5G Call Flow
The 5G call flow is a sequence of signaling and message exchanges that take place when a device connects to a 5G network and sets up a data session, such as for voice or internet access. This process involves multiple network components, including the 5G core network (5GC), access network (RAN), and user equipment (UE). Below is a detailed breakdown of the 5G call flow, including key steps, signaling procedures, and the roles of the relevant network functions.
Key Components in 5G Call Flow
Before diving into the call flow, it’s important to understand the major network components involved:
UE (User Equipment): The mobile device or terminal.
gNB (Next-Generation Node B): The 5G base station providing radio access.
AMF (Access and Mobility Management Function): Manages registration, authentication, and mobility.
SMF (Session Management Function): Manages session setup, modification, and teardown.
UPF (User Plane Function): Handles user data traffic routing between the 5G network and external data networks.
UDM (Unified Data Management): Stores subscription information and assists in authentication.
PCF (Policy Control Function): Manages policies for quality of service (QoS) and charging.
NSSF (Network Slice Selection Function): Helps with slice selection in the network.
NRF (Network Repository Function): Provides network function registration and discovery.
5G Call Flow Steps
Step 1: Initial Access (RRC Connection Establishment)
When the UE powers on or moves into a new 5G coverage area, it performs an initial access procedure to connect to the network.
Random Access Procedure: The UE initiates the Random Access Channel (RACH) procedure to request access to the network.
RRC Connection Request: The UE sends an RRC (Radio Resource Control) connection request to the gNB, indicating its intent to access the network.
RRC Connection Setup: The gNB allocates resources for the UE, assigns a temporary identifier (C-RNTI), and responds with an RRC Connection Setup message.
RRC Connection Complete: The UE confirms the setup by sending the RRC Connection Complete message to the gNB. At this point, the gNB knows the UE is connected.
Step 2: Registration with the 5G Core Network
Once the RRC connection is established, the UE registers with the core network (5GC):
NAS (Non-Access Stratum) Registration Request: The UE sends a registration request via NAS signaling, including its SUCI (Subscription Concealed Identifier), to the AMF.
Authentication: The AMF interacts with the UDM to authenticate the UE using the 5G-AKA or EAP-AKA’ procedure. The UDM fetches the necessary credentials from the Home Subscriber Server (HSS).
Security Mode Command: After successful authentication, the AMF initiates the security setup with the UE. This includes setting up encryption and integrity protection algorithms for signaling.
Registration Accept: The AMF sends a Registration Accept message to the UE, confirming its registration with the network.
Step 3: Network Slice Selection
5G allows for the use of different network slices for various services. During the registration process, the appropriate network slice is selected.
NSSF (Network Slice Selection Function): The NSSF helps select the right slice based on the UE’s subscription and service requirements.
S-NSSAI (Single Network Slice Selection Assistance Information): The selected slice information (S-NSSAI) is communicated to the UE.
Step 4: PDU Session Establishment (Data Session Setup)
A PDU (Protocol Data Unit) session must be established to enable the UE to send and receive data. This corresponds to the creation of a bearer in 4G/LTE.
PDU Session Request: The UE sends a PDU Session Request message via NAS signaling to the AMF, requesting a session for data services.
SMF Selection: The AMF interacts with the SMF to set up the data session. The SMF is selected based on factors like the UE’s data plan, slice, and service type.
SMF-SMF Communication: The SMF communicates with other core functions (e.g., UPF) to establish the session.
Session Establishment: The UPF (User Plane Function) is allocated to handle the data traffic for the session. The SMF sets up necessary policies through the PCF (Policy Control Function) for QoS and charging.
NAS Response: The AMF informs the UE that the PDU session is successfully established.
RRC Reconfiguration: The gNB configures radio resources for the data session and communicates this configuration to the UE via an RRC Reconfiguration message.
Data Path Established: The data path is set up, enabling data transfer between the UE and external networks, such as the internet, via the UPF.
Step 5: User Data Transfer
After successful PDU session setup, the user data can flow between the UE and external networks:
UE Traffic: The UE sends and receives data over the established session using the user plane resources managed by the UPF.
QoS Enforcement: The UPF enforces the QoS policies defined by the PCF to ensure that the data session meets the required service levels (e.g., latency, bandwidth).
Step 6: Handover (Mobility)
When the UE moves between cells, a handover may be required to maintain an ongoing session:
Measurement Report: The UE reports signal strength from neighboring cells to the gNB.
Handover Decision: If needed, the gNB coordinates the handover with the target cell, notifying the core network.
RRC Reconfiguration: The UE receives a reconfiguration message, instructing it to switch to the target cell.
Step 7: Session Modification and QoS Adjustments
The SMF or PCF can modify the PDU session dynamically, such as changing the QoS parameters or adding/removing network resources for different services (e.g., video streaming vs. file download). The UE and gNB are informed about the changes via NAS signaling and RRC messages.
Step 8: Session Release
Once the data session is no longer needed, the PDU session is released:
PDU Session Release Request: The UE or the network can trigger a session release. The UE sends a PDU Session Release Request via NAS signaling to the AMF.
Resource Cleanup: The SMF coordinates with the UPF and other network functions to release the allocated resources.
NAS Response: The UE is notified that the PDU session has been released.
RRC Reconfiguration: The gNB reconfigures the UE’s radio resources to terminate the data session.
Key Interfaces and Protocols in 5G Call Flow
The 5G call flow involves several interfaces and protocols, each serving a specific role:
NG-C (NG Core) Interface: Between the gNB and the 5GC (AMF and SMF) for control plane signaling.
NG-U (NG User Plane): Between the gNB and UPF for user plane data transfer.
N1, N2, N3 Interfaces:
N1: Between the UE and the AMF for NAS signaling.
N2: Between the gNB and the AMF for signaling and mobility.
N3: Between the gNB and the UPF for user plane traffic.
N4 Interface: Between the SMF and the UPF for session and policy control.
N6 Interface: Between the UPF and external data networks (e.g., the internet).
Conclusion
The 5G call flow is a complex process involving multiple steps, including registration, authentication, session establishment, data transfer, and session release. Each of these steps requires coordination between the UE, gNB, and various core network functions like AMF, SMF, and UPF. With enhanced features such as network slicing, QoS management, and ultra-reliable low-latency communication (URLLC), 5G offers more flexibility and better performance than its predecessors, enabling a wide range of applications from basic internet access to advanced IoT and industrial automation.