An AP determines an overall frequency bandwidth of an operating channel of a WLAN, and one or more punctured sub-channels for the operating channel. The AP transmits a first packet to one or more client stations of the WLAN, the first packet specifying i) the overall frequency bandwidth of the operating channel, and ii) the one or more punctured sub-channels. The AP receives a second packet that includes an RTS frame from a first client station. The AP determines that one or more sub-channels in the operating channel are not idle from the standpoint of the AP, and in response to receiving the second packet, the AP transmits a third packet that includes a CTS frame that indicates to the one or more client stations of the WLAN that only a subset of sub-channels in the operating channel is reserved for a transmit opportunity of the first client station.