Similar to the previous generation 4G (LTE) systems, the Path Switch Request is a signaling message sent by the target base station to the core network during handover to redirect the (user) data path of the terminal's (packet data) session. This message initiates a process where the session management unit instructs the user plane to change the downlink data endpoint from the old site (source) to the new site, ensuring uninterrupted data flow to the user's new location.
I. Path Switch Request In 5G, the path request process establishes a terminal (UE)-related signaling connection with the 5GC and, where applicable, requests switching the downlink terminal point of the NG-U transport bearer to a new terminal point. This process utilizes UE-related signaling.
II. Path Request Process As illustrated in Figure 8.4.4.2-1 below, the “PATH SWITCH REQUEST” is initiated by the target NG-RAN node to the AMF. Its specific definition is as follows:
The NG-RAN node initiates the process by sending a Path Switch Request (PATH SWITCH REQUEST) message to the AMF. Upon receiving the PATH SWITCH REQUEST message, the AMF shall transparently transfer the Path Switch Request Transfer IE to the SMF associated with each PDU session indicated in the PDU Session ID IE.
Upon receiving the PATH SWITCH REQUEST message, the AMF shall deactivate the activated MT communication processing as described in TS 23.502.
III. Path Request Message Processing If the PATH SWITCH REQUEST message contains an RRC Resume Cause IE, the AMF shall (if supported) use it in accordance with the user plane CIoT 5GS optimization provisions for NG-RAN nodes acting as ng-eNBs specified in TS 23.502. If the PATH SWITCH REQUEST message contains a RedCap Indicator IE or eRedCap Indicator IE, the AMF shall (if supported) treat the UE as a RedCap UE or eRedCap UE previously served by an E-UTRA cell, respectively, and use this IE according to TS 23.501. After all necessary updates (including uplink path switching) are successfully completed in the 5GC, the AMF shall send a Path Switch Request Acknowledge message to the NG-RAN node for at least one PDU session resource included in the Path Switch Request. The process then terminates.
IV. PDU Session Handling For an IAB-MT or mobile IAB-MT where the PDU session ID IE in the PATH SWITCH REQUEST message indicates an unassigned PDU session identifier (as defined in TS 24.007), the AMF shall (if supported) consider the IAB-MT or mobile IAB-MT to lack a PDU session and proceed as specified in TS 23.501. Subsequently, the NG-RAN node shall (if supported) ignore the PDU Session Resource Switched List IE in the Path Switch Request Acknowledge message. For each PDU session where the Path Switch Request Transfer IE within the Path Switch Request message contains an Additional DL QoS Flow per TNL Information IE, The SMF can use each included uplink transport layer information as the downlink termination point for the associated QoS flows split across different tunnels for this PDU session.
