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5G NAS PDU Session Reject Cause Values and Reasons

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the 5G (Fifth Generation) mobile network architecture, NAS (Non-Access Stratum) is responsible for control plane signaling between the UE (User Equipment) and the AMF (Access and Mobility Management Function) in the core network. The PDU (Protocol Data Unit) Session is established between the UE and the SMF (Session Management Function) to provide data connectivity services.

When a PDU Session establishment request is received from the UE, the AMF and SMF may decide to reject the request due to various reasons. The PDU Session Reject message contains a cause value that indicates the reason for the rejection. The cause value helps in understanding why the PDU Session establishment request was denied.

Here are some of the NAS PDU Session Reject cause values and their associated reasons:

  1. Insufficient Resources (0x01):
    • Reason: The network may not have enough resources to establish the requested PDU Session. This could be due to congestion or resource limitations in the network elements such as the SMF or UPF (User Plane Function).
  2. Invalid PDU Session ID (0x02):
    • Reason: The PDU Session ID provided in the request is not valid. This could happen if the UE or network tries to establish a PDU Session with an invalid or non-existing Session ID.
  3. Semantic Error in the TFT Operation (0x03):
    • Reason: There is a semantic error in the TFT (Traffic Flow Template) operation requested by the UE. TFT defines the packet filters for a PDU Session, and if there's an error in its configuration or operation, the request may be rejected.
  4. Syntactical Error in the TFT Operation (0x04):
    • Reason: The TFT operation requested by the UE contains syntactical errors. This means that the format or structure of the TFT provided is incorrect or not supported.
  5. Semantic Errors in Packet Filter(s) (0x05):
    • Reason: There's a semantic error in the packet filters provided by the UE. Packet filters define the criteria for filtering packets within a PDU Session, and any error in its definition can lead to rejection.
  6. Syntactical Errors in Packet Filter(s) (0x06):
    • Reason: The packet filters provided by the UE contain syntactical errors. This means that the format or structure of the packet filters is incorrect.
  7. Service Option Not Supported (0x07):
    • Reason: The requested service option for the PDU Session is not supported by the network. The network may not support certain features or configurations requested by the UE.
  8. Requested Service Option Temporarily Out of Order (0x08):
    • Reason: The requested service option is temporarily unavailable or out of order. This could be due to maintenance activities or temporary network issues.

These are some of the NAS PDU Session Reject cause values and their associated reasons in the 5G network. Each cause value provides specific information about why a PDU Session establishment request was rejected, helping in troubleshooting and resolving connectivity issues.