8.11 SRNS Context Transfer

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

8.11.1 General

The purpose of the SRNS Context Transfer procedure is to trigger the transfer of SRNS contexts from the source RNC to the CN (PS domain) in case of intersystem change or in some further cases described in TS 23.060 [21]. The procedure uses connection oriented signalling.

8.11.2 Successful Operation

Figure 12: SRNS Context Transfer procedure. Successful operation.

The CN initiates the procedure by sending an SRNS CONTEXT REQUEST message to the source RNC. The SRNS CONTEXT REQUEST message shall include the list of RABs whose contexts should be transferred, and may include the RAT Type IE, when available to indicate the RAT from which the context request originates.

The source RNC shall respond to the CN with an SRNS CONTEXT RESPONSE message containing all the referenced RABs, including both successful and unsuccessful RABs transfers. For each RAB whose transfer is successful, the following context information elements 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 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;

– 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.

Transmission and reception of the SRNS CONTEXT RESPONSE message terminate the procedure in the UTRAN and in the CN respectively.

8.11.3 Unsuccessful Operation

For each RAB for which the UTRAN is not able to transfer the RAB context, e.g. if the RAB ID is unknown to the RNC, the RAB ID is included in the SRNS CONTEXT RESPONSE message together with a Cause IE, e.g. "Invalid RAB ID".

8.11.4 Abnormal Conditions

Not applicable.