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5G NAS Registration Reject Cause

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5G (Fifth Generation) mobile networks, NAS (Non-Access Stratum) refers to the higher layer protocols that handle signaling between the user equipment (UE) and the core network. NAS Registration Reject Cause is a message sent by the network to the UE to indicate that the registration request has been rejected. The rejection can happen for various reasons, and each rejection cause is associated with a specific code.

Here's a technical explanation of NAS Registration Reject Cause in 5G:

  1. NAS Registration Request:
    When a UE connects to the 5G network or moves to a new location area, it initiates the registration process by sending a NAS Registration Request to the core network. This request includes information about the UE and its current location.
  2. Network Authentication and Authorization:
    The core network performs authentication and authorization checks based on the received registration request. It verifies the UE's identity, checks its subscription status, and ensures that the UE is allowed to access the network.
  3. Possible Rejection Causes:
    The network may reject the registration request for various reasons, and each rejection cause is identified by a specific code. Some common rejection causes include:
    • UE Identity Cannot Be Derived by the Network: The network is unable to derive the UE's identity based on the information provided.
    • Implicitly Detached: The UE is not registered in the network because it has been implicitly detached.
    • UE Identity Already in Use: The UE identity is already in use by another UE in the network.
    • UE Not Reachable for PS Services: The UE is not reachable for packet-switched services.
  4. NAS Registration Reject Cause Codes:
    The rejection cause is conveyed to the UE through the NAS Registration Reject message, and it includes a cause code. Each cause code corresponds to a specific reason for the rejection. These cause codes are standardized and defined in the 3GPP specifications.
  5. NAS Registration Reject Message:
    The NAS Registration Reject message is sent from the network to the UE, indicating that the registration request has been rejected and providing the cause code for the rejection. This message helps the UE understand the reason for the rejection and take appropriate actions, such as reattempting the registration or notifying the user.
  6. Handling NAS Registration Reject:
    The UE needs to interpret the cause code received in the NAS Registration Reject message and respond accordingly. Depending on the cause, the UE may attempt to re-register, adjust its configuration, or take other actions to resolve the issue.

NAS Registration Reject Cause in 5G involves the rejection of a UE's registration request by the network, with the rejection cause communicated through a NAS Registration Reject message containing a specific cause code. The cause codes help identify the reason for the rejection, enabling the UE to take appropriate actions.