MAC Wi-Fi MAC LTE PHY LTE PHY Wi-Fi LTE access Wi-Fi access Figure 11.3. Protocol architecture for LWA aggregation eNB and AP entities are distant On LTE access, the LCID is carried by the MAC layer. The recipient uses the LCID to reassemble the PDCP frames of the same bearer. The re-sequencing of the PDCP frames received by the two LTE and Wi-Fi accesses is performed by the PDCP. Frames transported on an LWA bearer are only those acknowledged and those corresponding to RLC frames using acknowledgment mode (AM) on the LTE interface. The Type field of the LLC header for Wi-Fi access is set to hexadecimal 9E65. The mobile uses this value to determine that the frame comes from an LWA bearer. When the eNB and AP entities are distant, the eNB entity can be connected to multiple AP entities via the Xw interface that supports the traffic and control data (Figure 11.3). The NAS signaling data is carried on the S1-MME interface, between the MME and eNB entities, and then on the Xw-C (Control) interface, between the eNB and AP entities. Carrier Aggregation Traffic data, corresponding to the IP stream, is transported in a GTP-U (GPRS Tunneling Protocol User) tunnel: - on the S1-U interface, between SGW and eNB entities; - on the Xw-U (User) interface, between the eNB and AP entities. Mutual authentication is based on the EAP-AKA (Authentication and Key Agreement) method described in Chapter 6. During the procedure of attaching the mobile to the 4G mobile network, the home subscriber server (HSS) retrieves the Ki key allocated to the mobile, draws a random (RAND) and calculates the integrity key (IK) and cipher key (CK). The Ki key is a secret key, shared between the HSS entity and the mobile, created during the subscription. The HSS entity computes the KASME key from the IK and CK and passes the KASME key and the random to the mobility management entity (MME). From the KASME key, the MME entity calculates the Kenb key and transmits it to the eNB entity. The MME entity transmits the random (RAND) to the mobile that performs 