the same transmission ranges, and does not solve the hidden terminal problem. The RTS/CTS frames can cause a new problem called the exposed terminal problem in which a wireless node that is nearby, but is associated with another access point, overhears the exchange and then is signaled to back off and cease transmitting for the time specified in the RTS. RTS/CTS is an additional method to implement ''virtual carrier sensing'' in

carrier sense multiple access with collision avoidance Carrier-sense multiple access with collision avoidance (CSMA/CA) in computer networking, is a network multiple access method in which carrier sensing is used, but nodes attempt to avoid collisions by beginning transmission only after the channel ...

(CSMA/CA). By default,

relies on ''physical carrier sensing'' only, which is known to suffer from the

. The RTS/CTS packet size threshold is 0–2347

octet Octet may refer to: Music * Octet (music), ensemble consisting of eight instruments or voices, or composition written for such an ensemble ** String octet, a piece of music written for eight string instruments *** Octet (Mendelssohn), 1825 compos ...

s. Typically, sending RTS/CTS frames does not occur unless the packet size exceeds this threshold. If the packet size that the node wants to transmit is larger than the threshold, the RTS/CTS handshake gets triggered. Otherwise, the data frame gets sent immediately. IEEE 802.11 RTS/CTS mechanism could help solve exposed node problem as well, only if the nodes are synchronized and packet sizes and data rates are the same for both the transmitting nodes. 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. If the nodes are not synchronized (or if the packet sizes are different or the data rates are different) the problem may occur that the exposed node will not hear the CTS or the ACK during the transmission of data of its neighbor.

References

External links

"A New Channel Access Method for Packet Radio"
* {{DEFAULTSORT:IEEE 802.11 RTS CTS RTS CTS de:Carrier Sense Multiple Access/Collision Avoidance#RTS/CTS Koordination

Frame format

See also

References

External links