RTS / CTS, What is it?

 RTS / CTS, What is it?

When learning about the .11 protocol you will probably come across these terms, RTS or Request to Send and CTS or Clear to Send. Both of these frames are used in conjunction with each other to further control the medium. While we still have CSMA/CA and EDCA to moderate and prioritize various transmissions over the medium, in areas of high utilization or when large frames are used we need additional help. RTS/CTS helps us 'reserve' the medium for a period of time. 
 
Initially a STA will send a RTS frame to an AP and include the duration it needs for transmission including the SIFS and PHY duration needed for the AP to respond. When the AP sends the CTS frame, its duration field will be the duration found in the RTS minus the CTS and minus the SIFS interval for that PHY. 

Ive added some packet captures I did on my home network below so we can see the duration fields of each frame type and further understand how it is calculated. 

We can clearly see that in the RTS frame we have a duration value of 179uS. This timer includes the SIFS interval as well as the duration for the PHY to transmit the CTS frame. 

Next, in the CTS frame we can see a duration value of 119uS which will be the time needed for the transmission from the requesting STA. Note this value is equal to the original duration field minus the CTS frame and SIFS interval. 
 
To prove it. We know this is a VHT PHY with a SIFS interval of 16uS. We can also see in the radio information field the duration of the frame which is 44uS. So 179 - 44 -16 = 119uS. 

By using RTS/CTS we can clear and reserve the medium to protect transmissions in highly congested areas or when hidden nodes are causing issues. Just another way the .11 protocol is robust and will 'work' even when poorly deployed.
 
 

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