 ACK control frame when the traffic frame has been correctly received; - EIFS (Extended Inter-Frame Space): this interval is used when the transmitter has not received an acknowledgment. Its duration is equal to SIFS + (8 X ACK) + (PLCP header) + DIFS. 2.2.2. Mobile registration Mobile registration at the access point is done in three phases, namely scanning, authentication and association. The purpose of scanning is to recover the characteristics of the radio interface, which can be either passive or active. When the scanning is passive, the station scrutinizes the BEACON channel on each radio channel. When the scanning is active, the station transmits a PROBE REQUEST management frame, containing the network identifier (SSID), broadcast to all the access points, using a broadcast BSSID. MAC Layer The access point recognizable in the SSID corresponds to the PROBE RESPONSE management frame containing the characteristics of the radio interface of the access point (Figure 2.7). Probe Station Request Probe Response Probe Response Figure 2.7. Active scanning In OSA mode, authentication is done in two steps: - the station sends the AUTHENTICATION management frame by mentioning the authentication mode; - the access point responds with the AUTHENTICATION management frame containing the status (success or failure). In SKA mode, authentication is done in four steps: - the station sends the AUTHENTICATION management frame by mentioning the authentication mode; - the access point sends the AUTHENTICATION management frame containing a bit string in the Challenge Text field; - the station sends the AUTHENTICATION management frame containing the encrypted bit string in the Challenge Text field; - the access point verifies the response of the station and sends the AUTHENTICATION management frame containing the status (success or failure). The aim of the association phase is to check that the transmission characteristics of each part (the station, the access point) are compatible. It is carried out in two phases: - the station se