priorized channel access (WiMedia) (PCA)
Prioritized Channel Access(PCA) is a simple, random channel access method for WLANs and WPANs in which all subscribers have equal rights. It corresponds to the quality of service( QoS) defined in 802.11e for WLANs.
The PCA method is a random listen-before-transmit scheme like CSMA. A station willing to transmit first listens to the transmission medium and checks if it is not busy. To prevent the transmission of large files from blocking the transmission medium for the transmission of real-time data or higher priority messages, WiMedia has implemented a priority scheme. In this scheme, each station draws a random number, the amount of which depends on the priority of the station. The higher the priority, the smaller the random number and the shorter the waiting time for channel access using CSMA. After the random waiting time, a station is allowed to start a transmission attempt.
Although the PCA procedure is stochastic, it excludes collisions as far as possible.