===== 5GC Call Flow =====

{{  :5g_call_flow.png?600  |(Assumes both UEs use the same SMF but different UPFs)}} <font 11.0pt/Calibri;;inherit;;inherit>(Assumes both UEs use the same SMF but different UPFs)</font>

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.