 with (CWmin, CWmax) = (15, 1023) because voice traffic will have a back- off interval (to try to access the medium again to avoid collision at the initial moment) after a random number of time slots (this is not the TDMA approach, but each slot is a predefined time interval), which in this case are a minimum of three and a maximum of seven. The same priority is given to video traffic with additionally set TXOP parameters, which provides the possibility for back-to-back transmission of video packets (needed due to the bursty nature of video) for the given time period (e.g. 3.008 ms according to Table 5.6 values) before it releases the wireless channel. TXOP is convenient for variable bitrate (i.e. bursty) traffic such as video or Internet traffic (e.g. Web-based). The packets Table 5.5 WiFi access categories and priorities for QoS provisioning. Access Designation Priority category (AC) (informative) Voice Voice Video Video Video probe Best effort Best effort Best effort 5 QoS for Mobile Ultra-Broadband Table 5.6 Default EDCA parameters for WiFi QoS provisioning. Access category CWmin CWmax AIFSN Max TXOP Background Best effort Video 3.008 ms Voice 1.504 ms AIFS (AC 0) Low priority AIFS (AC 1) Lower priority AIFS (AC 2) Middle priority AIFS (AC 3) priority Contention windows Virtual collisions (in WiFi access point) Transmission AC - Access category ACK - Acknowledgment AIFS - Arbitration interframe space Figure 5.9 Access categories in IEEE 802.11e for WiFi QoS support. belonging to different ACs are separately queued at the given AP (Figure 5.9), which implements virtual collisions to avoid physical collisions in the wireless channel. Another approach to provide QoS support in WiFi networks is by using HCCA, which extends the EDCA access rules. It is based on a hybrid approach which has a contention period and a contention-free period. 5.4 QoS and Giga Speed WiFi In the contention period, TXOP is given and used after AIFS + Backoff period. QoS polling is done after PIFS (based on the PCF access scheme for WiFi).