8.3.9 RRC Transfer

38.4233GPPNG-RANRelease 17TSXn Application Protocol (XnAP)

8.3.9.1 General

The purpose of the RRC Transfer procedure is to deliver a PDCP-C PDU encapsulating an LTE RRC message or NR RRC message to the S-NG-RAN-NODE that it may then be forwarded to the UE, or from the S-NG-RAN-NODE, if it was received from the UE. The delivery status may also be provided from the S-NG-RAN-NODE to the M-NG-RAN-NODE using the RRC Transfer.

The procedure is also used to enable transfer one of the following messages from the M-NG-RAN-NODE to the S-NG-RAN-NODE, when received from the UE:

– the NR RRC message container with the NR measurements;

– the E-UTRA RRC message container with the E-UTRA measurements;

– the NR RRC message container with the NR failure information;

– the NR RRC message container with the RRCReconfigurationComplete message;

– the NR RRC message container with the UE assistance information.

In case of RACH based SDT without UE context relocation, this procedure is also used to deliver a PDCP-C PDU encapsulating an NR RRC message between the new NG-RAN node and the old NG-RAN node.

The procedure uses UE-associated signalling.

8.3.9.2 Successful Operation

Figure 8.3.9.2-1: RRC Transfer procedure for dual connectivity, successful operation.

Figure 8.3.9.2-2: RRC Transfer procedure for SDT, successful operation.

Dual Connectivity

The M-NG-RAN-NODE initiates the procedure by sending the RRC TRANSFER message to the S-NG-RAN-NODE or the S-NG-RAN-NODE initiates the procedure by sending the RRC TRANSFER message to the M-NG-RAN-NODE.

If the S-NG-RAN-NODE receives an RRC TRANSFER message which does not include the RRC Container IE in the Split SRB IE, or the RRC Container IE in the NR UE Report IE, or the the RRC Container IE in the Fast MCG Recovery via SRB3 from MN to SN IE, or the the RRC Container IE in the Fast MCG Recovery via SRB3 from SN to MN IE, it shall ignore the message. If the S-NG-RAN-NODE receives an RRC TRANSFER message with the Delivery Status IE in the Split SRB IE, it shall ignore the message. If the S-NG-RAN-NODE receives the RRC Container IE in the Split SRB IE, it shall deliver the contained PDCP-C PDU encapsulating an RRC message to the UE. If the S-NG-RAN-NODE receives the RRC Container IE in the Fast MCG Recovery via SRB3 from MN to SN IE, the S-NG-RAN-NODE shall deliver the contained RRC container encapsulating an RRC message to the UE.

If the M-NG-RAN-NODE receives the Delivery Status IE in the Split SRB IE, the M-NG-RAN-NODE shall consider RRC messages up to the indicated NR PDCP SN as having been successfully delivered to UE by S-NG-RAN-NODE. If the M-NG-RAN-NODE receives the RRC Container IE in the Fast MCG Recovery via SRB3 from SN to MN IE, the M-NG-RAN-NODE shall consider MCG link failure detected at the UE as specified in TS 37.340 [8].

SDT

The new NG-RAN-NODE initiates the procedure by sending the RRC TRANSFER message to the old NG-RAN-NODE or the old NG-RAN-NODE initiates the procedure by sending the RRC TRANSFER message to the new NG-RAN-NODE.

If the new NG-RAN node receives the RRC Container IE in the SDT SRB between New NG-RAN node and Old NG-RAN node IE, it shall deliver the contained PDCP-C PDU encapsulating an RRC message to the UE. If the old NG-RAN-NODE receives the RRC Container IE in the SDT SRB between New NG-RAN node and Old NG-RAN node IE, it shall consider the contained PDCP-C PDU encapsulating an RRC message from the UE.

8.3.9.3 Unsuccessful Operation

Not applicable.

8.3.9.4 Abnormal Conditions

In case of the split SRBs, the receiving node may ignore the message, if the M-NG-RAN-NODE has not indicated possibility of RRC transfer at the bearer setup.