ce (AIFS) during which the mobile detects that the radio channel is free, before triggering the backoff mechanism or the transmission: AIFS[AC] = AIFSN[AC] X SlotTime + SIFSTime; - the minimum length of the contention window CWmin (Contention Window) and the maximum CWmax used for the backoff mechanism; - TXOP (Transmission Opportunity) time during which the mobile transmits when it has access to the radio channel. The EDCA parameters are stored in the mobile and can be updated by the access point in the EDCA Parameter Set information element transmitted in the BEACON, ASSOCIATION RESPONSE, REASSOCIATION RESPONSE and PROBE RESPONSE management frames. The modification of the EDCA parameters is indicated in the QoS Capability information element present in the BEACON management frame that does not contain the EDCA Parameter Set information element and in the (RE)ASSOCIATION REQUEST management frames (RE). The default values of the EDCA parameters are shown in Table 2.4. Access category CWmin CWmax AIFSN TXOP (ms) AC-BK 1,023 AC-BE 1,023 AC-VI 3,008 AC-VO 1,504 Table 2.4. Default values of EDCA parameters 48 Wi-Fi Integration to the 4G Mobile Network 2.4.2. Impact on the MAC header The MAC header of the traffic frame inserts the two-byte QoS Control field following the Address 4 field (Figure 2.24). Bit 0-3 Bit 4 Bit 5-6 Bit 7 Bit 8-15 Ack Policy Reserved Reserved Frame Duration Address 1 Address 2 Frame Address 3 Address 4 Control Control byte 2 0 2304 Figure 2.24. Evolution of MAC header structure The presence of the QoS Control field is indicated by the Subtype field (value equal to 1,000). The TID (Traffic Identifier) subfield, coded on three bits, provides the priority level (UP) of the access to the radio channel. The Ack Policy subfield, coded on two bits, identifies the MAC frame acknowledgment rule: - 00: the frame must be acknowledged. The receiver of the MAC frame must return an ACK frame after the SIFS interval; - 01: the frame must not be acknowledged. Acknowledgment is performed in the upper layers. Th