The Enhanced Distributed Coordination Function (EDCF) is an extended DCF function that supports the quality of service (QoS) in WLANs according to 802.11e in conjunction with the Hybrid Coordination
Function (HCF). Since the quality of service cannot be guaranteed in WLANs due to
the access procedure, the hybrid coordination function HCF is used in conjunction with the extended coordination function EDCF. EDCF provides a certain degree of priority control
inthe CSMA access method by controlling the window size
. The priorities are determined by up to eight trafficcategories (TC) whose parameters can be changed dynamically. This allows different service levels to be implemented. Each Traffic Category is granted access to the transmission medium for a certain duration. The start time and the interval for the transmission authorization are defined in the Transmission Opportunity (TXOP).