Core Network: Architecture and Signaling Procedures

The UE is the subscriber’s device. The UE communicates with the NG-RAN over the Uu radio interface and with the AMF over N1, which carries the Non-Access Stratum (NAS) protocol of TS 24.501.

The NG-RAN is the 5G radio access network, made up of NR Node Bs (gNBs) and ng-eNBs (TS 38.300). The NG-RAN relays control-plane signaling to the AMF over N2 and user-plane traffic to the UPF over N3, and NG-RAN nodes interconnect over Xn.

The AMF terminates N1 and N2 and handles registration, connection, reachability, and mobility management for the UE, together with access authentication and authorization (TS 23.501 clause 6.2.1). The AMF relays session management messages between the UE and the SMF and exposes the Namf SBI (TS 29.518).

The SMF establishes, modifies, and releases Protocol Data Unit (PDU) sessions, allocates the Internet Protocol (IP) address of the UE, and selects and controls the UPF over N4 (TS 23.501 clause 6.2.2). The SMF exposes the Nsmf SBI (TS 29.502).

The UPF is the anchor point of the PDU session in the user plane. The UPF routes and forwards packets between the NG-RAN and the DN and applies the packet detection, Quality of Service (QoS), and usage reporting rules that the SMF provisions over N4 (TS 23.501 clause 6.2.3). Two UPFs interconnect over N9, for example when an intermediate UPF serves a PDU session.

The DN is the data network that the PDU session gives access to, for example the internet, operator services, or third-party services. The UPF connects to the DN over N6.

The AUSF authenticates the UE for 3GPP access and for untrusted non-3GPP access (TS 23.501 clause 6.2.8). The AUSF exposes the Nausf SBI (TS 29.509).

The UDM generates the authentication credentials, handles the user identification, and manages the subscription data and the registration of the serving NFs for each UE (TS 23.501 clause 6.2.7). The UDM can keep the subscription data in the UDR and exposes the Nudm SBI (TS 29.503).

The UDR stores and retrieves the subscription data for the UDM and the policy data for the PCF, together with the structured data for exposure and the application data, for example the Packet Flow Descriptions (PFDs) for application detection (TS 23.501 clause 6.2.11). The UDR exposes the Nudr SBI (TS 29.504).

The PCF provides the policy rules for access and mobility management to the AMF and the policy and charging control rules for the PDU session to the SMF (TS 23.501 clause 6.2.4). The PCF exposes the Npcf SBI, whose services several stage 3 specifications define, for example TS 29.507, TS 29.512, and TS 29.514.

The AF interacts with the 5GC to influence traffic routing, to request policy control from the PCF, or to use the exposure framework of the NEF (TS 23.501 clause 6.2.10). An AF in a trusted domain can interact with the PCF directly over N5, and any other AF uses the NEF.

The NEF exposes the capabilities and events of the 5GC to the AF and translates the information that the AF provides into requests towards the other NFs (TS 23.501 clause 6.2.5). The NEF exposes the Nnef SBI, and TS 29.522 defines the northbound application programming interfaces (APIs) of the NEF towards the AF.

The NRF holds the profiles of the available NF instances and their services, and it supports NF registration and NF discovery (TS 23.501 clause 6.2.6). The NRF exposes the Nnrf SBI (TS 29.510).

The NSSF selects the set of network slice instances that serve the UE and determines the allowed and configured network slices (TS 23.501 clause 6.2.14). The NSSF exposes the Nnssf SBI (TS 29.531).

The NSSAAF performs network slice-specific authentication and authorization with an external authentication, authorization, and accounting (AAA) server, and it supports access to a standalone non-public network (SNPN) with credentials from a credentials holder (TS 23.501 clause 6.2.23). The NSSAAF exposes the Nnssaaf SBI (TS 29.526).

The NSACF monitors and controls the number of registered UEs and the number of PDU sessions per network slice (TS 23.501 clause 6.2.28). The NSACF exposes the Nnsacf SBI (TS 29.536).

The 5G-EIR is an optional NF that checks the status of the Permanent Equipment Identifier (PEI) of the UE, for example to check that the PEI is not prohibited (TS 23.501 clause 6.2.15). The 5G-EIR exposes the N5g-eir SBI (TS 29.511).

The CHF performs converged online and offline charging for the charging data that the SMF and the AMF report (TS 32.240, TS 32.255 for the SMF, and TS 32.256 for the AMF), and it provides the policy counters for spending limit control to the PCF (TS 29.594). The CHF exposes the Nchf SBI (TS 32.291).

The NWDAF collects data from the NFs, from the AF, and from operations, administration, and maintenance (OAM) and provides analytics information to the NFs and to the AF (TS 23.501 clause 6.2.18 and TS 23.288). The NWDAF exposes the Nnwdaf SBI (TS 29.520).

The UDSF is an optional NF in which any NF can store and retrieve its unstructured data, for example UE contexts (TS 23.501 clause 6.2.12 and clause 4.2.5). The UDSF exposes the Nudsf SBI.

The SCP is an optional NF that routes the service requests between NFs for indirect communication and can perform the NF discovery and selection on behalf of the consumer NF (TS 23.501 clause 6.2.19 and Annex E). When an operator deploys no SCP, the NFs communicate with each other directly.