by the access point. The PROBE REQUEST management frame is used by the station to request the characteristics of the radio interface of the access point. The PROBE REQUEST frame is a broadcast frame. The Subtype subfield is set to 0100. 28 Wi-Fi Integration to the 4G Mobile Network When the station has sent the PROBE REQUEST frame, it will arm a timer. If there is no response before expiration, then the station repeats the process on another radio channel. The access point provides its characteristics in the PROBE RESPONSE management frame when the value of the SSID contained in the PROBE REQUEST frame corresponds to that of the access point. The PROBE RESPONSE management frame is transmitted in unicast. The Subtype subfield is set to 0101. The AUTHENTICATION management frame is used for the authentication of the station (Figure 2.4). Authentification Authentification Status Challenge Algorithm Transaction Number Sequence Number Frame Destination Source Duration BSSID Message Control Address Address Authentication Figure 2.4. Structure of the AUTHENTICATION management frame Authentication Algorithm Number: this field is coded on 16 bits. It identifies the authentication mode. The following two modes are defined: - OSA (Open System Authentication): this mode corresponds to open access to the network. This mode is used for the WPA1 and WPA2 mechanisms; - SKA (Shared Key Authentication): this mode requires the station to send a seal to access the network. This mode is used for the WEP (Wired Equivalent Privacy) mechanism. Authentication Transaction Sequence Number: this field is coded on 16 bits. It contains the number of the authentication sequence. Status Code: this field is coded on 16 bits. It indicates whether the operation was successful or not. Challenge Code: this field, used for the WEP mechanism, has a variable size less than or equal to 255 bytes. It contains a string of bits, emitted in clear by the access point and then encrypted by the station. MAC Layer The association phase is implemented from four m