5G Core Network Architecture: Components & their functional descriptions

Introduction

The transition from 4G to 5G is not just a generational shift in radio technology but a complete transformation of the core network architecture. At the heart of this evolution lies the 5G Core (5GC) — a service-based, cloud-native platform designed to meet the demands of ultra-reliable low-latency communication (URLLC), massive IoT (mMTC), and enhanced mobile broadband (eMBB).

This article explores:

• Key 5GC components and their roles

• How 5GC improves upon 4G EPC

• New capabilities enabled by 5GC architecture

The 5G Core (5GC) architecture is a cloud-native, service-based system designed to enable high speed, low latency, and massive device connectivity. Unlike 4G’s monolithic EPC, 5GC disaggregates functions into specialized Network Functions (NFs) that communicate via standardized APIs. Key components include AMF (mobility/access control), SMF (session management), UPF (data forwarding), and others like PCF, UDM, AUSF, NRF, and NSSF. This modular approach supports network slicing, dynamic scaling, and multi-access use cases. 5GC enables smarter, more flexible mobile networks to power next-gen applications. Below diagram depicts the 5G core components in a 5G network deployment.

Image source: https://www.viavisolutions.com/en-uk/what-5g-architecture

1. 5G Core (5GC) vs 4G EPC – At a Glance

Aspect	4G EPC	5G Core
Architecture	Monolithic	Service-Based (SBA)
Deployment	Appliance/VM-based	Cloud-native, containerized
Mobility	Tight coupling of signaling/data	Decoupled control and user plane
Slicing	Not supported	Native support
Policy	Basic QoS via PCRF	Dynamic QoS via PCF & SMF
Roaming	Complex, node-based	Simplified, interface-driven

 2. Key Components of 5G Core (5GC)

1. AMF – Access and Mobility Management Function

• Equivalent to: MME in 4G

  Functionality:

• Manages UE registration, mobility, and access authentication.

• Receives signaling from the gNB and coordinates with other NFs.

• Stateless function, can scale independently 

During the attachment, the AMF records the TAI (Tracking Area Identity) location and private identity of the mobile and assigns a 5G-GUTI (5G Globally Unique Temporary Identifier) to the mobile.

5G-GUTI replaces the encrypted private identifier SUCI (Subscription Concealed Identifier) and the private identifier SUPI (Subscription Private Identifier).

Once the attachment procedure is completed, the AMF selects the SMF, according to the DNN (Data Network Name) and the network slice indicator NSSAI (Network Slice Selection Assistance Information).

Key Message Interactions:

• From gNB (NGAP): Initial UE Message, UL NAS Transport

• To NRF: NF Discovery Request for UDM, AUSF

• To AUSF: Nausf_UEAuthentication_Authenticate

• To UDM: Nudm_UECM_Registration, Nudm_SubscriberDataManagement

• To SMF: Nsmf_PDUSession_CreateSMContext

• To NSSF: Nnssf_NSSelection_Get

• To UE (NAS): Authentication Request, Security Mode Command, Registration Accept

Role in Handover:

• Receives Handover Required from source gNB.

• Sends Handover Request to target gNB and updates SMF/UPF via context transfer.

 Improvements over MME:

• Only manages signaling; no session context

• Works with all access types: 5G, Wi-Fi, LTE

2. SMF – Session Management Function

• Equivalent to: SGW + PGW Control Plane

Functionality:

• Manages PDU session lifecycle, IP address allocation, and interacts with UPF.

• Session setup, modification, release

• Allocates IP addresses to UEs

• Selects and controls UPF

• QoS policy enforcement (in coordination with PCF)

The SMF (Session Management Function) is responsible for creating, updating and removing PDU (Protocol Data Unit) sessions and managing session context with the UPF (User Plane Function). The SMF injects routing rules to the selected UPFs.

A routing rule corresponds to an entry in the context table of the UPF. This context table contains four fields:

• a correspondence field (PDR (Packet Detection Rule));

• a routing field NH (next hop: IP address, tunnel number TEID (Tunnel End Identifier) or SR (Segment Routing)) to find the next node;

• the quality of service to be applied to the flow (QER (QoS Enhancement Rules));

• the measurement reports to be applied to the flow (URR (Usage Reporting Rules)).

The SMF is responsible for the session management for each DNN and by network slice (S-NSSAI), based on the user profile stored at the UDR.

