Dual Connectivity involves two radio network nodes in providing radio resources to a given UE (with active radio bearers), while a single N2 termination point exists for the UE between an AMF and the RAN. The RAN architecture and related functions to support Dual Connectivity is further described in RAN specifications (e.g. TS 37.340
The RAN node at which the N2 terminates, performs all necessary N2 related functions such as mobility management, relaying of NAS signalling, etc. and manages the handling of user plane connection (e.g. transfer over N3). It is called the Master RAN Node. It may use resources of another RAN node, the Secondary RAN node, to exchange User Plane traffic of an UE. Master RAN node takes into account the RSN to determine if dual connectivity shall be set up and ensure appropriate PDU session handling ensures fully redundant user plane path as described in clause 126.96.36.199
If the UE has Mobility Restriction (either signalled from the UDM, or, locally generated by the Serving PLMN policy in the AMF) the AMF signals these restrictions to the Master RAN Node as Mobility Restriction List; This may prevent the Master RAN node from setting up a Dual Connectivity for an UE.
Subject to policies in the NG-RAN, configuration of Dual Connectivity for a Data Radio Bearer can also be based on the Network Slice that the PDU Session belongs to.
Dual Connectivity provides the possibility for the Master node RAN to request SMF:
For some or all PDU Sessions of an UE: Direct all the DL User Plane traffic of the PDU Session to the either the Master RAN Node or to the Secondary RAN Node. In this case, there is a single N3 tunnel termination at the RAN for such PDU Session.
The terminating RAN Node, can decide to keep traffic for specific QFI(s) in a PDU Session for a UE on a single RAT, or split them across the two RATs.
For some other PDU Sessions of an UE: Direct the DL User Plane traffic of some QoS Flows of the PDU Session to the Secondary (respectively Master) RAN Node while the remaining QoS Flows of the PDU Session are directed to the Master (respectively Secondary) RAN Node. In this case there are, irrespective of the number of QoS Flows, two N3 tunnel terminations at the RAN for such PDU Session.
The Master RAN may create and change this assignment for the user plane of a PDU Session at any time during the life time of the PDU Session;
In both cases, a single PDU Session Id is used to identify the PDU Session.
Additional functional characteristics are:
User location information reporting is based on the identity of the cell that is serving the UE in the Master RAN node.
Path update signalling related with Dual Connectivity and UPF re-allocation cannot occur at the same time.