A MAC (Medium Access Control) hierarchy module for performing a back-off procedure for real-time application in a station having a frame to be transmitted in a network including a plurality of stations, includes: a priority identification sub-module for identifying a priority of each data to be transmitted; a first CW (Contention Window) generation sub-module for pseudo-randomly generating a CW based on the priority of the data to be transmitted; a back-off time calculation/back-off process sub-module for calculating a back-off time based on the CW and performing the back-off procedure based on the back-off time; and a second CW generation sub-module for randomly generating another CW when there is a collision between transmissions of data having same-order priorities.