Briefly, a wireless communication system that includes an Initiator and a Responder to transmit an aggregation of data units in a reverse direction is provided. The Initiator includes a medium access controller capable of allocating a time slot for exchanging one or more transmissions of aggregation of data units with the Responder. The medium access controller includes a channel access scheme, which is able to allocate the time slot for the Responder to transmit the aggregation of data units in the reverse direction, and to grant Originator rights of the Initiator to the Responder within said time slot.