link packets that were not delivered to the gNB given that the HARQ buffers are flushed upon handover. In this case, some PDUs may be received in duplicate form, possibly from both the source and the target gNBs. In this case, the PDCP will remove any duplicates. The PDCP entity can also be configured to perform reordering to ensure in-sequence delivery of SDUs to higher layer protocols. In some cases, the PDUs can be duplicated and transmitted on multiple cells, increasing the likelihood of their correct reception at the receiving end, which can be useful for services requiring very high reliability. At the receiving end, the PDCP duplicate removal functionality removes any duplicates. The PDCP plays an important role in dual connectivity, where a device is connected to two cells, that is, an MCG and an SCG. The two cell groups can be handled by different gNBs. A radio bearer is typically handled by one of the cell groups, but there is also the possibility for split bearers, in which case one IETF: https://www.ietf.org/. 224 Chapter 2 radio bearer is handled by both cell groups. In this case the PDCP entity is responsible for distributing the data between the MCG and the SCG [17]. The PDCP entities are located in the PDCP sublayer. Several PDCP entities may be defined for a UE. Each PDCP entity carries the data of one radio bearer, which is associ- ated with either the control-plane or the user-plane. Fig. 2.14 illustrates the functional block diagram of a PDCP entity. For split bearers, the routing function is performed in the transmitting PDCP entity. The PDCP sublayer provides services to RRC and SDAP sub- layers. The PDCP provides transfer of user-plane and control-plane data; header compres- sion; ciphering; integrity protection services to the higher layer protocols. The maximum size of a data or control PDCP SDU supported in NR is 9000 bytes [8]. It must be noted that an NR system provides protection against eavesdropping and modification attacks. Signaling traffic (RRC messages) is encrypted and integrit