5G Core network have lots of functions and interfaces between those functions. When user connects a 5G core network, most of the processes are handled by different and separate functions. For example network functions explored by NRF, user's session handled by SMF, network sliced by NSSF, and data is routed by PCF.
Control Interfaces (Control Plane)
There are dashed lines and straight lines on the image given above Picture on page “[3] 5G Core Architecture (Essential)”. Dashed lines means user cannot access those layers/interfaces and those interfaces are used to gather information or handle some processes. As you can see on the image, APIs is written on the dashed lines. This is a pretty good definition. They are some discrete functions and communicate with APIs and report the situation/data.
AMF is one of the core function in 5G core network. We can call it as brain of the network. It handles the jobs and communicates with related functions and gathers information; like a brain.
AMF is crucial but it is worthless without other functions. For example SMF handles user's session, and UPF handles the routing and they both handles 5QI. For more detailed information you can examine the following link.
Data Interfaces (User Plane)
Straight line represents the data interfaces. As you can easily understand there is no user data between functions which are connected with dashed lines. But when a user attached to a 5G core network -after gNB connection- AMF handles the required operations. UE initiates a PDU session request subsequently AMF routes it SMF. SMF interacts with NRF to discover available UPFs. SMF selects optimal UPF and established the user-plane path (Unlike NSA, SA's SMF fully controls UPF selection in the native 5G core).
Note: You can examine the User & Control Planes page for more details about Control Interfaces (Control Plane) & Data Interfaces (User Plane).
!Note: You can examine the following link Free5GC Documents, Free5GC Blogs. These two webpages are highly detailed and pretty good places for studying and understanding the fundamentals of the core.
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