Core Network Function Testing – Comprehensive Validation of 5G Core, EPC, and IMS Network Functions

Core network function testing is a systematic validation process that verifies the protocol conformance, functional correctness, performance, and reliability of mobile core network functions such as AMF, SMF, UPF, MME, and IMS call session control servers. Based on 3GPP, ETSI, and IETF specifications, accredited third-party laboratories and engineering teams rely on these tests to confirm that 4G/5G core networks deliver stable registration, session management, mobility, voice, and policy services before commercial deployment. This article explains the technical scope, test samples, test items, methodologies, and typical applications of core network function testing.

Technical Overview

The core network is the central part of a mobile communication system that provides connectivity, mobility, session, subscriber, policy, and service management functions behind the radio access network (RAN). Core network function testing refers to the structured verification of these functions against formal specifications, ensuring that every signaling procedure, data flow, and network behavior operates exactly as defined by relevant standards before the network element is released to operators or deployed in live networks.

In 5G systems, the core network adopts a Service Based Architecture (SBA) defined in 3GPP TS 23.501 and 3GPP TS 23.502. Network functions such as the Access and Mobility Management Function (AMF), Session Management Function (SMF), User Plane Function (UPF), Authentication Server Function (AUSF), Unified Data Management (UDM), Policy Control Function (PCF), Network Repository Function (NRF), Network Slice Selection Function (NSSF), and Network Exposure Function (NEF) communicate through Service Based Interfaces (SBI) using HTTP/2 and RESTful principles. These interfaces are specified in the 3GPP TS 29.500 series, while Non-Access Stratum (NAS) signaling is defined in 3GPP TS 24.501.

For 4G LTE, the Evolved Packet Core (EPC) specified in 3GPP TS 23.401 consists of the Mobility Management Entity (MME), Serving Gateway (SGW), PDN Gateway (PGW), Home Subscriber Server (HSS), and Policy and Charging Rules Function (PCRF), which exchange signaling over Diameter and GTP protocols defined by 3GPP and IETF RFCs. The IP Multimedia Subsystem (IMS), which enables VoLTE, VoNR, and rich communication services, adds Call Session Control Functions (CSCF) that operate on the Session Initiation Protocol (SIP).

The objective of core network function testing is to validate conformance to these specifications, verify interoperability among network functions and multi-vendor equipment, confirm security mechanisms such as 5G-AKA and EAP-AKA', and prove that performance, capacity, and reliability targets are met under both normal and abnormal network conditions. Testing is typically performed in laboratory environments using simulated user equipment (UE), simulated base stations, protocol analyzers, and automated test harnesses.

Testing Samples

5G Core Network Functions (5GC)

Individual 5G network functions or complete cloud-native 5GC software stacks submitted for testing include AMF, SMF, UPF, AUSF, UDM, UDR, PCF, NRF, NSSF, NEF, SMSF, and NWDAF. Samples may be delivered as virtual machine images, containerized cloud-native network functions (CNFs), or pre-integrated hardware-software platforms.

Evolved Packet Core (EPC) Elements

4G core samples cover MME, SGW, PGW, HSS, PCRF, and combined gateway products, including virtualized EPC (vEPC) deployments. Testing verifies GTP-C and GTP-U handling, Diameter signaling, bearer management, and attach procedures defined in 3GPP TS 23.401 and related technical specifications.

IMS Network Functions

IMS samples include Proxy-CSCF, Interrogating-CSCF, Serving-CSCF, Emergency-CSCF, IMS-HSS, Multimedia Telephony Application Servers (MMTel AS), and SMS gateways. These are tested for SIP protocol conformance, registration flows, call setup and release, supplementary services, and emergency call handling.

Integrated Core Platforms and Network Slices

End-to-end core platforms, converged 4G/5G cores, private network cores, and slice-specific core instances are tested as complete systems. Samples in this category include pre-configured 5GC/EPC bundles intended for enterprise private networks, industrial IoT deployments, and public operator networks.

Testing Items

Core network function testing covers functional, conformance, security, performance, and reliability dimensions. Typical test items include:

• Registration and deregistration management: initial registration, registration update, periodic registration, implicit deregistration, and UE configuration updates.
• Session management: PDU session establishment, modification, and release; IPv4, IPv6, Ethernet, and unstructured PDU session types; SSC modes 1, 2, and 3.
• Mobility management: service requests, N2-based and N26-based handovers, tracking area updates, idle and connected mode transitions, and inter-gNB mobility.
• Authentication and security: 5G-AKA and EAP-AKA' procedures, key derivation and distribution, NAS and N3 user plane integrity protection, and SUPI concealment (SUCI) handling per 3GPP TS 33.501.
• Policy control and QoS: QoS flow establishment, 5QI mapping, session AMBR and UE AMBR enforcement, policy association management, and event-triggered policy updates.
• Charging: offline charging via Charging Data Records (CDR), online charging via N4 and Gy/N40 interactions, and charging accuracy verification.
• Voice and SMS services: VoNR and EPS fallback call flows, SMS over NAS, SIP registration, basic calls, hold, forwarding, and emergency sessions.
• Interworking and roaming: 4G/5G interworking via N26, roaming architectures (home-routed and local breakout), and inter-PLMN signaling compliance.
• Network slicing: slice selection procedures, slice-specific authentication, SST/SD handling, and slice-level resource isolation.
• Service Based Interface behavior: NRF registration and discovery, OAuth 2.0 token-based authorization, API conformance to 3GPP TS 29.500 series specifications, and OpenAPI schema validation.
• Performance and reliability: signaling throughput, session setup latency, attach and registration success rates, capacity scaling, stateful failover, redundancy, and long-duration stability.

