A method, comprising: determining a primary access category for a transmission opportunity, TXOP, to transmit data to a primary wireless communication device, the primary access category being an access category included in a group of access categories that are associated with different transmission priorities, respectively, wherein determining the primary access category comprises determining the TXOP based on a back-off timer associated with the primary access category; retrieving one or more primary data units belonging to the primary access category to transmit to the primary wireless communication device during the TXOP; determining a secondary access category for the TXOP, the secondary access category being an access category included in the group of access categories, the secondary access category having a transmission priority different from the primary access category, wherein determining the secondary access category comprises selecting the secondary access category having a transmission priority higher than the primary access category; retrieving one or more secondary data units belonging to the secondary access category to transmit to a secondary wireless communication device during the TXOP; transmitting spatially steered streams during the TXOP, the steered streams embodying (i) the one or more primary data units and (ii) the one or more secondary data units, wherein the steered streams concurrently provide separate data to the primary wireless communication device and the secondary wireless communication device; and controlling a second back-off timer associated with the secondary access category, wherein the controlled back-off timer is not penalized for a communication failure associated with the primary access category during the TXOP. determining a primary access category for a transmission opportunity, TXOP, to transmit data to a primary wireless communication device, the primary access category being an access category included in a group of access categories that are associated with different transmission priorities, respectively, wherein determining the primary access category comprises determining the TXOP based on a back-off timer associated with the primary access category; retrieving one or more primary data units belonging to the primary access category to transmit to the primary wireless communication device during the TXOP; determining a secondary access category for the TXOP, the secondary access category being an access category included in the group of access categories, the secondary access category having a transmission priority different from the primary access category, wherein determining the secondary access category comprises selecting the secondary access category having a transmission priority higher than the primary access category; retrieving one or more secondary data units belonging to the secondary access category to transmit to a secondary wireless communication device during the TXOP; transmitting spatially steered streams during the TXOP, the steered streams embodying (i) the one or more primary data units and (ii) the one or more secondary data units, wherein the steered streams concurrently provide separate data to the primary wireless communication device and the secondary wireless communication device; and controlling a second back-off timer associated with the secondary access category, wherein the controlled back-off timer is not penalized for a communication failure associated with the primary access category during the TXOP.