ximizing cell capacity for GBR bearers in 5G. 7.3.4 Header In the downlink, an SDAP header is configured for a DRB when RQoS is required. In the uplink, an SDAP header is configured for a DRB when multiple QoS flows are mapped onto that DRB: The QFIs are needed in the uplink for the gNB to identify the QoS flows and mark the UP PDUs on NG-U. The end markers are needed to handle QoS flow remapping as described in Section 7.3.2 above. 7 5G Radio Protocols Going back to the example given in Section 7.3.1 where a default bearer is used to han- dle the bulk of the traffic, that default bearer would then be configured with SDAP head- ers in both the uplink and the downlink, and the separate bearers additionally configured to individually handle specific QoS flows would not have any SDAP header configured. 7.4.1 Overview The PDCP layer handles DRBs and guarantees in-order delivery without duplicates to the upper layers, performs header compression when required, and is the layer respon- sible for enforcing security in the RAN through ciphering and integrity protection. In 5G, the PDCP layer also takes care of duplication for increased reliability and reduced latency. The configuration of the PDCP sublayer for handling one radio bearer is called a PDCP entity. There are as many PDCP entities as there are radio bearers to handle. In the case of LTE and 5G dual connectivity, the 5G-based PDCP layer is always used, as shown in Chapter 5. This allows the actual data routing (if via eNB or gNB) to be transparent to the UE implementation, since the user plane operation remains the same with both routing options. 7.4.2 Reordering The design process of the PDCP reordering window in LTE is interesting. First, a reordering window at handover was agreed upon in order to ensure in-sequence delivery during handovers. Then, to cope with possible forwarding losses during handover, the reordering window was changed to a duplicate discard window. Finally, to simplify the behavior, it was agreed to apply the window always (i.e. not only a