Testing Methods

Testing is executed through a combination of laboratory-based simulation, protocol analysis, and automated test execution. Key methodologies include:

• Protocol conformance testing: test suites derived from 3GPP specification clauses are executed against each network function, with signaling traces captured and compared message-by-message against expected procedure flows using TTCN-3 based or equivalent test harnesses.
• Interoperability testing (IOT): multi-vendor network functions are interconnected in a laboratory to verify that AMF, SMF, UPF, gNB simulators, UE simulators, and IMS elements interoperate correctly across N1, N2, N4, N6, N7, S1-MME, S5/S8, and Gm/Mw reference points.
• End-to-end functional testing: full user scenarios such as attach, PDU session establishment, internet browsing, VoNR calls, handovers, and deregistration are executed with simulated UEs and traffic generators to validate complete service behavior.
• Negative and robustness testing: malformed messages, protocol violations, invalid authentication attempts, timeout conditions, and abnormal release scenarios are injected to confirm that the core network rejects errors gracefully and recovers without crashing or memory leakage.
• Load and stress testing: thousands to millions of simulated subscribers and simultaneous signaling transactions are generated to measure maximum registration rate, session setup capacity, packets per second through the user plane, and system behavior beyond rated capacity.
• Stability and soak testing: sustained traffic and signaling loads run continuously for 24 hours to several weeks to detect memory leaks, resource exhaustion, timer drift, and gradual performance degradation.
• Failover and resilience testing: node termination, link interruption, and virtual machine or container migration scenarios verify stateful redundancy, session continuity, and alarm reporting.
• Automated regression testing: test cases are integrated into CI/CD pipelines using automation frameworks, allowing each software release of the network function to be revalidated quickly and consistently.

Applications

Core network function testing supports a wide range of stakeholders and deployment scenarios across the telecommunications industry:

• Network equipment manufacturers: verifying that 5GC, EPC, and IMS products meet 3GPP conformance requirements before commercial release and operator acceptance.
• Mobile network operators: validating multi-vendor core integration, confirming readiness for network launches, upgrades, migrations from 4G to 5G, and roaming expansions.
• Private 5G network integrators: ensuring that compact core platforms for enterprise campuses, factories, ports, and mines deliver reliable connectivity, slicing, and low-latency user plane performance.
• Cloud and infrastructure providers: certifying virtualized and containerized network functions on different virtualization infrastructure, aligned with ETSI NFV architecture standards.
• Research and certification programs: providing objective evidence for standard compliance in innovation projects, testbeds, and government-supported network initiatives.
• Operations and maintenance teams: reproducing field-reported defects in the laboratory, isolating root causes, and verifying fixes through targeted functional test campaigns.

FAQ

What is core network function testing?

Core network function testing is the process of verifying that the signaling, session, mobility, security, policy, and user plane functions of a mobile core network (5GC, EPC, or IMS) behave correctly and conform to 3GPP, ETSI, and IETF specifications. It combines conformance, interoperability, functional, performance, and stability testing.

Which standards govern core network function testing?

The primary references are the 3GPP specification series, including TS 23.501, TS 23.502, TS 23.503, TS 24.501, the TS 29.500 series for 5GC service based interfaces, TS 23.401 for EPC, and TS 33.501 for security. Protocol behavior additionally references IETF RFCs for Diameter, SIP, HTTP/2, and IP transport, while virtualization aspects align with ETSI NFV standards.

What is the difference between conformance testing and interoperability testing?

Conformance testing checks a single network function against the exact procedures and message formats defined in specifications, using a controlled test harness. Interoperability testing connects network functions or equipment from different vendors in a realistic configuration to verify that they work together correctly in practice.

What test environment is required?

A typical environment includes the core network function under test, UE and gNB/eNB simulators, signaling generators, protocol analyzers, traffic generators for user plane load, and orchestration tools for virtualized deployments. All testing is conducted in isolated laboratory networks with monitored reference clocks and trace capture.

What deliverables are provided after testing?

Standard deliverables include a detailed test plan mapped to specification clauses, executed test case reports with pass or fail verdicts, signaling traces and log files, defect reports for any observed anomalies, and a summary assessment of functional readiness, performance results, and recommendations.

How long does a core network function test campaign take?

Duration depends on scope. A focused conformance campaign for one network function may take one to three weeks, while full end-to-end system validation covering performance, stability, and interworking typically spans one to three months, with automated regression loops shortening subsequent retests significantly.