9.3.4.1 Handover with SRVCC operation

The source NR node decides to handover the UE with ongoing IMS voice from NR to UTRAN according the following principles:

• The source NR node determines that the UE supports UTRA and requests the UE to send its UTRA radio access capabilities to the NG-RAN;
• The source NR node configures target RAT measurement and reporting;
• The source NR node determines based on the radio conditions and the indication that SRVCC operation is possible that handover to UTRAN should be initiated;
• The source NR node initiates the handover preparation only for the ongoing IMS voice and provides the indication to AMF that the handover is towards UTRAN together with the target UTRAN Node ID. The source NR node also provides an indication to the target UTRAN that the incoming handover originates from 5G. The SRVCC proceeds as specified in TS 23.216 [34];
• The source NR node ensures that the size of the source-to-target container does not exceed the limits that can be handled by the interfaces involved in the handover;

NOTE: For SRVCC handover, the size limit of the inter-node source-to-target container is 2560 octets (see AN-APDU in TS 29.002 [44]).

• Radio resources are prepared in the target RAT before the handover;
• The RRC reconfiguration message from the target RAT is delivered to the source NR node via a transparent container and is passed to the UE by the source NR node in the handover command;
• In-sequence and lossless handovers are not supported;
• Only voice bearer is handed over to target RAT;
• Security procedures for handover to UTRA follows the procedures as specified in TS 33.501 [5];
• Only handover to UTRA-FDD is supported.