nds the ASSOCIATION REQUEST management frame; Wi-Fi Integration to the 4G Mobile Network - the access point sends the ASSOCIATION RESPONSE management frame containing the AID assigned to the station and the status (success or failure). The cell change is initiated by the station, by issuing REASSOCIATION REQUEST management frame to a new access point. This frame contains the MAC address of the old access point. The new access point responds with the REASSOCIATION RESPONSE management frame that contains the new identifier (AID) assigned to the station. In the meantime, the station must perform an authentication phase. 2.2.3. Data transfer The distributed coordination function (DCF) mode implements the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) mechanism. A station first listens to the radio channel before transmitting. To avoid collisions, the backoff mechanism is used before transmission of frame if the radio channel is busy. The use of RTS and CTS control frames makes it possible to limit the impact of a collision to the single short RTS frame (Figure 2.8). SIFS, traffic source destination Back-off traffic other station Figure 2.8. Use of control frames for data transfer The use of RTS and CTS control frames, for the transmission of unicast frames from the station and the access point, and multicast or broadcast frames from the station, depends on a configuration parameter, corresponding to the size of the frame. MAC Layer The multicast or broadcast frames transmitted by the access point are transmitted without RTS and CTS control frames. The transmitted unicast traffic frames must be acknowledged by an ACK control frame (Figure 2.8), as well as multicast or broadcast traffic frames sent by the station. The multicast or broadcast traffic frames sent by the access point are not acknowledged. When a frame is sent, the transmitter arms a timer. If the acknowledgment is not received when the latter expires, the transmitter will try retransmitting again using the EIFS interval. If the radio channel i