8.13 SRNS Context Forwarding from Source RNC to CN

25.4133GPPRelease 17TSUTRAN Iu interface Radio Access Network Application Part (RANAP) signalling

8.13.1 General

The purpose of this procedure is to transfer SRNS contexts from the source RNC to the CN (PS domain) in case of handover via the CN. The procedure uses connection oriented signalling. SRNS contexts are sent for each concerned RAB among those that are supported by the target system, and for which at least either GTP-PDU or PDCP sequence numbering is available. The SRNS contexts contain the sequence numbers of the next GTP-PDUs to be transmitted in the uplink and downlink directions, if available, and the next PDCP sequence numbers that would have been used to send and receive data from the UE, if available. The Source RNC PDCP context info shall be sent if available.

8.13.2 Successful Operation

Figure 14: SRNS Context forwarding from source RNC to CN. Successful operation.

The source RNC initialises the procedure by sending a FORWARD SRNS CONTEXT message to the CN. The FORWARD SRNS CONTEXT message contains the RAB Context information for each referenced RAB. For each RAB the following information shall be included:

– RAB ID IE;

– always when available, the sequence number for the next downlink GTP‑PDU to be sent to the UE, i.e. the DL GTP-PDU Sequence Number IE;

– always when available, the sequence number for the next uplink GTP‑PDU to be tunnelled to the GGSN, i.e. the UL GTP-PDU Sequence Number IE;

– always when available, the radio interface sequence number (PDCP) TS 25.323 [17] of the next uplink N-PDU (PDCP SDU) that would have been expected from the UE by a source system i.e. the UL N-PDU Sequence Number IE;

– always when available, the radio interface sequence number (PDCP) TS 25.323 [17] of the next downlink N-PDU (PDCP SDU) that would have been sent to the UE by a source system i.e. the DL N-PDU Sequence Number IE.

8.13.3 Abnormal Conditions

Not applicable.