8.23 Direct Transfer

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

8.23.1 General

The purpose of the Direct Transfer procedure is to carry UE – CN signalling messages over the Iu Interface. The UE ‑ CN signalling messages are not interpreted by the UTRAN, and their content (e.g. MM or CC message) is outside the scope of this specification (see TS 24.008 [8]). The UE – CN signalling messages are transported as a parameter in the DIRECT TRANSFER messages. The procedure uses connection oriented signalling.

8.23.2 Successful Operation

8.23.2.1 CN Originated Direct Transfer

Figure 25: Direct Transfer, CN originated. Successful operation.

If a UE – CN signalling message has to be sent from the CN to the UE, the CN shall send a DIRECT TRANSFER message to the RNC including the UE – CN signalling message as a NAS-PDU IE.

If the DIRECT TRANSFER message is sent in the downlink direction, it shall include the SAPI IE and shall not include the LAI + RAC IE and the SAI IE. The use of the SAPI IE included in the DIRECT TRANSFER message enables the UTRAN to provide specific service for the transport of the included NAS message.

Upon receipt of the DIRECT TRANSFER message the RNC shall

– store the Subscriber Profile ID for RAT/Frequency priority IE and use it as defined in TS 36.300 [52].

In case of rerouting in MOCN configuration:

If the CN can serve the network sharing non-supporting UE and CS/PS coordination is not required, the NAS-PDU IE – i.e. the accept NAS message – and the Redirection Completed IE shall be included in the DIRECT TRANSFER message for the downlink direction.

If the CN cannot serve the network sharing non-supporting UE, the NAS-PDU IE – i.e. the reject NAS message – and a Redirection Completed IE shall be included in the DIRECT TRANSFER message for the downlink direction, except for some particular reject causes, see Annex N TS 24.008 [8]. For these particular reject causes, the NAS-PDU IE and the Redirection Indication IE shall be included in the DIRECT TRANSFER message for the downlink direction.

If CS/PS coordination is required, the NAS-PDU IE – i.e. the reject NAS message – and a Redirection Indication IE shall be included in the DIRECT TRANSFER message for the downlink direction.

The Redirection Indication IE shall contain:

– The initial NAS-PDU IE received from the UE;

– The Reject Cause Value IE;

– The NAS Sequence Number IE, if available for CS;

– The Permanent NAS UE Identity IE, if available.

The Redirection Indication IE may also according to TS 23.251 [39] contain:

– The Additional CS/PS coordination information IE;

Upon reception of the downlink DIRECT TRANSFER message including the Redirection Indication IE, the RNC shall store as part of the Rerouting Function the associated Reject Cause Value IE and NAS-PDU IE related to this CN. In case the Reject Cause Value IE is set to "CS/PS coordination required", then the RNC shall perform CS/PS coordination based on the received Permanent NAS UE Identity IE and the Additional CS/PS coordination information IE if available. In this case the Reject Cause Value IE and the associated NAS-PDU IE shall not be stored. The CS/PS coordination may also use the UE Registration Query procedure towards the CN nodes of the other domain according to TS 23.251 [39].

In case all attempted CN operators have replied with the Redirection Indication IE, the RNC shall select the most appropriate NAS-PDU among the NAS-PDU IEs received from the attempted CN nodes based on the stored information as part of the Rerouting function and send it back to the UE.

Upon reception of the downlink DIRECT TRANSFER message including the Redirection Completed IE, the RNC shall send back the included NAS-PDU IE to the UE and terminate the Rerouting Function.

8.23.2.2 UTRAN Originated Direct Transfer

Figure 26: Direct Transfer, RNC originated. Successful operation.

If a UE – CN signalling message has to be sent from the RNC to the CN without interpretation, the RNC shall send a DIRECT TRANSFER message to the CN including the UE – CN signalling message as a NAS-PDU IE.

If the DIRECT TRANSFER message shall be sent to the PS domain, the RNC shall also add the LAI and the RAC IEs, which were the last LAI + RAC indicated to the UE by the UTRAN via the current RRC connection, or if the UTRAN had not yet indicated any LAI + RAC to the UE via the current RRC connection, then the LAI + RAC of the cell via which the current RRC connection was established. If the DIRECT TRANSFER message is sent to the PS domain, the RNC shall also add a Service Area corresponding to at least one of the cells from which the UE is consuming radio resources. If the DIRECT TRANSFER message is sent in uplink direction, the RNC shall not include the SAPI IE.

If the RNC has a co-located L-GW for LIPA operation, it shall include the L-GW Transport Layer Address IE in the DIRECT TRANSFER message.

If the SIPTO L-GW Transport Layer Address IE is received in the DIRECT TRANSFER message, the CN shall, if supported, use it for SIPTO@LN operation as specified in TS 23.060 [21].

If the LHN ID IE is included in the DIRECT TRANSFER message, the CN shall, if supported, use it as specified in TS 23.060 [21].

8.23.3 Abnormal Conditions

If the DIRECT TRANSFER message is sent by the RNC to the PS domain, and any of the LAI IE, RAC IE or SAI IE is missing, the CN shall continue with the Direct Transfer procedure, ignoring the missing IE.

If the DIRECT TRANSFER message is sent by the CN to the RNC without the SAPI IE, the RNC shall continue with the Direct Transfer procedure.

If Redirect Attempt Flag was sent in an INITIAL UE MESSAGE and the corresponding DIRECT TRANSFER message is sent by the CN to the RNC without Redirection Completed IE or Redirection Indication IE, the RNC shall send back the included NAS-PDU IE to the UE and terminate the Rerouting Function.

If the DIRECT TRANSFER message is sent by the CN to the RNC and it contains the SIPTO L-GW Transport Layer Address IE, the RNC shall consider the procedure as failed.

If the DIRECT TRANSFER message is sent by the CN to the RNC and it contains the LHN ID IE, the RNC shall consider the procedure as failed.