TS 23.501 – 5G System Architecture

1 Scope

The present document defines the Stage 2 system architecture for the 5G System. The 5G System provides data connectivity and services.

This specification covers both roaming and non-roaming scenarios in all aspects, including interworking between 5GS and EPS, mobility within 5GS, QoS, policy control and charging, authentication and in general 5G System wide features e.g. SMS, Location Services, Emergency Services.

ITU‑T Recommendation I.130 [11] describes a three-stage method for characterisation of telecommunication services and ITU‑T Recommendation Q.65 [12] defines Stage 2 of the method.

TS 23.502 [3] contains the stage 2 procedures and flows for 5G System and it is a companion specification to this specification.

TS 23.503 [45] contains the stage 2 Policy Control and Charging architecture for 5G System and it is a companion specification to this specification.

4 Architecture model and concepts

4.1 General concepts

The 5G System architecture is defined to support data connectivity and services enabling deployments to use techniques such as e.g. Network Function Virtualization and Software Defined Networking. The 5G System architecture shall leverage service-based interactions between Control Plane (CP) Network Functions where identified. Some key principles and concept are to:

- Separate the User Plane (UP) functions from the Control Plane (CP) functions, allowing independent scalability, evolution and flexible deployments e.g. centralized location or distributed (remote) location.

- Modularize the function design, e.g. to enable flexible and efficient network slicing.

- Wherever applicable, define procedures (i.e. the set of interactions between network functions) as services, so that their re-use is possible.

- Enable each Network Function and its Network Function Services to interact with other NF and its Network Function Services directly or indirectly via a Service Communication Proxy if required. The architecture does not preclude the use of another intermediate function to help route Control Plane messages (e.g. like a DRA).

- Minimize dependencies between the Access Network (AN) and the Core Network (CN). The architecture is defined with a converged core network with a common AN - CN interface which integrates different Access Types e.g. 3GPP access and non-3GPP access.

- Support a unified authentication framework.

- Support "stateless" NFs, where the "compute" resource is decoupled from the "storage" resource.

- Support capability exposure.

- Support concurrent access to local and centralized services. To support low latency services and local access to data networks, UP functions can be deployed close to the Access Network.

- Support roaming with both Home routed traffic as well as Local breakout traffic in the visited PLMN.

4.2 Architecture reference model

4.2.1 General

This specification describes the architecture for the 5G System. The 5G architecture is defined as service-based and the interaction between network functions is represented in two ways.

- A service-based representation, where network functions (e.g. AMF) within the Control Plane enables other authorized network functions to access their services. This representation also includes point-to-point reference points where necessary.

- A reference point representation, shows the interaction exist between the NF services in the network functions described by point-to-point reference point (e.g. N11) between any two network functions (e.g. AMF and SMF).

Service-based interfaces are listed in clause 4.2.6. Reference points are listed in clause 4.2.7.

Network functions within the 5GC Control Plane shall only use service-based interfaces for their interactions.

NOTE 1: The interactions between NF services within one NF are not specified in this Release of the specification.

NFs and NF services can communicate directly, referred to as Direct Communication, or indirectly via the SCP, referred to as Indirect Communication. For more information on communication options, see Annex E and clauses under 6.3.1 and 7.1.2.

In addition to the architecture descriptions in clause 4, the following areas are further described in other specifications:

- NG-RAN architecture is described in TS 38.300 [27] and TS 38.401 [42].

- Security architecture is described in TS 33.501 [29] and TS 33.535 [124].

- Charging architecture is described in TS 32.240 [41].

- 5G Media streaming architecture is described in TS 26.501 [135].

NOTE 3: The NFs listed in clause 4.2.2 are described in the following clauses or in the specifications above.

4.2.2 Network Functions and entities

The 5G System architecture consists of the following network functions (NF):

- Authentication Server Function (AUSF).

- Access and Mobility Management Function (AMF).

…