5G System Architecture

General concepts of system architecture of 5G

The System architecture of 5G is defined so that connectivity and services of data can be supported that enables the techniques such as Network Function Virtualization and Software Defined Networking to be used in the deployment. The service-based interactions are leveraged in the 5G system architecture between Control Plane (CP) Network Functions. There are some key principles and concept such as:

  • The User Plane functions are separated from the Control Plane functions that allows deployments that are scalability, evolution and flexible such as centralized location or distributed location.
  • The function design is modularized by enabling flexible and efficient network slicing.
  • The procedures are defined as services so that they can be reused.
  • Each Network Function can interact directly with other NF. The use of an intermediate function can not be precluded by the architecture so that the Control Plane messages can be routed easily.
  • Dependencies between the Access Network (AN) and the Core Network (CN) can be minimized.
  • A converged core network is used to define the architecture with a common AN - CN interface due to which different Access Types e.g. 3GPP access and non-3GPP access can be integrated.
  • A unified authentication framework can be supported.
  • Stateless NFs can be supported so that decoupling of the compute resource can be done from the storage resource.
  • Capability exposure can be supported.
  • Concurrent access to local and centralized services can be supported. There is deployment of UP functions close to the Access Network so that low latency services and access to local data networks can be supported.
  • Both Home routed traffic as well as Local breakout traffic are used to support roaming in the visited PLMN.

Architecture Reference Model

The architecture of 5G is defined as service-based and there are two ways of interaction between network functions, such as:

  • A service-based representation
  • A reference point representation

The following network functions are consisted in the 5G System architecture, i.e.:

  • Authentication Server Function (AUSF)
  • Access and Mobility Management Function (AMF)
  • Data Network (DN), e.g. operator services, Internet access or 3rd party services
  • Unstructured Data Storage Function (UDSF)
  • Network Exposure Function (NEF)
  • Network Repository Function (NRF)
  • Network Slice Selection Function (NSSF)
  • Policy Control Function (PCF)
  • Session Management Function (SMF)
  • Unified Data Management (UDM)
  • Unified Data Repository (UDR)
  • User Plane Function (UPF)
  • Application Function (AF)
  • User Equipment (UE)
  • Radio Access Network (RAN)
  • 5G-Equipment Identity Register (5G-EIR)
  • Security Edge Protection Proxy (SEPP)
  • Network Data Analytics Function (NWDAF)

The non-roaming reference architecture is shown in the figure below.

Non-Roaming 5G System architecture in reference point representation is shown in the figure below.

The figure below shows the non-roaming architecture for UEs in which two data networks have been accessed with the use of multiple PDU Sessions, using the reference point representation.

The architecture for multiple PDU Sessions is shown in this figure in which for the two different PDU Sessions, two SMFs have been selected. However, both local and central UPF, within a PDU Session, are controlled by each SMF. The figure below shows the non-roaming architecture when providing access to two data networks within a single PDU Session in reference point representation.

The figure below shows the 5G System roaming architecture having local breakout scenario in service-based interface in the control plane.

The figure below shows the home routed scenario in service-based interfaces in the Control Plane for the 5G System architecture.

The figure below shows the 5G System roaming architecture having local breakout scenario in reference point interface representation.

The figure below shows the home routed scenario in reference point interface for the 5G System architecture.

Each PLMN for the roaming scenarios has been implemented as proxy functionality to secure interconnection and hide topology on the inter-PLMN interfaces.

In the 5G System architecture, any NF can be allowed to store and retrieve its unstructured data to/from a UDSF as shown in the figure below.

The UDM, PCF and NEF are also allowed to store data in the UDR in which UDM and PCF subscription and policy data, structured data for exposure and NEF application data are included as shown in the figure below.

The following different functions are served by the UDR which has been deployed in each PLMN:

  • The NEF accesses the UDR which belongs to the same PLMN as the NEF.
  • The UDM accesses the UDR which belongs to the same PLMN as UDM if a split architecture is supported by the UDM.
  • The PCF accesses the UDR which belongs to the same PLMN as PCF.

For the network functions, there defined the Nudr interface so that a particular set of the stored data can be accessed and notification of relevant changes in data in the UDR can be read, updated, deleted, and subscribed.

The data that is authorised to change can be added, modified, updated or deleted by each NF Service Consumer when the UDR has been accessed, via Nudr. The UDR shall perform this authorisation on a per data set.

Nudr is used by the UDR sets to expose and store the following data:

  • Subscription Data
  • Policy Data
  • Structured Data for exposure
  • Application data

The data sets expose the 3GPP defined information elements whose content and format/encoding is defined by service based Nudr interface. In addition to this, the operator specific data sets from the UDR are also accessed by the the NF Service Consumers.

Supporting Non-3GPP Access

The non-3GPP access networks are used by the 5G to access the connectivity of the UE. Only untrusted non-3GPP accesses can be supported by 5G Core Network. A Non-3GPP InterWorking Function can be used by non-3GPP access networks to connect to the 5G Core Network. The CP and UP functions of the 5G Core Network are interfaced by the N3IWF via N2 and N3 interfaces, respectively. The standalone non-3GPP accesses are connected to 5G Core Network control-plane and user-plane functions by using the N2 and N3 reference points, respectively. NAS signalling has been supported by a UEin which the 5G Core Network has been accessed over a standalone non-3GPP access by using the N1 reference point with 5G Core Network control-plane functions. For the UE, there exists multiple N1 instances when a NG-RAN and a standalone non-3GPP access is used to connect a UE. A single AMF serves a UE that is simultaneously connected to the same 5G Core Network of a PLMN over a 3GPP access and a non-3GPP access if the location of the selected N3IWF is in the same PLMN as the 3GPP access.

The two PLMNs serve the UE separately when there is a connection of a UE to a 3GPP access of a PLMN if the location of the selected N3IWF is in different PLMN from the PLMN of the 3GPP access. Two separate AMFs are used to register the UE. V-SMFs serve the PDU Sessions over the 3GPP access and different V-SMF serves the PDU Sessions over the non-3GPP access.

There is no dependency of PLMN selection for the 3GPP access on the N3IWF selection. PLMN selection for the 3GPP access is performed by the UE independent of the PLMN which belongs to the N3IWF if a UE is registered over a non-3GPP. An IPSec tunnel shall be established by a UE with the N3IWF in order to get attached to the 5G Core Network over untrusted non-3GPP access. During the procedure of the IPSec tunnel establishment, there is an authentication and attachment of a UE to the 5G core network.

The figure shows the non-roaming architecture for 5G Core Network with non-3GPP access.

The figure shows LBO Roaming architecture for 5G core network with non-3GPP access.

The figure shows LBO roaming architecture for Non-3GPP Accesses when N3IWF is in different PLMN from 3GPP access.

Network Analytics architecture

The figure shows any NF can be allowed by the 5G System architecture so that network analytics information can be requested from NWDAF. The NWDAF is located in the same PLMN where there located the network function.