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RTS (Request to Send) & CTS (Clear to Send) frames are used to enhance the virtual carrier sense process.  It is possible for a client station to be able to communicate with a AP, but not able to hear or be heard by any of the other client stations. This will lead to possible collisions when a station transmits.

RTS/CTS is a mechanism that performs NAV distribution & helps to prevent collisions from occurring. So when RTS/CTS is enabled on a STA, every time STA want to transmit a frame, it must perform RTS/CTS exchange prior to  the normal data transmission. I have used below topology to see the RTS/CTS behavior.

CWAP-RTS-CTS-01If you capture wireless frames on CH149, you will see something similar to below & you can clearly see the STA send RTS & then wait for CTS prior to any data transmission.CWAP-RTS-CTS-02Here is the frame format of a RTS farme. It is 20 byte in length. Frame type is “Control or value 1” & subtype is “RTS or value 11“. Duration value of RTS frame include the time needed for the subsequent frames in the transmit operation to be transmitted.CWAP-RTS-CTS-03Here is the RTS frame capture. Note that it is 20 byte long Control Frame with subtype 11 (ie RTS). TA is the iPhone5 mac address (04:F7:E4:EA:5B:66) where as RA is the BSSID(1C:6A:7A:BC:4D:6E). Duration set to 124 μS which is the estimated time require to transmit the data frame which includes time for CTS & ACK as well (includes SIFS as well)

CWAP-RTS-CTS-11When the RTS received by the AP, it will respond with CTS after SIFS & if the medium is idle, to indicate client that he can transmit the data frame. When a STA in the BSS could not hear the original RTS, they may hear the CTS & then adjust their NAV to the duration set in CTS frame. Here is the frame format of a CTS frame.

CWAP-RTS-CTS-05CTS is a 14 byte long control frame. Subtype is 12 in this case. It simply get the TA of the RTS frame & set it to RA. Duration of the CTS frame is the duration field of RTS, adjusted by subtraction of aSIFS & time required to transmit the CTS frame.

Here is the frame capture of CTS frame. In this case duration is set to 80μS ( 124-44, indicate CTS frame require 44μS ). RA set to iPhoe5 mac address.

CWAP-RTS-CTS-12Once client get the CTS, he will transmit the Data frame. In this frame duration set to time required to ACK + aSIFS. In my case it is a DNS frame destined to a DNS server in wired side. Destination MAC address is the gateway MAC of the wireless client (Cisco HSRP address 00:00:0C:07:AC:8E in my case).nHere is the data frame captured.

CWAP-RTS-CTS-13Here is the Acknowledgement frame send to the client (iPhone5) to confirming the data frame received by AP. ACK is 14 byte long frame & duration always set to 0.

CWAP-RTS-CTS-14Below diagram (figure 9.4 IEEE-802.11-2012 std )summarize the RTS/CTS frame exchange & how each of them duration value is calculated.

RTS duration = SIFS + CTS + SIFS + Data + SIFS + ACK
CTS duration = SIFS + Data + SIFS + ACK

CWAP-RTS-CTS-15If a data frame is a fragmented MSDU or MMPDU then the Duration/ID field of a data and ACK frames specifies the total duration of the next fragment and acknowledgment as shown below.

CWAP-RTS-CTS-16In 802.11n where BlockAck is used, a client can send multiple frames before that block of data frames get BlockAck as shown below.

CWAP-RTS-CTS-18In this case RTS & CTS frame set its duration value to accommodate time required for all these frames.CWAP-RTS-CTS-19CTS-to-Self
CTS-to-Self is simply another method of performing NAV distribution & that use only CTS frames. It is used strictly as protection mechanism for mixed mode environment. The CTS-to-self NAV distribution mechanism is lower in network overhead cost than is the RTS/CTS NAV distribution mechanism, but CTS-to-self is less robust against hidden nodes and collisions than RTS/CTS. CTS-to-Self duration value is calculated as shown below (page 194- CWAP study guide).

CWAP-RTS-CTS-17STAs employing a NAV distribution mechanism should choose a mechanism such as CTS-to-self or RTS/CTS that is appropriate for the given network conditions. If errors occur when employing the CTS-to-self mechanism, STAs should switch to a more robust mechanism.

1. CWAP Official Study Guide – Chapter 5

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