A system is provided for allocating a transmission period in a wireless network system. An access point (AP) transmits a PSMP frame indicating a downlink period and an uplink period allocated to each station (STA), and at least one sub PSMP frame indicating an allocated downlink period for at least one of a retransmission of downlink data and a transmission of an ACK indicating successful receipt of uplink data. After exchanging data with the AP in the downlink and uplink periods indicated by the PSMP frame, an STA receives the each sub PSMP frame, and performs at least one of a reception of the retransmitted downlink data and a reception of the ACK in the downlink period indicated by the each sub MAP frame.