.4.6 Duplication For the support of Ultra-Reliable and Low Latency Communications (URLLC) services, duplication is introduced in PDCP in 5G. With duplication, in addition to sending the original PDUs on a "primary leg," another leg, the "secondary leg," is used to transmit duplicates of the original PDUs as depicted in Figure 7.9. As long as the two legs are independent of one another, duplication increases chances of getting the PDU through in the shortest time possible: the fastest of the two legs at any point of time always pulls the transmission forward. To guarantee that the two legs are independent of one another and are not transmitted on the same radio path, different cells or different cell groups are used for diversity. The former is called carrier aggregation (CA) duplication, and the latter is called DC duplication. In both cases, it is guaranteed that the original PDUs and their duplicates will not end up on the same transport block. Note that CA duplication relies on LCP restrictions to ensure that different transport blocks are used (see Section 7.6.6). Once a PDU is known to have reached the receiver, if the corresponding PDU still awaits transmission, it is discarded (the original PDU on the primary leg if the duplicate was received via the secondary leg, or the duplicate on the secondary leg if the original Figure 7.9 Duplication. primary secondary Cell/CGx Cell/CGy 7 5G Radio Protocols PDU was received on the primary leg). At the receiver, the second time the same PDU is received, it is discarded using the same mechanisms that were described in Section 7.4.5. 7.5.1 Overview The main tasks of the RLC sublayer are segmentation and error correction. In the trans- mitter, segmentation consists in cutting RLC SDUs (i.e. PDCP PDUs) into segments that fit into the payload allocated for transmission by the lower layers. This naturally occurs only if the SDU is too large to fit. In the receiver, the segments are then put back together through a process called SDU reassembly. Error correction is achieved