5G - id-AMF-UE-NGAP-ID

id-AMF-UE-NGAP-ID

id-AMF-UE-NGAP-ID (often simply called the AMF UE NGAP ID) is an identifier allocated by the Access and AMF. It is used on the 5G core side to uniquely identify a UE  within the context of NGAP-based signaling between the gNB and the AMF.. The main purpose this ID is :

• Uniquely identifies a UE within the context of the AMF during communication with the RAN (e.g., gNodeB) over the NG interface.
• Ensures proper coordination of UE-specific procedures (e.g., session setup, handovers, UE context release) between the AMF and RAN.

Key Characteristics and Functions

In short, id-AMF-UE-NGAP-ID is the core (AMF) side’s unique identifier for a UE, used in conjunction with the gNB’s RAN UE NGAP ID to ensure all NGAP messages can be properly associated with the correct UE context.

Allocation by AMF

• Whenever a new UE-associated signaling connection is established, the AMF generates and assigns the AMF UE NGAP ID to that UE.

Pairing with RAN UE NGAP ID

• On the gNB side, there is another ID called RAN UE NGAP ID.
• Together, (AMF UE NGAP ID, RAN UE NGAP ID) form a unique pair identifying the UE context for all NGAP procedures and messages.

Usage in NGAP Messages

• Once assigned, the AMF UE NGAP ID appears in subsequent NGAP messages—such as Uplink/Downlink NAS Transport and other UE-related signaling—so that the AMF can correctly link messages to the relevant UE.

Importance

• Consistent use of the AMF UE NGAP ID ensures the AMF can track and manage the UE’s signaling context, facilitating mobility management, session management, and any other procedures across the NG interface.

id-RAN-UE-NGAP-ID in Signaling

At the first NGAP message from gNB to AMF, gNB assigns a id-RAN-UE-NGAP-ID for the UE. From this point on, all the NGAP messages from gNB to AMF for this UE uses this ID.

Message: 127.0.1.1:33620 Initial UE message

 

initiatingMessage: {

  procedureCode id-InitialUEMessage,

  criticality ignore,

  value {

    protocolIEs {

      {

        id id-RAN-UE-NGAP-ID,

        criticality reject,

        value 1

      },

      {

        id id-NAS-PDU,

        criticality reject,

        value '7E014295C6102E7...0530101'H

      },

      {

        id id-UserLocationInformation,

        criticality reject,

        value userLocationInformationNR: {

          nR-CGI {

            pLMNIdentity '00F110'H,

            nRCellIdentity '001234501'H

          },

          tAI {

            pLMNIdentity '00F110'H,

            tAC '000064'H

          }

        }

      },

      {

        id id-RRCEstablishmentCause,

        criticality ignore,

        value mo-Signalling

      },

      {

        id id-UEContextRequest,

        criticality ignore,

        value requested

      }

    }

  }

}

At the first NGAP message from AMF to gNB, gNB assigns a id-AMF-UE-NGAP-ID for the UE. From this point on, all the NGAP messages from AMF to gNB for this UE uses this ID.

Message: 127.0.1.1:33620 Downlink NAS transport

 

initiatingMessage: {

  procedureCode id-DownlinkNASTransport,

  criticality ignore,

  value {

    protocolIEs {

      {

        id id-AMF-UE-NGAP-ID,

        criticality reject,

        value 100

      },

      {

        id id-RAN-UE-NGAP-ID,

        criticality reject,

        value 1

      },

      {

        id id-NAS-PDU,

        criticality reject,

        value '7E005B01'H

      }

    }

  }

}

Reference :

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