A first communication device transmits a first control frame to multiple second communication devices via a wireless communication medium, wherein the first control frame i) indicates to other communication devices that the wireless communication medium is reserved for a first time period, and ii) indicates that the second communication devices are requested to simultaneously transmit respective second control frames to the first communication device via the wireless communication medium, wherein the second control frames are to include information indicating to other communication devices that the wireless communication medium is reserved for a second time period that is a subset of the first time period.