Understanding WLAN RTS/CTS Frames: Request to Send and Clear to Send

This page discusses the application of RTS (Request to Send) and CTS (Clear to Send) frames in an IEEE 802.11 WLAN network. As we know, managing medium access is challenging when multiple stations are involved.

To provide a guaranteed reservation of the common medium and ensure uninterrupted data transmission, a station will use RTS/CTS message exchange.

RTS/CTS Frame Fields

Here are the frame fields for both the RTS and CTS frames:

As mentioned earlier, the RTS frame doesn’t contain any data fields. The NAV (Network Allocation Vector) field in the RTS frame allows the CTS frame to be completed. The CTS frame helps reserve access for the data part (i.e., the data frame).

WLAN RTS frame

The RTS/CTS mechanism is primarily used to obtain access to the medium for transmitting large-sized frames. This is based on the RTS threshold set in the WLAN card driver. Note that access to the medium is reserved for frames of the unicast type. Broadcast and multicast frames are simply transmitted without any access control.

As shown in the figure above, the RTS frame consists only of a header, with the FCS (Frame Check Sequence) appended at the end. No data is carried by the WLAN RTS frame. Four fields constitute the WLAN RTS frame:

• Frame Control: This field is the same as any generic WLAN MAC frame used for control. For the RTS frame, the subtype is set to 1011.

• Duration: The sender needs to estimate the time required for frame exchanges. Therefore, it calculates the time after the RTS frame ends. This time usually accounts for the CTS frame duration, the final ACK frame, and the time to transmit the frame or the first fragment. The RTS duration will be roughly equal to three SIFS (Short Interframe Space) intervals. The unit is microseconds.

• Address-1 (Receiver Address): The receiving station’s address, which will be the intended recipient of the larger frame.

• Address-2 (Transmitter Address): The address of the transmitter of the WLAN RTS frame.

WLAN RTS frame

WLAN CTS frame

The CTS frame is essentially a reply to the received RTS frame.

There are three main fields that make up the CTS frame:

• Frame control: The value is set to 1100.

• Duration: The transmitter of the CTS frame uses the duration field of the received RTS frame as a reference for duration calculation. The sender subtracts the time needed for the CTS frame and the SIFS prior to the CTS frame from the duration field mentioned in the RTS frame and inserts the result into the duration field of its own CTS frame.

• Address-1 (Receiver Address): The receiver station of the CTS frame is the same as the station address of the previously transmitted RTS frame. Therefore, the MAC layer copies the transmitter address as the receiver address.