ls the UE's PDCCH monitoring activity and is used, as in LTE, to save UE power consumption. When CA is configured, Secondary Cells (SCells) can be activated or deactivated using MAC control elements to save power. Figure 3.4.6 shows a MAC PDU structure consisting of MAC control elements and MAC sub-PDUs. Each MAC sub-PDU includes the MAC subheader and MAC SDU. This is unlike LTE where all the MAC subheaders are grouped and placed at the head of a MAC PDU. Such distribution of a MAC subheader to each MAC SDU in NR is again for faster generation of MAC PDU after receipt of an uplink grant by maximizing preprocessing. RACH procedure is handled by MAC. NR follows the contention-based and contention- less RACH procedures of LTE where successful completion of contention resolution in message 4 or successful Random Access Response (RAR) for contentionless access repre- sents successful completion of the RACH procedure. In addition to the RACH triggers as in LTE, such as for connection establishment, handover access, Timing Advance (TA) update in CONNECTED state, and SR, RACH in NR is also used for beam failure recovery and SI request. The RACH beam failure recovery procedure is used to indicate to the serving gNB when beam failure is detected on the serving Synchronization Signal Blocks (SSBs)/Channel State Information Reference Signals (CSI-RSs). A UE indicates beam failure to the network when the number of beam failure instance indications from the lower layers to the MAC entity exceeds a certain configured threshold. Successful completion of the RACH procedure rep- resents successful beam failure recovery. RACH can also be used to request on-demand System Information as discussed in Section 3.4.5.2.7. A single MAC entity in the UE can support multiple numerologies, transmission timings, and cells, such as in the CA. 3.4.4 Supporting QoS with 5GC 5GC uses a different QoS concept from 4G EPC. Each downlink data packet is marked by the 5GC with a QFI indicating the QoS requirement for the packet. In the uplink such mark