the same operations to generate the Kenb key. The pairwise master key (PMK) used for the four-way handshake procedure is the S-KWT key derived from the Kenb key. 11.2.2. LWIP aggregation The LWIP (LTE/WLAN radio level integration with IPsec tunnel) aggregation only applies to IP packets of the S1 bearer. The RRC (Radio Resource Control) and signaling messages, which are exchanged between the mobile and the eNB entity, are carried on the LTE interface (Figure 11.4). 206 Wi-Fi Integration to the 4G Mobile Network S1 Bearer LLC / MAC Wi-Fi PHY Wi-Fi Wi-Fi access MAC LTE PHY LTE LTE access Figure 11.4. Protocol architecture for LWIP aggregation IP packets are transported between the eNB entity and the mobile in the LWIP tunnel. The LWIPEP (LWIP Encapsulation Protocol) header contains the LCID of the radio bearer. The LWIP tunnel is protected between the mobile and the security gateway (SeGW) through an IP Security (IPSec) mechanism. Only one IPSec mechanism is mounted for all LWIP tunnels. The LWIP tunnel is transmitted in a GTP-U tunnel on the Xw interface, between the eNB and the SeGW entities. The IKE procedure for the IPSec mechanism is initialized after the association of the mobile to the Wi-Fi access point and authentication based on the EAP-AKA method. Each bearer is configured SO that the downstream direction or the upstream direction or both directions of transmission pass through the tunnel protected by the IPSec mechanism. For the downstream, IP packets are transmitted either on the LTE interface only, or on the Wi-Fi interface only, or simultaneously on both Carrier Aggregation LTE and Wi-Fi interfaces. In the latter case, the mobile can receive IP packets not in sequence. For the upstream, IP packets are transmitted either on the LTE interface only, or on the Wi-Fi interface only. 11.2.3. LAA aggregation LAA (Licensed Assisted Access) aggregation consists of using the 5-GHz U-NII (Unlicensed-National Information Infrastructure) band to transmit a 3GPP compliant LTE signal. The radio channel operating in