## 3 Background on 5G System
### NF Communication Procedures  
The architecture of the 5 GC users in a novel paradigm known as SBA. In this innovative architectural framework, a microservice structure is adopted, wherein each component (i.e., NF) is assigned a well-defined and specialized task to enhance modularity. These NFs communicate with one another through API requests via the SBI, facilitating the exchange of services. This architecture accommodates the utilization of NFs from diverse vendors, as the internal workings of these functions are externally exposed through interfaces standardized by the 3 GPP.  
The 3 GPP has laid out meticulous guidelines in TS 29.501 [22] to ensure consistent and interoperable API design across the 5G ecosystem. In this context, OpenAPI version 3 [23] serves as the standardized language for defining APIs. This description format allows for comprehensive interface specifications, enabling both human and machine comprehension of APIs without necessitating extensive source code or documentation scrutiny.  
All the Nxxx interfaces (communication between functions) are based on REST technologies: API requests in HTTP/2, encoded in JSON format [24]; see Figure 1. Authorization and access control within SBA are ensured by OAuth 2.0 [25]. As such, web technologies, particularly RESTful APIs, play a dominant role in managing control functions (e.g., mobility, access control, etc.) within the network.  
This integration of APIs within the SBA framework enables a transformative shift from proprietary protocols to a dynamic, service-oriented environment. This evolution enhances the network's interaction with third-party developers and vertical industries, fostering the development of tailored, on-demand services. The quintessential role of RESTful APIs as the backbone of this architectural model is indisputable, underpinning the technical realization and integration of the 5G ecosystem.  
Integral to the architecture is the integration of NFV, which shifts the network landscape away from dedicated hardware devices in favor of software-based solutions. This transition underscores the significance of adopting cloud-native technologies. Embracing this approach means that network functions like UPF and SMF can be developed using open-source software, such as [3; 4]. The software-hardware disaggregation of the network components working on the Uu air interface has pushed the open-source approach in the RAN, as shown by the Open Radio Access Network (O-RAN) Alliance [26]. This signifies the convergence of open-source and cloud-native principles in 5G deployment, distinguishing it as the pioneer in this domain.  
The interaction among NFs in 5 GC is characterized by a \"producer-consumer\" relationship. This relationship promotes dynamic collaboration, facilitating the smooth exchange of services between functions and thereby enhancing the overall network efficiency.  
Within the orchestration of NFs in the 5 GC, a dynamic model emerges in which each NF serves as both a \"producer\" and a \"consumer\" of services. This dynamic interaction model is based on \"request\" and \"subscribe\" interactions, providing flexibility and adaptability to the system. Figure 2 shows an example of an API call involving NRF. The example refers to the API-driven interaction for session establishment. The SMF discovery and Session Establishment are initiated by the AMF, serving as a key instance of the interaction flow. The NRF plays a pivotal role in facilitating SMF discovery, thereby exemplifying the essence of API-driven communication in SBA.