When requesting a session to be established, the SMF selects a UPF or queries the NRF (Network Repository Function) to obtain the address of the UPF.

The SMF grants an IPv4 or IPv6 address to the mobile. An IP address is provided for each PDU session, based on the address range of the PSA (PDU Session Anchor) selected to join the IP data network. The address range is obtained by either directly querying the selected UPF or by querying the NRF. If the assigned IPv4 address is a private address, the UPF entity performs NAPT (Network Address and Port Translation) in order to translate the IP address and TCP (Transmission Control Protocol) or UDP (User Datagram Protocol) port numbers.

At the end of the IP session, when the mobile enters the standby state, the SMF releases the session by removing the context at the UPF.

In the event of incoming packets, if the mobile is in the idle state, the SMF sends a notification to the AMF (Downlink Data Notification).

Key Message Interactions:

• From AMF: Nsmf_PDUSession_CreateSMContext

• To PCF: Npcf_SMPolicyControl_Create

• To UPF (via N4): Session Establishment, Session Modification

• To UDM: Nudm_SubscriberDataManagement_Get

• To UE (via AMF): PDU Session Establishment Accept

Role in Mobility:

• Updates UPF with new forwarding paths during handover.

• Can initiate a session anchor relocation for session continuity.

Improvement:

• Decoupled from user plane → scales independently

• Enables flexible session control across networks

3. UPF – User Plane Function

• Equivalent to: SGW + PGW User Plane

Functionality:

• Performs packet routing, data forwarding, QoS enforcement, and traffic shaping.

• Usage reporting

• Anchor point for mobility

Image:https://infohub.delltechnologies.com/en-us/p/the-5g-core-network-demystified/

The UPF is the anchor point for traffic when the mobile is moving from one NG-RAN node to another.

The UPF measures the quantity of data consumed for each UE.

The UPF can also implement traffic optimization functions and NAT (Network Address Translation), either from a private IPv4 address to a public IPv4 address, or from an IPv4 address to an IPv6 address and vice versa.

When the UPF receives data from the DN:

• in the absence of a routing context concerning the incoming flow, the UPF informs the SMF. The UPF either stores the data or transmits it to the SMF;

• in the presence of a routing context stored at the UPF level concerning the incoming flow, the flow is either transmitted to an NG-RAN node or to another UPF.

The UPF implements traffic routing rules by configuring the DSCP field of the IP based on the QFI. The QFI is defined by QoS rules which are injected by the SMF to the UPF when establishment of a session is requested. For each incoming piece of data, the UPF performs a traffic inspection (DPI (Deep Packet Inspection)) and classifies the packets into IP flow groups according to the SDF (Service Data Flow) service templates.

Key Message Interactions:

• From SMF (N4): Session Establishment Request, Modify Bearer, Usage Reporting

• To gNB: Uses GTP-U tunnels for data plane forwarding.

• To SMF: Sends Usage Report or buffer status on request.

Role in MEC/Edge:

• Deployed closer to users for low-latency applications (e.g., AR/VR).

• Routes traffic based on Data Network Name (DNN) and slice.

Improvement:

• Distributed close to the edge → supports low-latency use cases (MEC)

• Simplified and scalable user plane

4. PCF – Policy Control Function

• Equivalent to: PCRF

Functionality:

• Governs dynamic policy and QoS management.

• Provides policy rules to SMF:

  • QoS policies

  • Access rules

  • Charging rules

Key Message Interactions:

• From SMF: Npcf_SMPolicyControl_Create, Update, Terminate

• To AF: Receives application influence (e.g., preferred routing)

• To SMF/AMF: Sends QoS rules, session/flow decisions

Features:

• Allows differentiated QoS for network slices, applications, and user contexts.

Improvement:

• Richer context-awareness for per-slice policies

• Works in real-time with dynamic applications

5. AUSF – Authentication Server Function

• Equivalent to: HSS

Functionality:

• Authenticates the UE using the 5G-AKA protocol with support from UDM.

Key Message Interactions:

• From AMF: Nausf_UEAuthentication_Authenticate

• To UDM: Nudm_Authentication_GetAuthData, Nudm_Authentication_ConfirmAuthResult

• To AMF: Sends back Authentication Vectors, Result

Enhancements:

• Works with SUCI/SUPI for enhanced subscriber privacy (public key encrypted IDs).

