B for the selected beam (see Section 7.6.3). The process is controlled by many parameters, for instance, how many beam failures are detected with a given time window and what is the threshold (in the RSRP level) for considering SSB reception good enough to start the beam recovery process. If the SSB level is too weak, then beam recovery procedure is not started, and RLF follows. The process is illustrated in Figure 7.13. 7.6.12 Other Functions The MAC sublayer supports a large number of additional functions by means of MAC Control Elements (MAC signaling, including the following Timing Advance commands to maintain uplink timing in large cells (this was a basic LTE functionality already). 7 5G Radio Protocols CSI-RS (8 ports) 1. Detect CSI-RS resouce quality below threshold -> consider L1 CSI-RS (8 ports) beam failure 2. Exceed threshold of beam failures CSI-RS (8 ports) over given time 3. Consider SS block RSRP good enough for beam failure CSI-RS (8 ports) procedure CRI/RI/PMI/CQI 4. Start beam failure recovery procedure over RACH. L1 feedback reporting Figure 7.13 Beam failure detection and recovery. Activation and deactivation of SCells in CA to reduce power consumption in periods of low activity, for instance, when the PCell is enough to fulfill the QoS requirements or when the UE has no data to transmit (the functionality was introduced in Release 10 of LTE). Activation and deactivation of PDCP duplication (new 5G functionality described in Section 7.4.6). Bit rate recommendations (in DL) and recommendation queries for RAN-assisted codec adaptation (as introduced in the Release 14 of LTE). Further physical layer parameter or parameter set configurations and selections. 7.6.13 MAC PDU Structure As in LTE, a MAC PDU consists of MAC subPDUs, with the content of each MAC subPDU being indicated with a MAC subheader, pointing toward either control data (a MAC CE), user data (a logical channel), or padding. Unlike in LTE though, the MAC subheaders are not grouped together at the begin- ning of the MAC PDU. In 5G, MA