Method of transmitting or receiving frame in wireless lan system and apparatus therefor

A station (STA) for transmitting a frame in a wireless local area network (WLAN) system, comprising:

• a processor configured to set a transmission opportunity (TXOP) duration field and a uplink (UL) flag;
• and
• a transmitter configured to transmit a high efficiency physical layer protocol data unit (HE PPDU) containing the TXOP duration field and the UL flag under control of the processor,
• wherein the processor is configured to set the UL flag to a first value corresponding to uplink if the HE PPDU is addressed to an access point (AP) and
• wherein, when the TXOP duration field is set to a predetermined value other than a TXOP duration value for setting a network allocation vector (NAV) and the HE PPDU corresponds to a single user (SU) PPDU carrying an ready-to-send (RTS) frame or a clear-to-send (CTS) frame, the processor is configured to set the UL flag to a second value corresponding to downlink even if the HE PPDU is addressed to the AP.