n uplink grant, the UE MAC fills each MAC PDU with data from the highest priority logical channel up to the PBR and then cycles through the lower priority logical channels in sequence. This functionality is similar to LTE. NR MAC also includes a new functionality to restrict data from certain logical channels to be sent only on certain numerologies/cells. This is used for example to send the duplicate PDCP PDUs data of the RB on different logical channels of different cells in the case of CA. Semi Persistent Scheduling (SPS) functionality similar to LTE is also used in NR with some enhancements. Two types of Configured Grants are used - Type 1 where the period- icity and resources for the uplink grant is provided to a UE by RRC, and Type 2 where the resource is configured by a Layer 1 control channel, Physical Downlink Control Channel (PDCCH), and the periodicity by RRC. MAC is also responsible for handling Hybrid ARQ (HARQ) processing and retransmis- sions as in LTE. Apart from data transfer, the MAC protocol also supports several control elements that are used for signaling and configuration of the user plane. The Scheduling Request (SR) is used for requesting UL-SCH resources for new transmission. SR can be transmitted on Phys- ical Uplink Control Channel (PUCCH) resources if available or using the Random Access (RACH) procedure. Buffer Status reports are generated by MAC to inform the network 3 5G System Overview R/LCID Fixed-sized R/F/LCID/L Variable-sized R/F/LCID/L MAC SDU subheader MAC CE subheader MAC CE subheader MAC subPDU MAC subPDU MAC subPDU including MAC subPDU including MAC subPDU including including MAC CE 1 including MAC CE 2 MAC SDU MAC SDU padding (opt) Figure 3.4.6 Example of a downlink MAC PDU structure. (Source: Reproduced by permission of © 3GPP). about pending uplink data in the buffer. PDCP duplication can also be enabled/disabled using a MAC control element. Furthermore, MAC includes several power saving functionalities as in LTE. Discontin- uous reception (DRX), when configured, contro