5G (NR) RAN Learning - Path Request Failure During Handover

In the 5G system, a Path Switch Request (PATH SWITCH REQUEST) is a request for the terminal (UE) to establish a signaling connection with the 5GC and, if applicable, request the downlink of the NG-U transport bearer to be switched to a new service node. This request can fail for various reasons; 3GPP defines it in TS 38.413 as follows.

I. Path Request Operation Failure

As shown in Figure 8.4.4.3-1 below, a request failure is typically responded to by the AMF after the NG-RAN node issues a "PATH SWITCH REQUEST."

II. Request operation failure scenarios are typically as follows:

• If the 5GC fails to switch the downlink termination point of the NG-U transport bearer to the new (service) termination point for all PDU session resources, the AMF shall send a PATH SWITCH REQUEST FAILURE message to the NG-RAN node.

• The NG-RAN node shall release the corresponding QoS flows and consider the PDU Sessions indicated in the PDU Session Resource Release List IE contained in the PATH SWITCH REQUEST FAILURE message as released.

• The corresponding cause value for each released PDU Session is contained in the Path Switch Request Unsuccessful Transfer IE in the PATH SWITCH REQUEST FAILURE message.

III. Requesting Abnormal Operations

If the AMF receives a message containing multiple PDU Session ID IEs set to the same value (in the PDU Session Resource to be Switched IE in the downlink list), the AMF shall send a PATH SWITCH REQUEST FAILURE message to the NG-RAN node. In addition,

• As an exception, the AMF may generate a Path Switch Request Unsuccessful Transfer IE.

• If a Partially Allowed NSSAI IE is received in a PATH SWITCH REQUEST ACKNOWLEDGE message, and the total number of S-NSSAIs contained in the Allowed NSSAI and Partially Allowed NSSAI exceeds 8, the NG-RAN node shall consider the procedure to have failed.

• If any S-NSSAI present in the Partially Allowed NSSAI IE is also present in the Allowed NSSAI IE, the NG-RAN node shall consider the procedure to have failed.