A first communication device determines one or more parameters related to a format of a media access control layer (MAC) data unit for an uplink (UL) multi-user (MU) transmission. The format of the MAC data unit for the UL MU transmission is different than a format of a MAC data unit for an UL single user (SU) transmission. The first communication device generates one or more data units that include the one or more parameters. The first communication device transmits the one or more data units to a plurality of second communication devices to inform the plurality of second communication devices of the format of the MAC data unit for UL MU transmissions by the plurality of second communication devices to the first communication device.