Section: 8.2 Bearer type selection
Content: In EN-DC, for each radio bearer the MN decides the location of the PDCP entity and in which cell group(s) radio resources are to be configured. Once an SN terminated split bearer is established, e.g. by means of the Secondary Node Addition procedure or MN initiated Secondary Node Modification procedure, the SN may remove SCG resources for the respective E-RAB, as long as the QoS for the respective E-RAB is guaranteed. In case an SN terminated bearer is released or reconfigured to an MN terminated bearer, only the MN generates the corresponding configuration and the SN does not generate the release configuration. In MR-DC with 5GC, the following principles apply: - The MN decides per PDU session the location of the SDAP entity, i.e. whether it shall be hosted by the MN or the SN or by both (split PDU session); - If the MN decides to host an SDAP entity it may decide some of the related QoS flows to be realized as MCG bearer, some as SCG bearer, and others to be realized as split bearer; - If the MN decides that an SDAP entity shall be hosted in the SN, some of the related QoS flows may be realized as SCG bearer, some as MCG bearer, while others may be realized as split bearer. In this case, the SN decides how to realise the QoS flow, but if the MN does not offer MCG resources, the SN can only realize the QoS flow as SCG bearer. The SN may remove or add SCG resources for the respective QoS flows, as long as the QoS for the respective QoS flow is guaranteed - If the MN decides that an SDAP entity shall be hosted in the SN, coordination of DRB IDs between the MN and the SN is needed to ensure unique allocation of DRBs for a UE. The SN is responsible to assign the DRB IDs for the DRBs it terminates, based on the DRB IDs indicated by the MN. - For each PDU session, including split PDU sessions, at most one default DRB may be configured (see [3]). The MN decides whether the SN is allowed to setup the default DRB or not; - In case an SN terminated bearer is released or reconfigured to an MN terminated bearer, the MN generates the corresponding configuration and the SN does not generate the release configuration. The only exceptional case where SN generates the release configuration is for the DRB release due to QoS flow to DRB remapping within SN.