A method of transmitting data in a wireless local area network; and a device therefore are discussed. The method according to one embodiment includes generating, by a transmitting station, a physical layer convergence procedure (PLCP) protocol data unit (PPDU), the PPDU including a signal field and a data field; and transmitting, by the transmitting station, the PPDU to the receiving station. The signal field includes a class type indicating whether a transmission type of the PPDU is an uplink transmission or a downlink transmission.