A Complete Guide to CAPIF Architecture: How 3GPP Standardizes API Exposure in 5G Networks
CAPIF Architecture Explained: The Common API Framework for 5G and Beyond
As we dive deeper into the evolution of 5G networks, it’s clear that openness and programmability are key. This is where telecom APIs come into play; they’re becoming essential tools for innovation. They allow developers and companies to tap into advanced network features — whether it’s edge computing, QoS management, location services, or network slicing.
But there’s a catch: without standardization, we run the risk of creating fragmented API ecosystems, along with security issues and integration headaches.
That’s the role of CAPIF (Common API Framework) — a 3GPP-defined architecture (TS 23.222) aimed at providing a standardized, secure, and cohesive approach for how APIs are exposed and used in 5G networks.
In the image above, titled “CAPIF Architecture”, you can see how the CAPIF Core Function interacts with API Providers and API Invokers, which are spread across both the PLMN and third-party trust domains.
What Is CAPIF (Common API Framework)?
CAPIF is a framework put forth by 3GPP that standardizes the way network APIs are made available, managed, and used within telecom and external ecosystems.
It provides:
A common entry point for all APIs.
Secure onboarding and discovery mechanisms.
Uniform authentication and authorization processes.
Centralized management for API exposure and monitoring.
In essence, CAPIF allows telecom operators to securely open up their network capabilities to developers and outside partners — setting the stage for Network-as-a-Service (NaaS) and Open Gateway initiatives.
Why CAPIF Matters in 5G
5G networks are structured around a service-based architecture (SBA) where different network functions communicate using APIs.
However, things can get tricky when various operators, vendors, and external providers present APIs in different ways, leading to:
Interoperability issues
Security risks
Complicated onboarding for developers
CAPIF addresses these challenges by introducing:
Standardized interfaces (CAPIF-1 to CAPIF-7)
Consistent ways to expose APIs
A trust framework that applies to both PLMN and third-party domains
This setup makes sure every API interaction adheres to a secure, traceable process, no matter the operator or vendor involved.
Understanding the CAPIF Architecture (Based on the Image)
The diagram lays out the core components of the CAPIF framework, which are divided across two trust domains:
PLMN Trust Domain: This one is managed by the network operator.
Third-Party Trust Domain: This is overseen by outside service providers or developers.
Inside these domains, there are three main entities at work:
CAPIF Core Function
API Provider Domain
API Invoker
- CAPIF Core Function
The CAPIF Core Function is the central hub of the framework — it oversees all API activities within the PLMN trust domain.
Key Responsibilities:
Onboarding and authenticating both API Providers and API Invokers.
Managing the API catalog and exposure points.
Enforcing security measures, access control, and audit trails.
Providing CAPIF APIs for managing the API lifecycle.
Core Interfaces:
InterfacePurposeCAPIF-1eAPI Invoker onboarding and authenticationCAPIF-3Onboarding of API ProvidersCAPIF-4API publishing and catalog managementCAPIF-5Access control and authorizationCAPIF-2eExposing service APIs to API Invokers
These interfaces allow the CAPIF Core to ensure that every API, whether internal or external, goes through a secure and standardized workflow before it’s accessible to users.
API Provider Domain
The API Provider Domain includes all systems or network functions that offer APIs — think edge computing nodes, data analytics tools, or third-party service platforms.
As illustrated, each API Provider Domain comprises three key components:
a. API Exposing Function
This function deals with the exposure of service APIs to the CAPIF Core and invokers.
It implements security, rate limiting, and policy controls.
It ensures compliant exposure according to CAPIF standards.
b. API Publishing Function
This function registers APIs with the CAPIF Core Function using CAPIF-4.
It updates API metadata, versions, and availability.
It enables discoverability for authorized invokers.
c. API Management Function
Monitors API usage and performance.
Manages lifecycle events, access tokens, and logging.
Provides analytics and reporting back to the CAPIF Core.
Together, these functions guarantee that APIs are securely published, easily found, and constantly monitored within the telecom network.
- API Invokers
The API Invokers — labeled here as Invoker 1, 2, and 3 — are the ones who actually consume the APIs. They can be:
Internal applications within the PLMN domain.
External applications from third-party sources in the third-party trust domain.
These invokers interact with CAPIF using standardized interfaces:
CAPIF-1e: For authentication and onboarding.
CAPIF-2e: For accessing APIs (service calls).
CAPIF-7e: An optional interface for analytics, logging, and reporting.
Every API request from an invoker must navigate through CAPIF’s trust and authorization protocols, ensuring robust security and traceability.
Trust Domains in CAPIF Architecture
The CAPIF Architecture outlines two trust domains, each responsible for managing authentication, integrity, and authorization processes.
- PLMN Trust Domain
Operated by the Mobile Network Operator (MNO).
Houses the CAPIF Core Function and internal API Providers.
Safeguards the secure management of APIs within the network.
- Third-Party Trust Domain
Managed by external developers, partners, or businesses.
Contains external API Invokers and possibly external API Providers.
Relies on CAPIF’s authentication and authorization rules for secure access.
CAPIF acts as a bridge between these two trust domains, ensuring seamless interaction between operator-controlled and third-party environments.
How CAPIF Works: Step-by-Step Workflow
Here’s a simplified look at how CAPIF manages API exposure and usage:
Onboarding * API Invokers and Providers register with CAPIF Core (via CAPIF-1e and CAPIF-3).
API Registration * Providers use CAPIF-4 to publish their APIs.
API Discovery * Invokers check in with CAPIF Core to find available APIs.
Authorization * CAPIF Core verifies the invoker’s credentials and issues access tokens.
API Invocation * Invokers call APIs through CAPIF-2e, ensuring policy compliance.
Auditing and Reporting * CAPIF logs activities and shares usage stats via CAPIF-7e.
This end-to-end process guarantees secure onboarding, controlled access, and thorough monitoring for every API transaction.
Benefits of CAPIF in 5G Ecosystems
BenefitDescriptionStandardizationConsistent API exposure across operators and vendors.SecurityCentralized authentication, authorization, and auditing.ScalabilitySupports a variety of API providers and invokers.Simplified OnboardingEasier process for registration and discovery.InteroperabilityAllows smooth communication between PLMN and third-party services.Innovation EnablementEncourages developer-driven 5G services and applications.
By offering a unified API management framework, CAPIF empowers operators to safely open their networks, fostering innovation in areas like IoT, edge computing, and AI-driven telecom services.
Real-World Applications of CAPIF
Open Gateway APIs: A way for operators to offer standardized access to network intelligence.
Edge Computing Integration: CAPIF helps manage low-latency service APIs.
IoT Device Management: It provides secure onboarding and control APIs for connected devices.
Enterprise API Ecosystems: Enables businesses to dynamically manage and access network slices.
Essentially, CAPIF lays the groundwork for open, programmable networks, connecting telecom infrastructure with the wider developer ecosystem.
Conclusion
The CAPIF Architecture stands as a key element in the 5G service-based ecosystem, fostering a secure, standardized, and interoperable method for API exposure.
By clearly defining the roles of the CAPIF Core Function, API Providers, and API Invokers, it allows telecom networks to open up safely to external developers without compromising on performance or security.
As operators lean into Open Gateway models, CAPIF will keep being a fundamental part of how telecom APIs are shared, paving the way for new chances in collaboration, monetization, and digital innovation in the 5G landscape.