• Stateless, API-based service

6. UDM – Unified Data Management

• Equivalent to: HSS + HLR

Functionality:

• Stores subscriber data, slice profiles, and authentication credentials.

• Subscription and roaming policies

Key Message Interactions:

• From AMF/SMF/AUSF: Nudm_SubscriberDataManagement, Nudm_Authentication, Nudm_UECM_Registration

• To AUSF: Provides authentication vectors

• To AMF: Sends UE context, access rights, and network slice profile

Improvement:

• Modular, accessible via service-based interfaces

• Enables 5G user identity privacy (via SUCI/SUPI)

7. NSSF – Network Slice Selection Function

Functionality:

• Assists in selecting the appropriate slice for the UE session.

• Assigns UEs to appropriate network slices based on:

  • Subscription profile

  • Requested service

  • Operator policy

Key Message Interactions:

• From AMF: Nnssf_NSSelection_Get

• To AMF: Responds with Allowed NSSAI (slice set), S-NSSAI configuration

Role in Attach:

• Guides AMF in selecting appropriate SMF and other slice-specific NFs.

Use Case:

• Automotive apps use ultra-reliable slices, while video apps use high-throughput slices.

8. NEF – Network Exposure Function

Functionality:

Exposes internal network services to third-party apps securely.

Key Message Interactions:

• From AF/3rd party: Service requests via REST API

• To PCF/UDM/SMF: Policy control, exposure of events (e.g., UE reachability)

• Exposes capabilities of 5GC to external apps:

• QoS info

• Events like UE location, data usage

Use Case:

·       3rd-party IoT platforms get real-time device insights.

9. NRF – Network Repository Function

Functionality:

• Enables NF discovery and tracks the status of registered NFs.

Key Message Interactions:

• From All NFs: NRF Register NFInstance, Heartbeat, Deregister

  • To NFs: NFDiscovery Response including service endpoints

• Use Case:

  • AMF discovers AUSF, SMF discovers PCF, all via NRF queries.

• Benefit:

  • NF instances register/deregister in real-time, enabling microservice elasticity.

10. AF – Application Function

Influences 5GC policy behavior based on application requirements. Interfaces with PCF and NEF.

• To NEF: Application function requests

• To PCF (via NEF or direct): Influence session policy (e.g., request QoS flow for video)

• Allows 3rd-party applications to influence 5G core behavior

  Example: video streaming app can request specific QoS

3. Service-Based Architecture (SBA)

Unlike EPC, which uses point-to-point interfaces, 5GC uses a Service-Based Interface (SBI) model. All core functions expose services via APIs (typically RESTful over HTTP/2). 

Feature	Benefit
Decoupled control/user plane	Flexible scaling
API-based	Faster integration
NF Discovery (via NRF)	Resilience and dynamic deployment
Cloud-native support	CI/CD, containerization, orchestration (K8s)

 4. Advanced Capabilities Enabled by 5GC

Network Slicing

• Each slice has its own AMF, SMF, UPF, PCF

• Tailored for eMBB, URLLC, or mMTC

Edge Computing (MEC)

• UPFs closer to users reduce latency

• Supported natively via distributed architecture

Stateless Functions

• Functions like AMF and SMF are stateless

• Enable better load balancing and failover

Multi-Access Support

• Works across 5G, LTE, Wi-Fi

• Seamless handovers between access networks

 5GC Deployment Models

Deployment	Description
Standalone (SA)	5G NR + 5GC; full capabilities
Non-Standalone (NSA)	5G NR + 4G EPC; transitional mode
Dual Connectivity	UE connects to 5G NR and LTE simultaneously
Option 3/3x, Option 2	Deployment paths defined by 3GPP for gradual migration

 Real-World Deployment Example

 Operator X deploys:

• Distributed UPFs at metro edge → supports AR/VR with <10ms latency

• Centralized SMF, PCF, UDM in cloud → scalable control plane

• NEF interfaces with fleet management app → optimized vehicle tracking

Benefits:

• Cost efficiency

• Low latency

• API-driven service control

Conclusion

The 5G Core Network (5GC) is a leap forward in network design, offering flexibility, cloud-native scalability, and service-aware intelligence. It departs from the static and rigid EPC model of 4G and embraces microservices, APIs, and distributed computing.

References

• NG-RAN and 5G-NR 5G Radio Access Network and Radio Interface, Frédéric Launay