an awareness of the Application data Application frames assigned to traffic class queues Voice data Video data Best efforts Background Internal collision resolution Priority frame sent to PHY layer PHY layer Figure 6-7: EDCF Traffic Class Queues Wireless LAN Standards DIFS=34 us Voice SIFS 2 slots Backoff 0 3 slots Video DIFS=34 us SIFS 2 slots Backoff 0 7 slots Best effort AIFS[2]=43 us SIFS 3 slots Backoff 0 15 slots AIFS[3]=79 us Background Backoff 0 15 slots SIFS 7 slots Interframe spacing per traffic class Backoff period traffic class Figure 6-8: AIFS and Backoff Timing per WMM Traffic Class QoS requirements of each station. Stations report the lengths of their queues for each traffic class and the hybrid coordinator uses this to determine which station will receive a transmit opportunity (TXOP) during a contention free transmission period. This HCF controlled channel access (HCCA) mechanism considers several factors in determining this allocation; The priority of the TC The QoS requirements of the TC (bandwidth, latency and jitter) Queue lengths per TC and per station The duration of the TXOP available to be allocated The past QoS given to the TC and station. HCCA allows applications to schedule access according to their needs, and therefore enables QoS guarantees to be made. Scheduled access requires a client station to know its resource requirements in advance and scheduling concurrent traffic from multiple stations also requires the access point to make certain assumptions regarding data packet sizes, data transmission rates and the need to reserve surplus bandwidth for transmission retries. The Wi-Fi Alliance adopted a subset of the 802.11e standard in advance of the IEEE's September 2005 approval. This subset, called Wi-Fi multimedia (WMM), describes four access categories as shown in Table 6-11, with EDCF timings as shown in Figure 6-8. The prioritisation mechanism certified in WMM is equivalent to the EDCF coordination mode defined in 802.11e but did not initially include the scheduled access capabil