005. The 802.11e specification defines two new coordination functions for controlling and prioritising media access, which enhance the original 802.11 DCF and PCF mechanisms described above in Section "Wireless Media Access, p. 144". Up to eight traffic classes (TC) or access categories (AC) are defined, each of which can have specific QoS requirements and receive specific priority for media access. The simplest of the 802.11e coordination functions is enhanced DCF (EDCF) which allows several MAC parameters determining ease of media access to be specified per traffic class. An arbitrary interframe space (AIFS) is defined which is equal to DIFS for the highest priority traffic class and longer for other classes. This provides a deterministic mechanism for traffic prioritisation as shown in Figure 6-6. The minimum backoff time CWmin is also TC dependent, SO that, when a collision occurs, higher priority traffic, with a lower CWmin, will have a higher probability of accessing the medium. Chapter Six Device A Interframe spacing for control frames and continuation of high priority transmission Device B Interframe spacing for Point Coordination Function Device C High priority traffic; DIFS and short contention period Device D AIFS[2] Medium priority traffic; AIFS[2] and intermediate contention period Device E AIFS[3] Low priority traffic; AIFS[3] and longer contention period Figure 6-6: EDCF Timing Each station maintains a separate queue for each TC (Figure 6-7), and these behave as virtual stations, each with their individual MAC parameters. If two queues within a station reach the end of their backoff periods at the same time, data from the higher priority queue will be transmitted when the station gains access to the medium. Although the EDCF coordination mode does not provide a guaranteed service for any TC, it has the advantage of being simple to configure and implement as an extension of DCF. The second enhancement defines a new hybrid coordination function (HCF) which complements the polling concept of PCF with 