5GC Call Flow
(Assumes both UEs use the same SMF but different UPFs)
Phase-by-Phase Explanation
1. Registration & Authentication
- UE-A and UE-B send Registration Requests to the AMF via the gNB.
- AMF authenticates both devices with the AUSF/UDM (using SUCI/SUPI identifiers).
- Key Interface: N1 (UE ↔ AMF), N2 (gNB ↔ AMF).
2. PDU Session Establishment
- Each UE requests a data session:
- AMF forwards the request to the SMF.
- SMF selects UPF-A for UE-A and UPF-B for UE-B (based on location/slice).
- SMF configures QoS rules via PCF (e.g., 100 Mbps bandwidth for a video call).
- Key Interface: N4 (SMF ↔ UPF).
3. Service Request (Call Initiation)
- UE-A sends a service request to call UE-B (e.g., via SIP signaling).
- SMF identifies UE-B’s PDU session and establishes a GTP-U tunnel between UPF-A and UPF-B over N9.
- Key Interface: N11 (AMF ↔ SMF).
4. Data Transfer
- User Plane Path:
- UE-A’s data → gNB → UPF-A → N9 (GTP-U tunnel) → UPF-B → gNB → UE-B.
- Control Plane:
- SMF monitors sessions and enforces QoS (e.g., prioritizing voice packets).
- Key Protocol: GTP-U (encapsulation/tunneling on N9).
5. Call Termination
- Either UE sends a PDU Session Release.
- SMF instructs UPF-A and UPF-B to tear down the GTP-U tunnel.
- Resources are freed, but UEs remain registered until they detach.
Key 5G Components & Their Roles
| Function | Role |
| AMF | Manages registration, mobility, and authentication. |
| SMF | Controls session setup, UPF selection, and QoS policies. |
| UPF | Routes user data, enforces QoS, and collects billing records. |
| PCF | Provides policy rules (e.g., bandwidth limits). |
| UDM | Stores subscriber data and authentication credentials. |
Why 5G’s Architecture is Unique
- Control/User Plane Separation:
- AMF/SMF handle signaling, while UPF handles data. This allows independent scaling (e.g., deploy more UPFs for video traffic).
- Network Slicing:
- A dedicated slice (e.g., for emergency services) could assign both UEs to the same UPF/SMF for guaranteed performance.
- Edge Computing:
- UPFs can be deployed at the network edge (near gNBs) to reduce latency for AR/VR calls.
Real-World Example
Scenario: A 4K video call between two users in different countries.
- UE-A (Tokyo): Uses UPF-Tokyo (optimized for low latency).
- UE-B (Berlin): Uses UPF-Berlin (local breakout to reduce international hops).
- SMF creates a GTP-U tunnel between UPF-Tokyo and UPF-Berlin, ensuring QoS for 4K streaming.