A Beginners Guide for 5G Core Network Architecture

5G Core Network Architecture

We are witnessing floods of information on potential 5G services, use cases, implementations, and their impact across different verticals. 5G is providing connectivity and services to Consumers, Enterprises, and Things (Devices, Machines, Sensors, etc.).

Certainly, 5G looks very promising in terms of parameters like high bandwidth, high capacity, high speed, low/ultra-low latency, high-density coverage, high availability, low device energy consumption, high throughput, different treatments to different network slices, etc.

5G Core (5GC) network architecture implementation doesn't exactly follow the network evolution upgrade path as previously followed by 2G, 3G, and 4G networks and this is what makes 5G Architecture very different from its predecessors. In 5G, Network Management is envisioned to be Software-driven, and Network functions & resources are virtualized at the Edges and Core.

(Click to enlarge) Evolution of Mobile Network

How 5G is different from its predecessors?

Unlike previous cellular generations, 5G implementation is based on:-

• Cloud-native applications.
• REST services-based integration.
• Virtualized Network Functions.
• Softwarization of Network & IT.
• Support for Stateless Network functions by decoupling Compute and Storage.
• Microservices-based design patterns.
• DevOps, CI/CD methodologies for faster time-to-market offerings.
• Network slice-based approach of utilizing the physical network resources.
• Mobile access edge computing for delivering & processing low latency contents & data.
• Providing cellular connections to things & devices and supporting very high density.
• Handling advanced analytics.
• Separation of Control & User planes.
• Network capability exposure via APIs and Service Bus.
• Support for Centralized and Distributed processing.
• etc.

All these characteristics make 5G quite different and effective than older generations. However, it's worth noting that some 4G Operators were early adopters of the virtualization of network elements and separating their EPC User & Control planes for better service delivery & optimizing control signaling.

5G System includes 5G New Radio (NR) as the Access network and 5G Core (5GC) Network.

5G Core Network can be based on:-

1. Reference-Based Architecture

2. Service-Based Architecture 

If you follow 3GPP specs, then you can easily relate to point-to-point Reference-based Architecture. It is a traditional way of representing the point-to-point network elements' integration with each other based on their reference points. Here, more focus is given to interfaces between different network elements, protocols used, and information exchanges between them. Basically, the approach is to offer Products & Services to Consumers based on Network capabilities or limitations. This is how Operators were architecting their Networks in the 2G, 3G, and 4G eras.

In this article, we will put our focus on Service-Based Architecture (SBA) which is the recommended and programmable architecture design for the interconnection of 5G network functions in the Core and exposure of network capabilities & resources via Service message bus. SBA is more aligned with the 5G's Service-centric requirements of the Cloud-native model for Softwarized Network functions (NFs), Virtualized deployments of NFs at the Edges & Core, and their integration with the OSS/BSS layers.

A typical 5G Core Service-Based Architecture (SBA) is depicted below: -

(Click to enlarge) 5G Core Service-Based Architecture

In SBA, the approach is to adapt/evolve/expose/develop the Network capabilities based on the new generation of Services offered in the 5G world. As shown in Figure 2, Network functions based on the Control Plane are interconnected via the Service message interface Bus for exposing the network capabilities within and outside the Core Network.

Network functions are connected via a Service Bus in the Control Plane from the Service-Based Architecture (SBA) in the 5G Core Network.

In SBA, Network Functions (NFs) capabilities are exposed via REST APIs and based on HTTP2.0 protocol. Interconnection between NFs can be based on the Request/Response model or Subscribe/Notify model for availing the different 5G Services.

This is in line with the very requirements of today's Application-driven Digital Service Providers which require agility and quicker time-to-market for the products/services rollouts.

Details about 5G Network Functions:-

UPF (User Plane Function): It performs operations like maintaining PDU Sessions, Packet routing & forwarding, Packet inspection, Policy enforcement for User plane, QoS handling, etc. When compared with 4G EPC, its functionalities resemble SGW-U (Serving Gateway User Plane function) and PGW-U (PDN Gateway User Plane function) of 4G Network.

AMF (Access and Mobility Management Function): It performs operations like Mobility Management, Registration Management, Connection Management, etc. It acts as a single entry point for the UE connection. Based on the Service requested by the Consumer, AMF selects the respective SMF for managing the user session context. When compared with 4G EPC, its functionalities resemble with MME of the 4G Network.

AUSF (Authentication Server Function): It allows the AMF to authenticate the UE. When compared with 4G EPC, its functionalities resemble with HSS/AAA Server of the 4G Network.

SMF (Session Management Function): Performs operations like Session Management, IP Address allocation & management for UE, User plane selection, QoS & Policy enforcement for Control Plane, used for Service registration/discovery/establishment, etc. When compared with 4G EPC, its functionalities resemble MME, SGW-C (Control Plane), and PGW-C (Control Plane) of the 4G Network.

NSSF (Network Slice Selection Function): It maintains a list of the Operator defined network slice instances. AMF authorizes the use of Network slices based on the Subscription info stored in UDM or it can query NSSF to authorize access to a Network slice based on the service requirements. NSSF redirects the traffic to an intended Network slice.

NEF (Network Exposure Function): It exposes services and resources over APIs within and outside the 5G Core. 5G Services exposure by NEF is based on RESTful APIs. With the help of NEF, 3rd party applications can also access the 5G services.

NRF (NF Repository Function): It maintains the list of available Network Functions instances and their profiles. It also performs Service registration & discovery so that different Network functions can find each other via APIs. As an example, SMF which is registered to NRF; gets discoverable by AMF when UE tries to access a Service type served by the SMF. Since Network functions are connected via Service Message Bus in SBA, any authorized Consumers can access the services offered via registered Network functions (Producers).

PCF (Policy Control Function): It supports the Policy control framework, applying Policy decisions, accessing Subscription information, etc. to govern the Network behavior. When compared with 4G EPC, its functionalities resemble with PCRF of the 4G Network.

Unified Data Management (UDM): It performs operations like User identification handling, Subscription Management, User Authentication, and Access Authorization for operations like Roaming, etc. When compared with 4G EPC, its functionalities resemble with HSS/AAA Server of 4G Network.

AF (Application Function): It performs operations like accessing the Network Exposure Function for retrieving resources, interaction with PCF for Policy Control, exposing services to End users, etc. When compared with 4G EPC, its functionalities resemble AF of 4G Network.

The below figure shows how the functionalities present in the 4G EPC Network Elements are evolved and mapped against respective 5G Core Network Functions.

Mapping of 4G EPC Network Elements with 5G Core Network Functions:-

(Click to enlarge) EPC Network Elements Mapping with 5G Core Network Functions

SBA provides the much-required flexibility to register & discover NFs and exposes 5G Services over a Cloud-native framework. And herein, lies the strength & might of a 5G Network Architecture to support a connected digital world with the required flexibility, agility, and scalability.

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Glossary: SBA (Service Based Architecture), UE (User Equipment), UPF (User Plane Function), AMF (Access Management Function), AUSF (Authentication Service Function), SMF (Session Management Function), NSSF (Network Slice Selection Function), NEF (Network Exposure Function), NRF (NF Repository Function), PCF (Policy Control Function), UDM (Unified Data Management function), AF (Application Function), MME (Mobility Management Entity), SGW (Serving Gateway), PGW (PDN Gateway), HSS (Home Subscriber Server), AAA (Authentication, Authorization & Accounting Server), EPC (Evolved Packet Core), PCRF (Policy & Charging Rules Function), CN (Core Network), NFs (Network Functions)

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