f the channel is free, then the data is transmitted immediately to the beginning of the next frame. If the channel is busy, then another CCA check is performed at the next frame period (Figure 11.11). Frame period Frame period Frame period Idle period Traffic failure success failure success Figure 11.11. LBT mechanism -FBE option 214 Wi-Fi Integration to the 4G Mobile Network For the LBE option, the device performs CCA control whenever there is data to transmit. If the channel is free, then the data is transmitted immediately. If the channel is busy, then the device must wait until the timer for the backoff mechanism expires (Figure 11.12). This timer is decremented when the radio channel is free. Fixed window: 5 CCA period Traffic Traffic channel channel Figure 11.12. LBT mechanism -LBE option The LBE option is relatively similar to the backoff mechanism of Wi-Fi access. Unlike Wi-Fi access, which adopts an exponential backoff mechanism, the LBE option opts for a backoff mechanism with a fixed window. 11.4. PDCP The Packet Data Convergence Protocol (PDCP) is used for RRC (Radio Resource Control) messages, relating to dedicated control data, and IP (Internet Protocol) packet related to the traffic. The PDCP performs the following functions: - compression of traffic data headers using the ROHC (Robust Header Compression) mechanism; - security of traffic data (confidentiality) and of RRC messages (integrity and confidentiality); - delivery in sequence of RRC messages and IP packets; - recovery of PDCP frames lost during the handover. Carrier Aggregation The PDCP defines headers to encapsulate the RRC signaling data, the traffic data and the control messages associated with the traffic data. The structure of PDCP frames is described in Figure 11.13 for frames containing traffic data and RRC signaling data. PDCP frame containing RRC messages PDCP frame containing IP packets Byte 1 PDCP SN Byte 1 PDCP SN Byte 2 Byte 2 RRC message IP packet Byte N-4 Byte N Byte N-3 MAC-I Byte N-2 MAC-I Byte 1 PDCP SN Byte N-1 MAC-I Byt