Hello engineers 
5G networks rely on multiple interfaces to handle signaling, data transport, and mobility.
NG-C’‘Control Plane’’ NG-U ‘‘User Plane’’ Xn-C & Xn-U, To see how these interfaces work together, let’s break down a WhatsApp video call over 5G data.
UE initiates a WhatsApp video call over 5G data
The UE sends a NAS signaling message over the Uu ‘Air Interface’ to request a PDU session for internet access, enabling data communication for the video call. The gNB forwards this request to the AMF over NG-C, without modification.
‘‘Interface: Uu ⟶ NG-C (gNB ⟷ AMF)’’
The AMF checks the subscriber profile in the UDM to verify whether the user is authorized for internet access. After verification, the AMF forwards the session request to the SMF over N11, where the SMF assigns a QoS Flow Identifier ‘QFI’ based on the user’s data plan and network policies.
‘‘Interface: NG-C (AMF ⟷ SMF) over N11’’
The SMF sends QoS rules (QFI, ARP, 5QI) to the AMF, ensuring the network prioritizes video traffic accordingly. The AMF relays this information to the gNB over NG-C via an NGAP message, instructing it to allocate radio resources to match the QoS requirements.
‘‘Interface: N11 (SMF ⟶ AMF) ⟶ NG-C (AMF ⟶ gNB)’’
gNB Assigns PDU Session & Informs the UE ‘Signaling via Uu & NG-C’
The gNB sends an NGAP message confirming the PDU session setup to the UE. The UE receives this confirmation and is now connected to the internet, allowing the WhatsApp video call to proceed.
‘‘Interface: NG-C (gNB ⟷ UE) over Uu’’
The UE begins sending and receiving video call packets over the established PDU session. The gNB encapsulates these packets in GTP-U tunnels and transmits them to the UPF over NG-U, which then forwards them to the internet ‘Data Network: DN’ to reach WhatsApp servers.
‘‘Interface: NG-U (gNB ⟷ UPF)’’
Note:
If the UE moves from one gNB to another, the Xn-C ‘‘Control Plane’’ manages the handover process by transferring the UE’s session context, including QoS parameters, security keys, and bearer information, to the target gNB. At the same time, the Xn-U ‘‘User Plane’’ enables data forwarding from the source gNB to the target gNB to ensure continuous video transmission without packet loss or service interruption.
‘‘Interfaces: Xn-C& Xn-U (gNB ⟷ gNB)’’
When the UE or the network decides to end the video call, a session release request is sent via NAS signaling over NG-C. The AMF notifies the SMF, which instructs the UPF to release the PDU session. Finally, the gNB releases radio resources, officially terminating the video call session.
‘‘Interfaces: NG-C (UE ⟷ AMF) ⟶ N11 (AMF ⟷ SMF) ⟶ NG-U (gNB ⟷ UPF)’’
That’s it 
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