5GC service-based model
5GC control-plane functions expose services through service-based interfaces. A service producer publishes operations and an authorized consumer invokes them according to the discovered instance profile.
NRF registers function profiles and supports discovery. Communication can be direct or can traverse an SCP when the deployment uses a signaling proxy.
AMF and access handling
AMF terminates UE NAS signaling. It handles registration, access, mobility, reachability, and forwarding of session-related messages to SMF. It cooperates with AUSF for authentication and UDM for subscription data.
AMF does not carry application packets. AMF selection considers area, slice, and load. Serving-function changes require transfer of appropriate control context.
SMF and session control
SMF establishes, modifies, and releases PDU sessions. It selects UPF, manages forwarding rules, consults PCF, and participates in UE address-parameter allocation.
The N4 relationship connects SMF and UPF. SMF installs packet-classification and action rules, while UPF reports events or usage information required for session control.
UPF and packet path
UPF is the user-plane function. It terminates the N3 tunnel from NG-RAN, forwards packets to a DN through N6, and applies rules prepared by SMF.
- It classifies traffic against Packet Detection Rules.
- It forwards, drops, buffers, or marks packets.
- It can measure usage and anchor mobility.
- It can be placed near an application to reduce latency.
Subscriber data and authentication
- UDM manages subscriber data, identifiers, and subscription logic.
- UDR stores data exposed to authorized functions.
- AUSF participates in authentication and verifies the subscriber response.
Separating UDM logic from UDR storage lets several functions share a data layer under controlled access.
Policy and slice selection
PCF provides access, session, and QoS policy. SMF uses this policy when creating UPF rules. A policy update can modify an active session.
NSSF assists with slice-instance and AMF selection. It considers requested slice information, area availability, and subscription data.
Network capability exposure
NEF exposes selected 5GC capabilities through a controlled external point. It validates requests, limits information scope, and maps external operations to core services.
AF represents application influence on traffic handling, including routing and QoS requirements. A trusted AF can reach PCF directly, while other applications use NEF.
PDU-session establishment
- UE sends a NAS session request through gNB to AMF.
- AMF selects SMF according to subscription, slice, and Data Network name.
- SMF obtains policy, selects UPF, and creates N4 rules.
- SMF returns information needed by NG-RAN and UE through AMF.
- NG-RAN establishes the N3 tunnel to the selected UPF.
- UPF forwards packets between UE and the DN under session rules.
Function state and scaling
supports scaling but requires consistent external context data. Not every function and procedure is completely stateless.
A cloud deployment can run several instances of one function. NRF, load distribution, recovery, and data repositories must preserve procedure continuity when an instance fails.