Explaining the 5G SA Core network as distinct from 5G RAN (5G NR)

At this week’s ITU-R WP5D meeting, a Russian contribution (5D/998) proposed a liaison statement to Global Core Specification (GCS) Proponents to clarify 5G NSA (Non-Stand Alone) and 5G SA (Stand Alone) architectures in Recommendation ITU-R M.2150 (formerly know as IMT 2020.specs). Those architectures are described in 3GPP TR 21.915 (Summary of Rel-15 Work Items).  While 5G NSA uses LTE for everything other than the RAN (5G NR), 5G SA introduces the 5G Core network which facilitates new 5G features and functions.  3GPP has decided NOT to liaison their 5G SA Core network TR’s to ITU-T for consideration as ITU recommendations for 5G non radio aspects.

Here’s the functional split between the 5G RAN (5G NR) and the 5G core network, as per 3GPP Release 15:

From 3GPP TR 21.915:

5.5.2 The 5G Core Network

5.5.2.1 Main Network Functions (NFs)

The AMF (Access and Mobility management Function) support UEs with different mobility management needs. It performs the following main tasks:

– The Non-Access Stratum (NAS) signalling termination;

– The NAS signalling security;

– The Access Stratum (AS) Security control;

– Inter CN node signalling for mobility between 3GPP access networks;

– Idle mode UE Reachability (including control and execution of paging retransmission);

– Registration Area management;

– Support of intra-system and inter-system mobility;

– Access Authentication;

– Access Authorization including check of roaming rights;

– Mobility management control (subscription and policies);

– Support of Network Slicing;

– SMF selection.

The SMF (Session Management Function) can support, together with the AMF, customized mobility management schemes such as “Mobile Initiated Connection Only” (MICO) or RAN enhancements like “RRC Inactive” state. It performs the following main tasks:

– Session Management;

– UE IP address allocation and management;

– Selection and control of UPF;

– Configures traffic steering at UPF to route traffic to proper destination;

– Control part of policy enforcement and QoS;

– Downlink Data Notification.

The UPF (User Plane Function) performs the following main tasks:

– Anchor point for Intra-/Inter-RAT mobility (when applicable);

– External PDU session point of interconnect to Data Network;

– Packet routing & forwarding;

– Packet inspection and User plane part of Policy rule enforcement;

– Traffic usage reporting;

– Uplink classifier to support routing traffic flows to a data network;

– Branching point to support multi-homed PDU session;

– QoS handling for user plane, e.g. packet filtering, gating, UL/DL rate enforcement;

– Uplink Traffic verification (SDF to QoS flow mapping);

– Downlink packet buffering and downlink data notification triggering.

The other main Network Functions are:

– The “Network Repository Function” (NRF): it provides support for NF services management including registration, deregistration, authorization and discovery.

– The “Network Exposure Function” (NEF): it provides external exposure of the capabilities of the network functions. External exposure can be categorized as Monitoring capability, Provisioning capability, Application influence of traffic routing and Policy/Charging capability.

– The “Unified Data Management” (UDM): the 5GC supports Data Storage architecture for Compute and Storage separation. The Unified Data Repository (UDR) is the master database. The Unstructured Data Storage Function (UDSF) is introduced to store dynamic state data.

5.5.2.2 Specificities of the 5G Core Network and associated NFs

5.5.2.2.1 Local hosting of services and Edge Computing

5.5.2.2.2 Network slicing

5.5.2.2.3 Unified access control

5.5.2.2.4 Support of 3GPP and non-3GPP access

5.5.2.2.5 Policy framework and QoS support

5.5.2.2.7 Other specific services

The following services are also supported by 5G SA core network:

Short Message Service (SMS). This is supported by “SMS over NAS” (including over non-3GPP access).

IP-Multimedia Subsystem (IMS) and its services, although, this might not be available in all initial 5G deployments. If IMS services are invoked by a UE connected to an IMS-less 5GS, this triggers a network-based handover towards an appropriate RAT and related EPS. This applies also to the support of IMS emergency services.

Multi-Operator Core Network (MOCN), in which a RAN is shared by multiple core networks.

Public Warning System (PWS). This is supported by either using Service-based interfaces between the Cell Broadcast Centre (CBC) Function (CBCF) and the AMF, or using an interworking function between the CBC and the AMF.

Multimedia Priority Services (MPS). They are supported by MPS-specific exemptions for 5GS mobility management and 5GS session management.

Mission Critical Services (MCS). They are supported by having a subscription in place for both 5G QoS profile and the necessary policies. Some standardized QoS characteristics are defined for MCS.

PS Data Off. The 5G’s “PS data off” functionality is backward-compatible and provides Control Plane Load Control, Congestion and Overload control. This includes AMF Load balancing, AMF Load-rebalancing, TNL (Transport Network Layer between 5GC and 5G-AN) Load (re-)balancing, as well as AMF Overload Control, SMF Overload Control.

It should be noted that, in 5GS Phase 1 (3GPP Release 15), Location Services are optional and restricted to regulatory (emergency) services.  5GS Phase 2 (3GPP Release 16) was frozen in June 2020 without either URLLC in the RAN or URLCC in the Core specs completed.

References:

https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=3389

IMT 2020: Concept of Global Core Specification (GCS) and Transposing Organization(s)

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