Respective sub-channels of an OFDM channel are allocated by a first device to second devices. A first control frame, transmitted from the first device to the second devices, indicates that the second devices are requested to transmit a second control frame to the first device. Respective second control frames are received from at the first device from least some of the second devices. A second control frame received from a particular second device indicates that at least a portion of the sub-channel allocated to the second device is available. An OFDMA data unit is transmitted by the first device. The OFDMA data unit includes respective OFDM data units transmitted to the at least some of the second devices. Each OFDM data unit is transmitted to a particular second device in the portion of the sub-channel indicated to be available by the second control frame received from the second device.