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).
In this Release of the specification, the Dual Connectivity function does not apply to the NR RedCap UE.
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 and/or PDU Session Pair ID 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 188.8.131.52.
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.
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.
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 is based on the identity of the cell that is serving the UE in the Master RAN node. The cell identity of the Primary cell in the secondary RAN node may also be included. NG-RAN includes the user location information in NGAP messages where the contents of the user location information may change during the corresponding procedure.
Path update signalling related with Dual Connectivity and UPF re-allocation cannot occur at the same time.
5GC supports interactions towards CHF for network resource usage, as defined in TS 32.240. The CHF and the Nchf service are defined in TS 32.290.
The SMF supports the interactions towards the CHF, as defined in TS 32.255. The UPF supports functionality to collect and report usage data to SMF. The N4 reference point supports the SMF control of the UPF collection and reporting of usage data.
The AMF supports interactions towards the CHF, as defined in TS 32.256.
The SMSF supports interactions towards the CHF, as defined in TS 32.274.
The NEF supports interactions towards the CHF, as defined in TS 32.254.
When NG-RAN is deployed in dual connectivity configuration, the HPLMN or VPLMN operator may wish to record the data volume sent and received on the Secondary RAT.
In order to reduce the complexity of this procedure, the following principles are used in this release:
The PLMN locally activates the Secondary RAT Usage Data Reporting by NG-RAN OAM. The activation is based on configuration in NG-RAN and NG-RAN determines whether the data volume report will contain data volumes consumed for the whole PDU Session or for selected QoS Flows or both as described in TS 38.413.
The activation can happen separately for Data Volume Reporting of NR in licensed or unlicensed spectrum and E-UTRA in licensed or unlicensed spectrum. If the PLMN uses this feature, it should ensure that this functionality is supported by all NG-RAN nodes that support NR or E-UTRA as a Secondary RAT.
Depending on its configuration the NG-RAN reports uplink and downlink data volumes to the 5GC for the Secondary RAT (including the using of unlicensed spectrum for NR or E-UTRA) for the PDU Session or for selected QoS Flows or both and per time interval.
During Xn handover and N2 handover, the source NG-RAN node reports the data volume to the 5GC. The reported data volume excludes data forwarded to the target RAN node.
At the time of NG connection release, Secondary Node change/release, deactivation of UP connection for a PDU Session, the NG-RAN node reports the data volumes to the 5GC.
To assist "partial CDR" generation, NG-RAN OAM can instruct the NG-RAN to also make periodic reports (as described in clause 5.12.3) if no event has triggered a report before the period expires.
Periodic reporting of the Secondary RAT usage data is an optional function. When NG-RAN, as defined in bullet e) of clause 5.12.2, is configured with a "time interval for Secondary RAT usage data reporting", the NG-RAN shall send a RAN Usage Data Report message for periodic reporting purposes to the SMF only when the timer expires for a UE for which Secondary RAT usage data reporting is ongoing. The timer runs from the last usage reporting for the UE.
Edge computing enables operator and 3rd party services to be hosted close to the UE's access point of attachment, so as to achieve an efficient service delivery through the reduced end-to-end latency and load on the transport network. Edge Computing support by 5GC is specified in this specification and in TS 23.548.
The 5G Core Network selects a UPF close to the UE and forwards traffic to enable the local access to the DN via a N6 interface according to the provided traffic steering rules to the UPF. This may be based on the UE's subscription data, UE location, the information from Application Function (AF) as defined in clause 5.6.7, the EAS information reported from EASDF (as defined in TS 23.548), policy or other related traffic rules.
Due to user or Application Function mobility, the service or session continuity may be required based on the requirements of the service or the 5G network.
The 5G Core Network may expose network information and capabilities to an Edge Computing Application Function.
Edge computing can be supported by one or a combination of the following enablers:
User plane (re)selection: the 5G Core Network (re)selects UPF to route the user traffic to the local part of the DN as described in clause 6.3.3;
Local Routing and Traffic Steering: the 5G Core Network selects the traffic to be routed to the applications in the local part of the DN;
this includes the use of a single PDU Session with multiple PDU Session Anchor(s) (UL CL / IP v6 multi-homing) as described in clause 5.6.4 and the use of a PDU Session with Distributed Anchor Point using SSC mode 2/3.
Session and service continuity to enable UE and application mobility as described in clause 5.6.9;
An Application Function may influence UPF (re)selection and traffic routing via PCF or NEF as described in clause 5.6.7;