ultaneously access the physical medium. Particular MAC frames can be used for management functions (radio channel scanning, authentication, association) or transmission control (acknowledgment of received frames). The PLCP sub-layer allows adaptation of the MAC sub-layer to the PMD sub-layer, providing signal-processing parameters for the receiver and indicating the bit rate of the frame. The PMD sub-layer defines the characteristics of the radio transmission. I.2.2. Security architecture The 802.1x mechanism defines the mobile access control to the Wi-Fi network that is performed between the mobile and the RADIUS (Remote Authentication Dial-In User Service) server. The 802.1x mechanism relies on EAP-Method (Extensible Authentication Protocol) authentication messages, for which several protocols are defined: - EAP-CHAP (Challenge Handshake Authentication Protocol) protocol allows the authentication of the mobile by the RADIUS server, based on a password; - EAP-TLS (Transport Layer Security) protocol allows mutual authentication of the RADIUS server and the mobile, based on certificates; - EAP-TTLS (Tunneled Transport Layer Security) protocol allows mutual authentication of the RADIUS server based on certificate and of the mobile based on password. Data protection on the radio interface introduces an extension of the MAC header: - TKIP (Temporal Key Integrity Protocol) extension for the WPA (Wi-Fi Protected Access) mechanism based on RC4 (Rivest Cipher) algorithms for encryption and MICHAEL for integrity checking; Wi-Fi Integration to the 4G Mobile Network - CCMP (Counter-mode/CBC-MAC-Protocol) extension for the WPA2 mechanism based on the AES (Advanced Encryption Standard) algorithm for encryption and integrity checking. I.2.3. Physical layers The 802.11a interface defines the OFDM (Orthogonal Frequency Division Multiplexing) physical layer operating in the U-NII (Unlicensed- National Information Infrastructure) frequency band at 5 GHz. The 802.11g interface defines the ERP (Extended Rate Physical) physical layer