iability and reduce the latency, which is especially advantageous to URLLC services [8]. The PDCP control PDUs are not duplicated and are always submitted to the primary RLC entity. When configuring duplication for a DRB, the RRC sublayer sets the initial state to be either activated or deactivated. After the configuration, the state can be dynamically con- trolled by means of a MAC CE. In dual connectivity, the UE applies the MAC CE com- mands regardless of their origin (MCG or SCG). When duplication is configured for an SRB, the state is always active and cannot be dynamically controlled. When activating duplication for a DRB, NG-RAN ensures that at least one serving cell is activated for each logical channel of the DRB. When the deactivation of SCells leaves no activated serving cells for the logical channels of the DRB, NG-RAN ensures that duplication is also deactivated [8]. When duplication is activated, the original PDCP PDU and the corresponding duplicate are not transmitted on the same carrier. The primary and secondary logical channels can either belong to the same MAC entity (referred to as CA duplication) or to different ones (referred to as DC duplication). In CA duplication, logical channel mapping restrictions are applied to the MAC sublayer to ensure that the primary and secondary logical channels are not sent on the same carrier. When duplication is deactivated for a DRB, the secondary RLC entity is not reestablished, the HARQ buffers are not flushed, and the transmitting PDCP entity should indicate to the secondary RLC entity to discard all duplicated PDCP PDUs. In addition, in case of CA duplication, the logical channel mapping restrictions of the primary and secondary logical channels are relaxed as long as duplication remains deac- tivated. When an RLC entity acknowledges the transmission of a PDCP PDU, the PDCP entity notifies the other RLC entity to discard that PDU. When the secondary RLC entity 226 Chapter 2 Voice datagrams with IPv6/UDP/RTP headers Voice datagrams with ROHC headers Voice