The present invention provides a method for transmitting data stream via wireless medium for contention based medium access across a wireless network having a plurality of stations and an access point communicating to the stations via wireless medium. The method includes: (i) transmitting data stream from the station according to prescheduled wireless medium occupancy reservation for the station within only prescheduled medium occupancy period of a selective contention period; (ii) performing a contention between the stations for wireless medium occupancy for a contention successful station to transmit data stream within only contention medium occupancy period of the selective contention period; (iii) transmitting data stream from the contention successful station after the contention; and (iv) prescheduling a wireless medium occupancy reservation for the contention successful station to transmit within a prescheduled medium occupancy period of next selective contention period.