Section: 4.9.1.3 Inter NG-RAN node N2 based handover 4.9.1.3.1 General
Content: Clause 4.9.1.3 includes details regarding the inter NG-RAN node N2 based handover without Xn interface. The source NG-RAN decides to initiate an N2-based handover to the target NG-RAN. This can be triggered, for example, due to new radio conditions or load balancing, if there is no Xn connectivity to the target NG-RAN, an error indication from the target NG-RAN after an unsuccessful Xn-based handover (i.e. no IP connectivity between T-RAN and S-UPF), or based on dynamic information learnt by the S-RAN. NTN NR supports additional trigger conditions i.e. time-based trigger condition, upon which UE may execute conditional handover to a candidate cell, as defined in TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [12]. The availability of a direct forwarding path is determined in the source NG-RAN and indicated to the SMFs. If IP connectivity is available between the source and target NG-RAN and security association(s) is in place between them, a direct forwarding path is available. If a direct forwarding path is not available, indirect forwarding may be used. The SMFs use the indication from the source NG-RAN to determine whether to apply indirect forwarding. If both source NG-RAN and source AMF support DAPS the source NG-RAN may decide that some of the DRBs are subject for DAPS handover as defined in TS 38.300[ NR; NR and NG-RAN Overall description; Stage-2 ]  [9]; in this case, the source NG-RAN provides the DAPS information indicating the request concerns a DAPS handover for the DRB as part of the Source to Target (NG-RAN) Transparent Container. If the target NG-RAN accepts that the request concerns DAPS handover and both Target NG-RAN and Target AMF support DAPS, the DAPS handover will be performed and target NG-RAN provides DAPS response information as part of the Target to Source (NG-RAN) Transparent Container. In the case of handover to a shared network, the source NG-RAN determines a PLMN (or PLMN ID and NID, see clause 5.30 of TS 23.501[ System architecture for the 5G System (5GS) ]  [2]) to be used in the target network as specified by TS 23.501[ System architecture for the 5G System (5GS) ]  [2]. The source NG-RAN shall indicate the selected PLMN ID to be used in the target network to the AMF as part of the Tracking Area sent, or the selected SNPN ID to be used in the target network to the AMF, in the HO Required message. If the AMF generates the N2 downlink signalling during the ongoing handover and receives a rejection to a N2 interface procedure (e.g. DL NAS message transfer; Location reporting control; etc.) from the NG-RAN with an indication that an Inter NG-RAN node handover procedure is in progress, the AMF may reattempt the same N2 interface procedure either when the handover is complete or the handover is deemed to have failed if the AMF is still the serving AMF, when possible. If the Inter NG-RAN node handover changes the serving AMF, the source AMF shall terminate any other ongoing N2 interface procedures except the handover procedure. In order to minimize the number of procedures rejected by NG-RAN, the AMF should pause non-handover related N2 interface procedures (e.g. DL NAS message transfer, Location Report Control, etc.) while a handover is ongoing (i.e. from the time that a Handover Required has been received until either the Handover procedure has succeeded (Handover Notify) or failed (Handover Failure)) and continue them once the Handover procedure has completed if the AMF is still the serving AMF. If during the handover procedure the AMF detects that the AMF needs be changed, the AMF shall reject any SMF initiated N2 request received since handover procedure started and shall include an indication that the request has been temporarily rejected due to handover procedure in progress. Upon reception for an SMF initiated N1 and/or N2 request(s) with an indication either from the NG-RAN (via N2 SM Info) or AMF that the request has been temporarily rejected due to handover procedure in progress, the SMF starts a locally configured guard timer. The SMF should hold any signalling messages targeted towards AMF for a given UE during the handover preparation phase unless it detects that the handover execution is completed or handover has failed/cancelled. The SMF may re-attempt, up to a pre-configured number of times, when either it detects that the handover is completed or has failed using message reception or at expiry of the guard timer. In the case of N2 handover within the VPLMN in a home routed roaming scenario, the SMF in the Inter NG-RAN node N2 based handover procedure (Figure 4.9.1.3.2-1 and Figure 4.9.1.3.3-1) interacting with the S-UPF, T-UPF, S-AMF and T-AMF is the V-SMF and the SMF (Figure 4.9.1.3.3-1) interacting with the UPF (PSA) is the H-SMF. In the case of inter-PLMN N2 handover in a home routed roaming scenario, the procedures in clause 4.23 apply.