 latency for lossless mobility (waiting for status reports to be received and processed might unnecessarily delay the transmis- sion). Figure 7.8 illustrates an example of how status reports work. ((p)) ((p)) a,b,c,d,e,f,g,h b,c,e,f,g,h forwarding 1,2,4,5,6 TX: b,c,e,f,g,h 4,5,6 RX: 4,5,6 source target up to 2 and 4,5,6 up to a and d,e,f,g 1,2,3,4,5,6,7,8 handover 3,4,5,6,7,8 a,d,e,f,g d,e,f,g RX: X delivered to upper layers/TX: X acknowledged by lower layers Figure 7.8 PDCP status reports. 7.4 PDCP As depicted, in the downlink, before handover, the source gNB transmits PDUs {a, b, c, d, e, f, g, h}, but only {a, d} are acknowledged by the UE. In the receive window of the UE, PDUs {b, c} are known to be missing, blocking {d, e, f, g} from being delivered to upper layers while PDU {a} was already delivered to upper layers. In the uplink, before handover, the UE transmits PDUs {1, 2, 3, 4, 5, 6, 7, 8}, but only {1, 2, 4, 6} are acknowl- edged by the gNB. The first missing PDU is therefore {3}. In the receive window of the gNB, PDU {3} is known to be missing, blocking {4, 5, 6} from being delivered to upper layers while PDUs {1, 2} were already delivered to upper layers. At handover, the source gNB forwards the contents of its receive and transmit windows to the target gNB, that is, PDUs {b, c, e, f, g, h} for the downlink and {4, 5, 6} for the uplink, while the UE prepares the retransmission of all PDUs starting from the first known missing one, that is, PDUs {3,4,5,6,7,8}. After handover, the uplink status report generated by the gNB signals to the UE that PDUs up to {2} were correctly received, as well as PDUs {4, 5, 6}. The UE can then clear those PDUs from its transmission buffer and only retransmit PDUs {3, 7, 8} instead of {3, 4, 5, 6, 7, 8}. The downlink status report generated by the UE signals to the gNB that PDUs up to {a} were correctly received, as well as PDUs {d, e, f, g}. The gNB can then clear those PDUs from its transmission buffer and retransmit only PDUs {b, c, h} instead of {b, c, e, f, g, h}. 7