A physical AP manages a plurality of BSSs through VAPs. The AP sends a trigger frame to reserve a TXOP including resource units the nodes access to transmit data during the reserved TXOP. To improve channel utilization, the trigger frame identifies a plurality of groups, nodes of which are allowed to access the resources units to transmit data during the reserved TXOP. Thus, the AP receives, during the reserved TXOP, data from one node of a first group identified in the trigger frame and data from one node (separate from the first one) of a second and separate group identified in the trigger frame. A single access by the AP to the medium is thus required, reducing the channel occupation due to control frames. In some embodiments, the number of trigger frames can be drastically reduce, while offering resource units to several BSSs.