WLAN RTS/CTS frames
This page on WLAN RTS vs CTS mentions application of RTS frame and CTS frame in IEEE 802.11 WLAN network.
As we know medium access is very challenging when there are multiple stations. To provide guaranteed reservation of the common medium and hence uninterrupted data transmission, a station will use RTS/CTS message exchange. Following are the frame fields of both the RTS frame and CTS frame. As mentioned, RTS does not have any data fields. NAV field in the RTS frame allows CTS frame to be completed. CTS frame will help reserve access for the data part (i.e. data frame).
WLAN RTS frame type
It is mainly use to obtain access of the medium to transmit large size frames. This works based on RTS threshold set in the wlan card driver.
Pls. note that access of the medium is reserved for frame of unicast type. For other types such as broadcast and multicast frames are simply transmitted and no access control is needed.
As shown above in figure-1, RTS frame is made of only header and atlast FCS is appended. No data is carried by the WLAN RTS frame.
Four fields will form WLAN RTS frame.
• Frame Control:This is same as any generic wlan mac frame used for control. For RTS frame subtype is set to 1011.
• Duration:Sender need to estimate time required for frame exchanges. Hence it will calculate time after RTS ends. This time usually takes care of CTS frame duration, final ACK frame and time to transmit frame or first fragment. RTS duration will be roughly equal to three SIFS. The unit is microsecond. This is depicted in fig-2 below.
• Address-1 receiver address: The receiving station address which will be intended recipient of the larger frame.
• Address-2 transmitter address: The address of the transmitter of the WLAN RTS frame.
WLAN CTS Frame type
CTS frame is basically reply of the received RTS frame.
There are basically three fields which forms the CTS frame.
• Frame control: It is set to value 1100.
• Duration: The transmitter of CTS frame uses duration field of received RTS frame as reference for duration calculation. The sender substracts time needed for CTS frame, SIFS prior to CTS frame from the duration field mentioned in RTS frame and inserts this field in the duration field of its own.
• Address-1 receiver address: Receiver station of the CTS frame is same as previously transmitted RTS frame station address. Hence MAC layer copies transmitter address as receiver address.
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