02.11 standard defines the hybrid coordination function (HCF), which has two working modes: Enhanced distributed coordination function channel access (EDCA). Similar to DCF, but provides different priority levels for different services. Hybrid coordination function controlled channel access (HCCA) is CSMA/CA compatible polling technique (like PCF, but with improvements). 5.4 QoS and Giga Speed WiFi The QoS support with EDCA is based on traffic differentiation by using traffic priori- tization, SO higher priority traffic has a greater probability of being sent over the wireless interface than lower priority traffic. The access to the wireless channel in ECDA is con- trolled with four parameters: CWmin. It is the minimal size of the CW. CWmax. It is the maximal size of the CW. Arbitration interframe space (AIFS). It is variable DIFS (shorter AIFS is used for higher priority packets and vice versa). Transmission opportunity (TXOP). It specifies the maximum time interval during which wireless stations can transmit WiFi layer-2 frames back-to-back (contention- free access). The WiFi QoS support typically is provided with EDCA, which offers service differen- tiation of traffic into four (ACs and eight priority levels with the aim of providing traffic differentiation, as shown in Table 5.5. The defined ACs in IEEE 802.11e EDCA can be used over different physical layers for WiFi (e.g. 802.11a/g, 802.11n, 802.11ac) because the QoS support is implemented on the MAC layer (i.e. layer 2). WiFi multimedia (WMM) APs have enabled EDCA and also TXOP. In general, QoS support for WiFi is optional. Default values for EDCA parameters per access/category are given in Table 5.6. The QoS support in WiFi is relative, not absolute. For example, considering the given values for parameters in Table 5.6, voice traffic has CW for backoff in the range (CWmin, CWmax) = (3, 7), which gives voice traffic the possibility to access the wireless channel before video traffic which has (CWmin, CWmax) = (7, 15) and best effort and back- ground traffic