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5g nsa attach call flow

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The 5G Non-Standalone (NSA) architecture allows 5G networks to be deployed on existing 4G LTE infrastructure. The NSA approach involves using both 4G LTE and 5G NR (New Radio) for communication. The following is a high-level overview of the 5G NSA attach call flow, explaining the key steps involved:

  1. UE (User Equipment) Attachment to LTE Network:
    • The UE initially attaches to the LTE network, just like in traditional LTE networks.
    • The UE goes through the Attach procedure with the LTE Evolved Packet Core (EPC).
    • The EPC assigns the UE an IP address and establishes the default EPS (Evolved Packet System) bearer.
  2. Registration with 5G NR:
    • After attaching to LTE, the UE initiates the registration process with the 5G NR.
    • The 5G NR node involved in this step is the gNB (gNodeB), which is responsible for handling 5G radio communication.
  3. Access Registration Request (RRC Connection Request):
    • The UE sends an RRC (Radio Resource Control) Connection Request message to the gNB.
    • This message includes information like the UE's capabilities, identity, and the type of connection requested (e.g., Initial registration).
  4. Initial Context Setup:
    • The gNB sends an RRC Connection Setup message to the UE, indicating that the initial context setup is successful.
    • The Initial Context Setup includes information necessary for radio resource allocation and establishment.
  5. Bearer Establishment with 5G Core:
    • The gNB initiates the establishment of a 5G NR bearer with the 5G Core (5GC).
    • The gNB sends a message to the 5GC, known as the NGAP (Next-Generation Application Protocol) message, indicating the need for a new Radio Bearer Setup.
  6. PDU Session Establishment:
    • The 5GC responds to the NGAP message by initiating the establishment of a PDU (Packet Data Unit) session.
    • The PDU session is a concept in 5G that represents the end-to-end IP connectivity between the UE and the data network.
  7. Data Transmission:
    • With the 5G NR bearer and PDU session established, data can be transmitted between the UE and the 5G network using 5G NR.
  8. Dual Connectivity (LTE and 5G):
    • The UE may continue to use the LTE connection concurrently with the 5G NR connection for certain services.
    • This dual connectivity allows for seamless mobility and service continuity as the UE moves between LTE and 5G coverage areas.