5G Service-Based Architecture (SBA)

The 5G Core Network has been designed around services that are invoked using a standard API. On the surface, the 5G architecture looks very different from the 4G EPC but on close inspection, you can see the evolution from the 4G architecture to the 5G architecture.

We will look at the forcing driving the evolution of the 5G core (5GC) and look at the different functions that are defined in the 5G architecture.

Towards the 5G core network architecture

Evolution from 4G EPC

The 5G core has evolved from the 4G EPC in two steps:

  • Control and User Plane Separation (CUPS) of the 4G EPC
  • Reorganizing the 4G EPC CUPS functions into services

CUPS
The introduction of control and user plane separation in the 4G EPC is the first step towards the 5G architecture. The SGW and PGW functions were split into a control and data plane component.

  • SGW → SGW-C and SGW-U
  • PGW → PGW-C and PGW-U

Reorganization to services

4G EPC to 5GS architecture evolution (Credit: ITU)

As shown in the above figure, the 4G EPC components have been reorganized into service-oriented functions.

Service-based architecture

Service-based interface (request-reply and subscribe-notify) (Credit: ITU)

Service-based architectures have been in use in the software industry to improve the modularity of products. A software product can be broken down into communicating services. With this approach, the developers can mix and match services from different vendors into a single product.

The Wikipedia article on service-oriented architecture summarizes the concept quite well:

A service-oriented architecture (SOA) is a style of software design where services are provided to the other components by application components, through a communication protocol over a network. The basic principles of service-oriented architecture are independent of vendors, products and technologies. A service is a discrete unit of functionality that can be accessed remotely and acted upon and updated independently, such as retrieving a credit card statement online.

Cloud Computing and Virtualization

Virtualization and cloud computing have resulted in lowering the cost of computing by pooling resources in shared data centers.

  • 5G core networks can be shrunk in size by using virtualization. Varies components of the core network can be run as communicating virtual machines.
  • Moving the control plane of the 5G core network to a cloud provider lowers the deployment cost.

5G core network architecture

The 5G core is a mesh of interconnected services as shown in the figure below.

5G System Architecture (credit: Cisco)

AMF — Access and Mobility Function

The AMF performs most of the functions that the MME performs in a 4G network.

  • Terminates the RAN CP interface (N2)
  • NAS signaling
  • NAS ciphering and integrity protection
  • Mobility Management (MM) layer NAS termination
  • Session Management (SM) layer NAS forwarding
  • Authenticates UE
  • Manages the security context
  • Registration management.
  • Connection management.
  • Reachability management.
  • Mobility Management.
  • Apply mobility related policies from PCF (e.g. mobility restrictions)

AMF role in 5G standalone registration provides a good overview of the AMF functions. Click on the diagrams below to learn more.

AMF — Access and Mobility Function Context Diagram
AMF — Access and Mobility Function Sequence Diagram

SMF — Session Management Function

The SMF performs the session management functions that are handled by the 4G MME, SGW-C, and PGW-C.

  • Allocates IP addresses to UEs
  • NAS signaling for session management (SM)
  • Sends QoS and policy information to RAN via the AMF
  • Downlink data notification
  • Select and control UPF for traffic routing. The UPF selection function enables Mobile Edge Computing (MEC) by selecting a UPF close to the edge of the network.
  • Acts as the interface for all communication related to offered user plane services. SMF determines how the policy and charging for these services is applied.
  • Lawful intercept — control plane

SMF interactions in 5G standalone registration are shown below.

SMF — Session Management Function Interactions

UPF — User Plane Function

The UPF is essentially a fusion of the data plane parts of the SGW and PGW. In the context of the CUPS architecture:

  • EPC SGW-U + EPC PGW-U → 5G UPF

The UPF performs the following functions:

  • Packet routing and forwarding
  • Packet inspection and QoS handling. The UPF may optionally integrate a Deep Packet Inspection (DPI) for packet inspection and classification. The following figure shows the classification and QoS handling at the UPF
UPF and UE level classification and marking for QoS Flows (credit: 3GPP TS 23.501)
  • Connecting to the Internet POP (Point of Presence). The UPF may optionally integrate the Firewall and Network Address Translation (NAT) functions.
  • Mobility anchor for Intra RAT and Inter-RAT handovers
  • Lawful intercept — user plane
  • Maintains and reports traffic statistics

UPF functions in 5G standalone registration are illustrated in the following sequence diagram.

UPF — User Plane Function Interactions

PCF — Policy Control Function

The 5G PCF performs the same function as the PCRF in 4G networks.

  • Provides policy rules for control plane functions. This includes network slicing, roaming and mobility management.
  • Accesses subscription information for policy decisions taken by the UDR.
  • Supports the new 5G QoS policy and charging control functions.

PCF interactions in 5G standalone registration provide an overview of the PCF role in the 5G core.

PCF — Policy Control Function Interactions

AUSF — Authentication Server Function

The AUSF performs the authentication function of 4G HSS.

  • Implements the EAP authentication server
  • Stores keys

AUSF role in 5G standalone registration is shown below.

AUSF — Authentication Server Function Interactions

UDM — Unified Data Management

The UDM performs parts of the 4G HSS function.

  • Generation of Authentication and Key Agreement (AKA) credentials
  • User identification
  • Access authorization
  • Subscription management

UDM actions in the 5G standalone registration procedure are illustrated below.

UDM — Unified Data Management Interactions

AF — Application Function

Performs the same function as the EPC AF.

  • Application influence on traffic routing
  • Accessing NEF
  • Interaction with the policy framework for policy control.

NRF — NF Repository Function

Network Repository Function service registration and discovery (Credit: 3GPP)
  • Service registration and discovery function so that Network Functions can discover each other.
  • Maintains NF profile and available NF instances

NEF — Network Exposure Function

NEF provides a mechanism for securely exposing services and features of the 5G core.

  • Exposes capabilities and events
  • Secure provision of information from an external application to 3GPP network
  • Translation of internal/external information
  • Control plane parameter provisioning
  • Packet Flow Description (PFD) management. A PFD is a tuple of protocol, server-side IP and port number.

NSSF — Network Slice Selection Function

Network Slice Selection Function (Credit: Affirmed Networks)

NSSF redirects traffic to a network slice. Network slices may be defined for different classes of subscribers (see the above figure).

The NSSF performs the following functions:

  • Selecting of the Network Slice instances to serve the UE
  • Determining the allowed NSSAI
  • Determining the AMF set to be used to serve the UE

Learn more

The following video by 3G4G describes in the 5GS service-based architecture.

Understand the motivation behind the 4G EPC control and user plane separation.

The system architecture spec builds on what we have learnt here.

3GPP TS 23.501 System Architecture for the 5G System

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