Chapter 2: RTS-to-CTS and CTS-to-Self

RTS -CTS Method:

RTS-CTS mechanism is an optional method that is used in Virtual carrier sensing to avoid hidden node problems. First, let us understand what is a hidden node?

In the below diagram, there is an access point node A indicated by blue. Nodes B and C are wireless devices within the AP – A’s BSS. However, B and C cannot hear each other due to network congestion or they are outside each other’s BSS and are called hidden nodes. Due to this, physical carrier sensing by B and C will never indicate that medium is busy when either one of them is transmitting in the air and could result in corruption and distortion of signal. To avoid this situation, we can use RTS-CTS mechanism.

Any 802.11 device that wishes to transmit in the medium should send RTS (Request to Send) frame – requesting for medium access to the AP. The latter then responds with CTS (Clear to Send) frame that includes the Duration field, which helps the station to set its NAV timer. Once the medium is free, the station can access the medium for wireless transmissions. For every frame transmitted in air, there should be an ACK from the AP. There is a SIFs delay between each frame: RTS, CTS, DATA and ACK frames. At times, this mechanism can create a lot of overhead in the network leading to a lot of congestion. And hence, this can be enabled only if the frame size is equal or above a specific configured threshold that depends on the network.

CTS-to-Self

This is another mechanism that is useful in mixed environments with 802.11 b clients supporting HR/DSSS and 802.11g supporting ERP or OFDM modulations. Due to different modulations used, they do not understand each other and might end up transmitting the frames at the same time. In order to avoid this situation, CTS-to-Self method is implemented. When a new client device that supports 802.11g wishes to transmit in the medium, it sends a CTS frame without RTS frame preceding to it using the modulation technique of legacy 802.11b devices. This will help the old clients in the network to understand that there will a transmission and will prompt them to be quiet and continue to wait to access the medium.